The potential failure of the liquid-tightness of the energy storage liquid cooling pack involves multiple aspects, such as: leakage, corrosion and deposition, condensation water and other failure modes.

1- Fluid interconnection and composition

In the energy storage liquid cooling system, the fluid interconnection is responsible for transferring the coolant between the various components. Through effective fluid interconnection, the coolant is ensured to circulate efficiently in the system, thereby removing the excess heat generated during the battery charging and discharging process. 

A well-sealed system can effectively prevent coolant leakage. Leakage will not only lead to coolant loss and require frequent replenishment, but also affect the heat dissipation performance and stability of the system. In energy storage, coolant leakage may also cause battery Short circuit, causing safety problems.

2-Liquid-tight design of fluid interconnection system

The liquid-tight design of the fluid interconnection system is the key link to ensure that the system maintains sealing and prevents fluid leakage under various operating conditions.

Figure 1: Typical deployment of energy storage liquid cooling system

(1) Analyze possible leakage sources and risk points in the system:

l The self-sealing property of the liquid cooling assembly. For example, in the integrated design of the liquid cooling channel system and the Pack box, the components are connected by welding. Welding quality defects, poor welding, pores, cracks, etc. may all lead to liquid seepage problems.

l The structural design is unreasonable. For example, the positioning holes or threaded holes of the liquid cooling box are too close to the flow channel, and the poorly welded parts can easily become channels for liquid seepage.

l Connection parts: The pipe connections, valves and joints of the liquid cooling system are common leakage points. If the connection structure is not designed properly or the manufacturing process is not sophisticated, there are tiny defects inside the joints, and the coolant may also leak from these defects.

l Leakage caused by improper installation, material aging or damage, etc.

(2) Sealing structure design:

l The liquid-cooled PACK uses a dry-wet separated cold plate cooling method. Under normal working conditions, the battery cells have no contact with the coolant, which can ensure the normal operation of the battery cells. One solution for the energy storage liquid cooler is to form it through an extrusion process, integrate the flow channel directly on the cold plate, and then use mechanical processing to open up the cooling circulation path. In this process, choosing the right welding process is an important step to ensure sealing. For details, please refer to "Design of welding process for lower box for energy storage".

l Liquid cooling pipelines are mainly used for transitional soft (hard) pipe connections between liquid cooling sources and equipment, between equipment, and between equipment and pipelines. The main connection methods are:

Quick-connection: One of the connection methods for energy storage liquid cooling systems is to use VDA or CQC quick-connection.

Threaded connection: Both ends of the connection structure are slidably connected with pipes, and the threaded connection between the internal thread ring and the threaded sleeve increases the firmness of the connection.

Limiting pipe and nut connection: A connecting pipe is clamped at one end of the pipe, and limiting pipes are fixedly installed on both sides of the connecting pipe. Rubber washers and convex rings are fixedly installed inside the limiting pipes, and a limiting ring groove is opened on the surface of the connecting pipe head. A nut is rotatably connected to the top of the limiting tube and is rotatably connected to the limiting tube through threads.

Sealing ring connection: A sealing ring is adhered to the inner wall of the threaded sleeve by strong glue, and the inner wall of the sealing ring is movably connected to the outer surface of the pipe to prevent leakage during use.

(3) The PACK liquid cooling plate, cabin interface, cabin pipeline, etc. are all designed with long-term corrosion protection under common coolant, common temperature and flow rate conditions to ensure long-term operation without corrosion. Effect of operating conditions on liquid tightness:

l Temperature. Influence of high temperature: As the temperature increases, the viscosity of the liquid generally decreases, which may cause the sealing performance of the liquid to decrease, thereby affecting the liquid tightness. For example, certain sealing materials may deform or deteriorate at high temperatures, causing leakage. Influence of low temperature: In a low temperature environment, the liquid may become viscous, increasing the difficulty of flow, but it may also improve the performance of the sealing material, thereby enhancing liquid tightness to a certain extent.

l Pressure. High pressure environment: Under high pressure, the density and viscosity of the liquid may increase, thereby improving the sealing performance of the liquid. However, excessive pressure may also damage the sealing material and cause leakage. Low-pressure environment: Under low pressure, the sealing performance of the liquid may be relatively weak, especially if the sealing material itself is defective or aged, it is more likely to leak.

l Flow rate. High flow rate: When the liquid flows at high speed, it may produce a large impact force on the sealing surface, causing wear or deformation of the sealing material, thereby affecting the liquid tightness. Low flow rate: At low flow rate, the sealing performance of the liquid is relatively good, but this may also mask some potential sealing problems such as minor material defects.

3-Corrosion and deposition problems

l Effects of blocking on confidentiality:

Coolant, deposits or boiler growth may cause internal blockages, poor coolant flow and reduced cooling efficiency.

Fouling and scaling: Minerals in the coolant may form deposits on the inner wall of the pipe after long-term operation, which is called "scale". Fouling may also be formed due to solid particle precipitation, crystallization, corrosion or microbial activity. These dirt will clog pipes and cold plates, increase flow resistance and reduce heat transfer efficiency.

Foam problem: Foam may be generated in the liquid cooling system. The foam will adhere to the surface of the cold plate, resulting in a decrease in heat transfer effect, and may increase the resistance in system operation, cause cavitation corrosion to the pump, etc., and damage the equipment.

l The influence of eddy current on air tightness:

When a fluid flows in a pipe or gap, changes in velocity can cause eddies to form, especially when the fluid passes through narrow parts or obstacles, eddies are more likely to form. The viscosity and density of the fluid also affect the generation of vortices. Fluids with higher viscosity are more likely to form vortices, while fluids with higher density may weaken the formation of vortices.

Leakage paths: Eddy currents form vortices on contact surfaces, which may form tiny leakage paths in gaps or irregular surfaces, resulting in leakage of gas or liquid.

Surface wear: Eddy flow can cause wear of contact surfaces, especially in high-speed flow conditions. This wear can further reduce air tightness because the worn surfaces are more likely to form new leakage channels.

Thermal effects: Eddy current flow generates heat, which may cause deformation or thermal expansion of the contact surface material, thus affecting the airtightness, especially in systems with large temperature changes.

4-Condensation water problem

Under certain conditions, condensation may form in the liquid cooling lines, which may cause damage to the equipment or reduce efficiency. Insulation failure: If the insulation material of the pipe is damaged or aged, heat will be lost and the cooling effect will be affected. Especially in low temperature environments, insulation failure can cause frost or ice to form on the pipe surface. Frost cracking: In cold environments, if proper antifreeze measures are not taken, the coolant in the pipes may freeze and cause the pipes to rupture.

Solutions

l Sealing measures: Ensure that the inlet and outlet of the liquid cooling pipeline are completely blocked to prevent external humid air from entering the battery compartment.

l Dehumidification equipment: Install a dehumidification air conditioner or use the dehumidification function to maintain the humidity in the battery compartment within an appropriate range.

l Temperature control: By installing air conditioning or ventilation systems, the temperature and humidity of the environment where the energy storage cabinet is located can be controlled. For example, the temperature can be kept at 20-25 degrees Celsius and the relative humidity can be controlled at 40%-60%.

l Isolation measures: Simple isolation of empty battery racks to prevent moisture from entering the compartment containing the battery cluster.

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The fully sealed design of the energy storage pack is the key to ensuring its safety and long-term stable operation. Sealing is essentially the use of a device to close (seal) a gap or make a joint leak-proof. The fully sealed design can effectively prevent liquid and gas leakage within the battery cell, which is crucial to ensuring the safe and stable operation of the energy storage system. Therefore, when designing, both air tightness and the sealing of the liquid medium must be considered.

In actual operation, the energy storage pack sealing design needs to comprehensively consider multiple factors such as materials, processes, testing equipment, environmental conditions, and manufacturing processes to ensure that its sealing performance can meet the expected standards. This article explains the application practice and key points of energy storage Pack sealing design in actual engineering from the aspects of Pack box airtightness, liquid cooling cycle liquid tightness and liquid cooling medium.

Previous article: Energy storage pack box airtightness design

The sealing design helps maintain the temperature and pressure inside the energy storage pack stable, which plays a key role in the normal operation and performance of the battery; and the sealing design can reduce the impact of the external environment on the internal battery, such as moisture, dust and other pollutants, etc., thereby improving the reliability and service life of the system. In addition, the use of appropriate sealing materials and structures can effectively improve the wear resistance and aging resistance of seals, enhance the durability of the entire energy storage system, and reduce maintenance costs.

The general idea of airtight design is to analyze the box structure to find out the key areas where leakage may exist, and then take targeted measures according to the specific performance and functional requirements of different areas.

1-Box structure analysis

The box is not only the physical carrier of battery modules and electrical components, but also an important guarantee for the safe and reliable operation of the entire energy storage system. It is the "skeleton" of the energy storage pack, which is generally composed of an upper cover, a lower box, support components, and a sealing Parts and bolts, etc.

Figure 1: Schematic diagram of the energy storage pack box and key areas of focus in the sealing design (e.g., marked with red arrows)

As shown in the figure above, find out where potential leaks may occur:

l Multiple parts connection points, such as: the assembly interface between the upper cover and the lower box, the installation interface between the high and low voltage connectors and the box, the installation interface between the exposed components and the battery box, etc.

l If bolts are used for connection, there may also be a risk of leakage at the installation and fixing point, such as the electrical interface and the front panel installation interface of the box.

l There must be no holes or gaps in the upper cover and lower body of the box to ensure the sealing and protective performance of the box.

Figure 2: Immersion liquid cooling lower box (sheet metal frame + aluminum liquid cooling bottom plate)

2- Sealing design of the installation interface between the upper cover and the lower box

The upper cover can generally be divided into two types: flat type and special-shaped type. Their structural characteristics are also different. For example, SMC composite material, aluminum, no matter what material, in order to reduce the complexity of the self-sealing structure, the upper cover of the battery shell The cover is usually of one-piece design. In addition, the opening requirements of the upper cover should also meet the requirements of the interface, and should be independent of the sealing interface to reduce the impact on the battery pack sealing. The upper cover seal design generally follows the following principles:

l The integrated parts design is adopted to avoid the design of separate parts, thus ensuring the stability of the "self-sealing" performance of the upper cover.

l The positioning holes and positioning features are designed on the edge of the upper cover (outside the sealing interface between the upper cover and the lower tray).

l The sealing interface between the upper cover and the lower box body requires the matching surface to meet the "uniform" and "continuous" sealing requirements.

At present, the mainstream solutions for the lower box of the energy storage pack are: sheet metal box + liquid cooling plate, die-cast box + liquid cooling plate, profile integrated box, die-cast integrated box, etc. Among them, the profile integrated box and other solutions In contrast, it has the advantages of good flow channel bearing capacity and low mold opening cost, and is widely used. The choice of welding process has a great influence on the sealing performance. For weldments of different materials and thicknesses, choosing a suitable welding method can effectively improve the weld quality to ensure the overall strength and sealing performance of the system.

In addition, the sealing design of the lower box should follow the following sealing principles:

l Closed section profiles are used for frame construction, and self-sealing linear connection technology, such as CMT welding technology, is used at the joints.

l Battery trays made of aluminum profiles need to be designed with one or more continuous layers of sealing colloid.

l In the case of an integrated liquid cooling plate in the lower box, it is necessary to consider using colloid seals or self-sealing linear connection technologies, such as FSW friction stir welding technology.

l The sealing interface between the upper cover and the lower box body needs the matching surface to meet the "uniform" and "continuous" sealing requirements. If necessary, the sealing interface should be machined and polished.

Figure 3: Common sealing forms between the upper cover and the lower box

Usually, the upper cover and lower box of the energy storage Pack box adopt a bent flange and a sealing gasket design, as shown in Figure 2. The upper cover, lower box body and sealing gasket are fully compacted and coupled by fastening bolts to ensure that the Pack box meets the relevant requirements of IP67.

3- Sealing design of electrical and communication interfaces and the front panel installation interface of the lower box

The front panel of the box (as shown in Figure 3) is machined with holes on the extruded profile for installing electrical and communication interfaces to achieve functions such as current transmission, communication interaction, and safety control.

Figure 4: Electrical, communication interface and lower cabinet front panel installation interface

The air tightness of the installation interface between the box and the electrical, communication and other interfaces shall follow the following principles:

l The interface shape is designed to be streamlined to reduce the possibility of gas and liquid accumulation and penetration at the interface.

l Precise alignment avoids gaps caused by misalignment of interfaces during installation.

l Pre-seal the interface before installation and add anti-vibration pads or sealants to enhance the initial sealing effect or reduce sealing failure caused by vibration.

In addition, in terms of the selection of fasteners, high-strength, high-torque fasteners are used, and they are tightened multiple times during the installation process to ensure the tightness of the interface. For example, if a butt-weld nut is used, its characteristic is that it can be directly connected to the wall hole of the connected part (the front panel of the box) for butt welding. This structural design can significantly improve the airtightness of the connection part.

Figure 5: Using butt-welded nuts to increase airtightness

4-Seal selection

Seal design and selection are critical as they directly affect the reliability and service life of the system. The following are key factors to consider when designing and selecting seals for energy storage liquid cooling systems:

l The sealing material must have certain chemical and pressure compatibility, and be able to withstand the system operating temperature range, including high and low temperature environments. The material selection of the seal depends on the use environment and service life requirements. Common sealing materials include rubber, polytetrafluoroethylene (PTFE), nylon, metal, etc.

l Leakage freedom: The seal must be able to adapt to the slight deformation that may occur in the system during operation to ensure good sealing effect under various working conditions. Generally, the deformation of the gasket should be greater than 30% and less than 60%, and the sealing interface pressure should be greater than 30kPa.

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In pure electric vehicles, the weight of the battery pack accounts for about 30% of the curb weight. The lightweight of the battery pack is of great significance to improving the cruising range of the vehicle. Therefore, research on the high specific energy of battery packs is one of the current main research directions for new energy vehicles, and it is also the main way to achieve lightweight electric vehicles. The lightweighting of power battery packs can be carried out in two directions: improving single cells The energy density of the battery pack is optimized, and the related accessories of the battery pack are optimized.

The development of multi-material lightweight battery packs aims to reduce the weight of the battery pack, increase energy density and cruising range, while ensuring safety and reliability by using a variety of lightweight materials. Among the main components of the battery pack, the battery cell body has the highest mass, followed by the lower box of the pack, the upper cover, and BMS integrated components.

1-Lightweight design of the battery pack cover

The upper cover of the battery box is located above the power battery box and is not affected by the sides of the power battery box and will not affect the quality of the entire battery pack. Its functions mainly include sealing and protection. In order to improve the energy efficiency of the entire vehicle, lightweight materials will also be considered in the design. The use of lightweight materials, such as aluminum alloys and composite materials (SMC, FRP, etc.), can significantly reduce the weight of the battery pack.

In addition, the structural design of the upper cover also needs to consider manufacturing efficiency and mass production requirements. When the structure is very irregular, it may be difficult to use stamping forming or bending and tailor welding. The design of the upper cover also needs to consider the connection and cooperation with other components such as the lower box and sealing structural parts to ensure the structural stability and reliability of the entire battery box.

2-Lightweight design of the battery pack lower shell

Aluminum alloy is an ideal material for battery pack shells because of its low density, high specific strength, good thermal stability, strong corrosion resistance, good thermal conductivity, non-magnetic, easy molding and high recycling value. Commonly used aluminum alloy materials include 6061-T6, 6005A-T6 and 6063-T6. These materials have different yield strengths and tensile strengths and can meet different structural needs.

The battery pack shell is usually composed of an aluminum alloy profile frame and a bottom plate, which is welded using 6 series aluminum alloy extruded profiles. Factors such as the size and complexity of the profile section, wall thickness, etc. need to be considered during design to adapt to different structural and functional requirements. For example, components such as frames, middle partitions, floor panels, beams, etc. may adopt different cross-section designs.

Through reasonable structural design and connection methods, the overall strength of the shell can be effectively ensured, the processing difficulty can be reduced, and the weight of the shell can be reduced:

l Thin-walled: By adopting a thin-walled design and using stiffeners to meet strength requirements, the weight of the material can be effectively reduced.

l Hollowing: Introducing hollow sections into structural design to reduce material density.

l Size optimization: Optimize the size of the battery pack to reduce unnecessary material usage.

l Topology optimization: Reduce the space occupied by materials by optimizing the layout of internal components of the battery pack.

l Integrated modular design: Integrate cooling plates, battery pack lifting lugs and other components into the box to reduce the number and weight of individual components.

3-Manufacturing lightweight technology

l Material shaping

At present, there are three main categories of battery box material forming processes: stamping, aluminum alloy die-casting and aluminum alloy extrusion. The overall process flow of the power battery box includes material molding and connection processes, among which the material molding process is the key process of the power battery box. At present, the upper casing is mainly stamped, and the main processes of the lower casing are extrusion molding and aluminum alloy die-casting.

l Connection technology

The battery box connection process is crucial in new energy vehicle manufacturing and involves a variety of technologies and methods to ensure the structural strength and sealing of the battery box.

Welding is the main connection process in battery box processing and is widely used. Mainly include the following methods:

① Traditional fusion welding: such as TIG (tungsten inert gas welding) and MIG (metal inert gas welding). TIG welding has low speed and high quality. It is suitable for spot welding and complex trajectory welding. It is often used for frame tailor welding and side beam small piece welding. MIG welding has high speed and strong penetration ability, and is suitable for full-circle welding inside the frame bottom plate assembly.

② Friction stir welding: Welding is achieved by generating heat through friction. It has the characteristics of good joint quality and high production efficiency.

③Cold metal transfer technology: It is suitable for thin plate materials. There is no heat input during the welding process, reducing deformation.

④Laser welding: high precision, high speed, suitable for complex structure welding.

⑤ Stud welding and projection welding: used for quick connection of specific parts. Stud welding is fastened by studs and nuts, and projection welding is connected by pressing bumps.

The mechanical connection method mainly solves the problems of easy welding and thread slippage of thin plate materials during welding, including:

① Blind rivet nut: used to connect the sealing surface of the box frame and the inner cavity bottom plate. It has the advantages of high fastening efficiency and low use cost. Suitable for threaded connections between thin plates and other components.

②Wire thread insert: used to strengthen the screw holes of aluminum or other low-strength bodies, improve the load-bearing capacity of the screws and the force distribution of the threads, and is suitable for battery module mounting holes and sealing surface mounting holes. Compared with blind rivet nuts, wire thread inserts are stronger and easier to repair, but are generally not suitable for thin-wall installations.

We will regularly update you on technologies and information related to thermal design and lightweighting, sharing them for your reference.

Thank you for your attention to Walmate. 

In order to cope with market demands such as large spans, fast iterations, and rich product lines, while ensuring cost reduction, efficiency improvement, and quality assurance, for the automotive industry, product standardization - vehicle platformization is undoubtedly a good strategy. Through battery platformization, the same battery pack solution can be matched for different models, or battery pack solutions composed of the same type of battery cells and similar structures can be matched. This means that as many parts as possible can be standardized, which can shorten the development cycle, save costs, streamline production lines, and improve production efficiency.

ONE:Battery Platformization

The battery platform solution is conducive to the overall planning of products, cost reduction and optimization of production capacity. According to the battery platform strategy of the vehicle platform, it is necessary to consider the intersection and bandwidth of the requirements of each model of the platform, and use as few batteries and battery solutions as possible to be compatible with as many models as possible. In the architecture development of pure electric projects, it is crucial to reasonably arrange the integrated power battery pack. Specific work elements include power and power performance requirements, collision safety, layout location and space, etc.

1-Spatial size boundaries and battery cell standardization

l Available battery pack locations

At present, the mainstream power battery layout is under the floor, including under the front seats, under the rear seats, in the middle channel and at the footrest. This layout can maximize the available area, help lower the center of gravity of the vehicle, improve the handling stability of the vehicle and optimize the collision force transmission path.

Figure 1: Battery pack layout during the development of electric vehicles

l Evolution of battery pack space layout

Split battery pack: A split battery pack space layout is adopted, such as the JAC Tongyue series. The energy module consists of two battery packs, one placed at the original fuel tank position, and the other placed in the trunk where the spare tire is stored.

In addition, engineers are continuously exploring usable space within the original architecture of fuel vehicles, resulting in battery pack layouts appearing in "工" (gong), "T," and "土" (tu) shapes.

This type of design is a minor modification of a traditional fuel vehicle. The space is very limited, and the volume and weight of the battery pack that can be loaded are very limited, so the capacity is difficult to increase and the cruising range is not high.

Integrated battery pack: This is a new product design concept. The design of the entire vehicle revolves around the core component - the battery pack. The battery pack is modularly designed and laid flat on the vehicle chassis to maximize available space.

l Battery pack installation point layout

Reasonable layout of the battery pack is crucial, and the limiting factors in the design are ground clearance, passability, collision safety, power requirements and many other aspects.

Figure 2: Battery pack size design constraints

The vehicle platform needs to define the category, level and positioning of each vehicle model within the platform, and then determine the size and wheelbase of the vehicle. The vehicle layout decomposes the size envelope of the battery pack in the X, Y and Z directions according to the vehicle space. The battery needs to be arranged within the given envelope of the vehicle to ensure that there is no interference between the various systems of the vehicle. The curb weight index can decompose the system quality requirements of the battery pack.

In terms of battery size, the design of power battery packs cannot avoid rigid reference indicators such as vehicle space and curb weight, which means that there is a threshold for the design of battery cells. Constrained by this threshold, the battery cell size will be concentrated in a certain range, such as: the length of square battery cells ranges from 150-220mm, the width ranges from 20-80mm, and the height ranges around 100mm. The changing trend of battery cell size specifications is the result of the complementary relationship between vehicle platformization and battery standardization.

However, the battery platform strategies, vehicle models and understanding of standardization of various automakers are different, resulting in significant differences in the current product solutions. For example, BYD's standardization strategy is to fully replace the blade battery, the size of which is locked at 960*13.5 (14)*90 (102) mm, and the single cell voltage is 3.2/3.3V.

2- Development of Endurance Boundaries and Battery Capacity Solutions2- Development of Endurance Boundaries and Battery Capacity Solutions

The power battery provides energy for the vehicle to travel: battery capacity, discharge depth, and energy density affect the amount of power available. In order to meet the needs of different models, the difference in power consumption of models has become an important concern. The vehicle's cruising range will be affected by factors such as electric drive, battery, curb weight, wind resistance, mechanical resistance, low-voltage power consumption, and energy recovery. The possibility of sharing battery solutions between models with large differences in power consumption is weak, so it is necessary to develop personalized battery power solutions, including battery size, quality, power, and power performance optimization to meet the requirements of cruising performance.

Under the constraints of the pure electric range of the vehicle manufacturing platform, the net discharge required by the battery will be affected by the power consumption of different models. It is necessary to confirm the power consumption distribution of each model on the platform in order to further convert the power consumption bandwidth into the battery demand distribution, and then determine the battery power plan required by the platform.

3-Power performance boundary

The dynamic performance of the whole vehicle includes the power performance under acceleration conditions, constant speed conditions and power preservation conditions at different SOCs and ambient temperatures. The variables corresponding to the battery are the power-voltage characteristics of the battery at different SOCs and temperatures. The power of the battery corresponds to the power requirement of the vehicle's power system, and the voltage corresponds to the rated voltage requirement of the drive motor.

Generally, the evaluation of battery solutions for the entire vehicle platform starts from the acceleration time of 100 kilometers at normal temperature and high power and its battery indicator decomposition, and gradually extends to the battery indicator decomposition over the entire range and under all operating conditions.

SECOND:Battery box development

1-Battery integration and modularization

Optimize the design of battery modules, improve the integration and modularity of battery packs, reduce inactive components, and increase the energy density of battery packs.

Currently popular battery pack integration technologies include CTP, CTB, CTC and other forms. The shape, material and combination of parts have changed with the advancement of integration technology. The overall direction is integration and integration. By reducing the number of independent parts and using one large part to replace several parts, larger and more functional components are formed.

2-Battery box design

The battery case is the carrier of the power battery system assembly, plays a key role in the safe operation and protection of the product, and directly affects the safety of the entire vehicle. The structural design of the battery case mainly includes the selection of shell materials for the upper shell, lower shell and other components of the battery case, and the selection of manufacturing process solutions. The upper cover of the battery case mainly plays a sealing role and is not subject to much force; the lower case of the battery case is the carrier of the entire power battery system product, and the battery module is mainly arranged in the lower case. Therefore, there must be structural measures such as embedded grooves and baffles inside the battery case to ensure that the battery module is reliably fixed when the vehicle is driving, and there is no movement in the front, back, left, right, up and down directions, so as to avoid impact on the side walls and upper cover and affect the life of the battery case.

Figure 3: Battery lower box solution, a-skin frame, b-FSW welding + frame, c-FSW welding + frame

l Battery pack installation point structure design and connection fixation

The battery pack installation point usually adopts a mounting beam structure, which runs through the front and back, and the front end is connected to the front cabin longitudinal beam to form an effective and coherent closed beam structure. The installation points are reasonably arranged according to the weight distribution of the battery pack. The battery pack and the vehicle are fixed in various ways, including bolt fixing, mechanical fixing + adhesive joint hybrid connection, snap-on connection, etc.

Figure 4: Battery pack layout and installation section

The power battery pack is generally installed on the vehicle through multiple lifting lug structures. In addition to the large weight of the power battery pack itself, the lifting lugs must also withstand the road excitation brought by the vehicle's movement, such as stone roads and deep potholes. Such durable working conditions and misuse conditions place higher requirements on the strength of the lifting lug structure.

Figure 5: Different lifting lug connection solutions: a Welded lifting lug b Aluminum extruded frame lifting lug

l Battery box safety and protection structure

Mechanical strength and protection: The battery box should have sufficient mechanical strength to protect the batteries inside from mechanical shock and impact. The battery box needs to be able to withstand vibration, extrusion, and mechanical shock to ensure the safety of the battery under various conditions.

Collision protection: The design of the battery case must take collision safety into consideration, especially for side collisions and bottom collisions. It is usually made of aluminum or steel and connected to the lower tray through an outer frame to provide structural rigidity and enhance collision energy absorption capabilities. In addition, appropriate collision absorption structures should be designed to prevent deformation of the battery case and damage to the battery cells.

Waterproof, dustproof and corrosion-resistant: The battery box needs to be waterproof and dustproof, and usually uses IP67-level sealing gaskets to ensure airtightness. In addition, anti-corrosion measures should also be considered, such as spraying PVC coating on the outside to enhance corrosion resistance.

Explosion-proof and pressure relief design: When a battery explodes, the energy should be released in a concentrated and directional manner through devices such as balanced explosion-proof valves to avoid entering the customer cabin. In addition, explosion-proof measures (such as partial structural destruction) should be taken to prevent the overall rupture of the equipment.

l Sealing design

The design of the sealing surface between the upper cover and the lower case of the battery box plays an important role in the sealing performance, and its design needs to be designed together with the battery box structure and the sealing ring. The sealing surface should be kept in the same plane as much as possible to avoid too many curved structures. Since the upper cover and the lower case are connected by bolts, a large number of bolts are used, so it is particularly important to ensure the coaxiality of the holes. While arranging the bolt hole positions reasonably, the position dimensions should be as round as possible and arranged symmetrically in the X and Y directions. The selection of the number of connecting bolts needs to be comprehensively considered based on the level of sealing and the amount of disassembly and assembly workload.

Figure 6: Upper and lower box sealing design, 1-battery upper cover 2-sealing gasket 3-battery lower cover 4-metal conduit

l Electrical safety and short circuit protection

Connection reliability: The connectors inside the battery box should have the correct polarity connection to ensure the overcurrent capacity of the battery box and the reliability of the electrical/mechanical connections, including relaxation measures, etc.

Electrical insulation and voltage resistance design: The module design adopts double insulation protection. The battery cell itself has a layer of battery cell blue film and a battery cell top patch to meet the insulation and voltage resistance requirements. Insulation and voltage resistance protection is set between the end/side plate and the battery cell, and between the battery cell and the bottom mounting surface.

l Thermal management design

Battery thermal management development runs through the entire cycle of battery pack system design and development, including the design of battery temperature control, cold plate, piping system, etc. The main goal of battery thermal management system design is to ensure that the battery system operates at a relatively suitable operating temperature through heating or cooling control while considering space layout, design cost, lightweight, etc., while reducing the temperature difference between cells to ensure consistency.

We will regularly update you on technologies and information related to thermal design and lightweighting, sharing them for your reference.Thank you for your attention to Walmate.

The battery pack is the core energy source of new energy vehicles, providing driving power for the entire vehicle. We generally evaluate the advantages and disadvantages of battery pack technology from the dimensions of efficiency (energy density), safety, manufacturing and maintenance costs.

In battery design, the voltage of a single cell is only about 3-4V, while the voltage required by electric vehicles is at least 100V. New cars now even have a voltage of 700V/800V, and the output power is generally 200W, so the battery needs to be boosted. In order to meet the current and voltage requirements of electric vehicles, different cells need to be connected in series or parallel.

The battery pack is composed of battery cells, electronic and electrical systems, thermal management systems, etc., which are enclosed by a battery frame structure - base plate (tray), frame (metal frame), upper cover plate, bolts, etc. How to "package" these components and systems into a whole more efficiently and safely has always been a topic of continuous research and exploration for the entire industry.

Previous article: Innovation and development of battery integration technology

The origin of power battery group technology can be traced back to the 1950s, and originated in the former Soviet Union and some European countries. This technology was originally used as an engineering and manufacturing concept to determine the physical similarities of parts (universal process routes) and establish their efficient production.

The core of Group Technology (GT) is to identify and explore the similarities of related things in production activities, classify similar problems into groups, and seek relatively unified optimal solutions to solve this group of problems to achieve economic benefits. In the field of power batteries, group technology mainly involves the technology of integrating batteries from single cells into battery packs (Packs), including structure, thermal management, electrical connection design and battery management system (BMS) technology.

The earlier grouping technology in the automotive field is MTP (Module To Pack), which means that the cells are first integrated into modules, and then the modules are integrated into Packs. This technology is characterized by detachable and replaceable modules, which have good maintainability, but the grouping efficiency is low. With the development of technology, the grouping technology has undergone a transformation from MTP to CTP (Cell To Pack). CTP technology refers to the technology of directly integrating cells into Packs, eliminating the traditional module structure and improving grouping efficiency and production efficiency. In recent years, the industry is also exploring grouping technologies such as CTC (Cell To Chassis), CTB (Cell To Body & Bracket) and MTB (Module To Body) with higher integration efficiency.

In the field of power batteries and electrochemical energy storage, the main technological advances of lithium batteries come from structural innovation and material innovation. The former is to optimize the structure of "cell-module-battery pack" at the physical level to achieve the goal of both improving the volume energy density of the battery pack and reducing costs; the latter is to explore battery materials at the chemical level to achieve the goal of both improving the performance of single cells and reducing costs. This article focuses on the impact of different structural integration technologies on battery pack manufacturing technology and the direction of innovative development from the perspective of battery pack structural integration. The current key technologies for power battery integration are shown in the figure below:

1-MTP has been eliminated

At the beginning of the current wave of electric vehicle development, many oil-to-electric new energy vehicle models have been launched. They continue the spatial layout and styling design of traditional gasoline vehicles. Engineers have assembled a relatively large battery cell module by connecting a certain number of individual battery cells in series/parallel, and then placed several such battery cell modules into the battery pack, which is the familiar "MTP" battery pack. Since the battery pack needs to be "packaged" more than twice, the number of required components is extremely large, and the battery pack appears as "three layers inside and three layers outside", with too many redundant parts occupying more system volume and weight, resulting in poor volumetric energy density and gravimetric energy density of the "MTP" battery pack. Additionally, since the design of gasoline vehicles did not specifically reserve space for the battery, the battery system can only be "squeezed in wherever it fits", leading to poor product competitiveness and user experience.

Since the launch of new intelligent electric vehicle platforms represented by Tesla, native pure electric vehicles have enabled battery packs to be installed in ideal spatial locations in a more efficient and regular manner, the three-electric systems can be more reasonably laid out, and the vehicle's electronic and electrical architecture and thermal management design can be more efficiently integrated. The vehicle's product strength in terms of energy efficiency, endurance, and intelligence has been greatly enhanced.

2-Integrated Technology 2.0 Era——CTP

The MTP structure battery pack has a significant space utilization problem. The space utilization of the battery cell to the module is 80%, the space utilization of the module to the battery pack is 50%, and the overall space utilization is only 40%. The module hardware cost accounts for about 14% of the total battery cost. This low space utilization structure cannot meet the development requirements of new energy vehicles. Under the framework of the battery cell → module → battery pack → body integration idea, if the vehicle wants to load as much power as possible in the limited chassis space and improve the volume utilization, it is necessary to consider the standardization of each integration step. As the market demand for driving range continues to rise, the volume of a single battery module continues to increase, which indirectly leads to the emergence of the CTP solution.

CTP structure technology was born for the consideration of safety, packaging complexity, cost reduction, etc. Under the premise of ensuring the safety of the battery cell, CTP technology reduces the internal cables and structural parts. Compared with MTP technology, CTP technology has no module structure and directly packages the battery cell into a battery pack before installing it on the vehicle.

There are currently two main ideas. One is to regard the Pack as a complete large module that replaces the structure of multiple small modules inside, represented by CATL; the other is to consider using a module-free solution during design, and design the battery itself as the strength participant, such as BYD's blade battery.

The core point of CTP technology is to cancel the module design. The battery cell is directly combined with the shell, reducing the use of end plates and partitions. The problems that follow are the fixation of the battery pack and thermal management.

In fact, the original product of the CTP battery pack was not a pure module-free design, but a design that merged the original small modules into three large modules and two medium modules, and there were also aluminum end plates at both ends, so in theory it is still MTP, but there are indeed great improvements in structure.

After the introduction of CTP 3.0, CATL presented a more advanced manufacturing method, achieving a completely module-free design. The battery cells have been changed from a vertical orientation along the height to a horizontal position. Additionally, a new cooling solution has been implemented between the battery cells, which not only dissipates heat but also provides support, cushioning, insulation, and temperature control functions. The bottom shell has also been designed with a limiting fixation feature.

Figure 1: Comparison between CATL Kirin Battery CTP2.0 and CTP3.0

3-Integrated Technology 3.0 Era——CTB, CTC

a.CTB technology

CTP technology is a major step forward in battery structure innovation, but it has not made a breakthrough in the battery pack itself. In CTP technology, the battery pack is still an independent component. Compared with CTP's streamlined strategy for battery packs, CTB technology combines the body floor panel and the battery pack cover into one. The flat sealing surface formed by the battery cover, the door sill, and the front and rear beams seals the passenger compartment with sealant, and the bottom is assembled with the body through the installation point. When designing and manufacturing the battery pack, the battery system is integrated with the body as a whole, the sealing and waterproof requirements of the battery itself can be met, and the sealing of the battery and the passenger compartment is relatively simple, and the risks are controllable.

In this way, the original sandwich structure of "battery pack cover-battery cell-tray" is transformed into a sandwich structure of "body underbody integrated battery pack cover-battery cell-tray", reducing the space loss caused by the connection between the body and the battery cover. In this structural mode, the battery pack is not only an energy source, but also participates in the force and transmission of the whole vehicle as a structure.

Figure 2: Schematic diagram of CTB technology structure

b.CTC technology

After adopting the CTC method, the battery pack is no longer an independent assembly, but is integrated into the body of the vehicle, which optimizes the product design and production process, reduces the number of vehicle parts, especially reduces the internal structural parts and connectors of the battery, has the inherent advantage of lightweight, maximizes the space utilization, and provides space for increasing the number of batteries and improving the driving range. Under the condition that the electrochemical system itself remains unchanged, the driving range can be increased by increasing the number of batteries.

Figure 3: Tesla CTC technology structure diagram

For example, Tesla and other automakers have successively launched CTC technology models. At the cell level, they can use multifunctional elastic sandwich structures and large-area water cooling technology, and superimpose the anti-collision space reuse technology at the bottom of the battery pack brought by integrated development, taking into account the grouping efficiency, heat dissipation and safety, and promoting the application of CTC technology from the two dimensions of cell optimization and vehicle structure protection. At the level of vehicle integrated development, the battery cell is directly integrated into the chassis, eliminating the links of modules and battery packs. The integration of the three major electric systems (motor, electronic control, battery), the three minor electric systems (DC/DC, OBC, PDU), the chassis system (transmission system, driving system, steering system, braking system) and autonomous driving related modules is realized, and the power distribution is optimized and energy consumption is reduced through the intelligent power domain controller.

4-Changes in specific requirements for battery boxes for CTP, CTB and CTC technologies

In the traditional battery pack structure, the battery module plays the role of supporting, fixing and protecting the battery cell, while the battery box body mainly bears the external extrusion force. The application of CTP, CTB and CTC technologies puts forward new requirements for battery boxes, which are specifically reflected in:

The strength requirements of the battery box body are improved: Since the module link is reduced or eliminated in the CTP, CTB and CTC structures, the battery box body must not only withstand the external extrusion force, but also the expansion force from the battery cell originally borne by the module. Therefore, the strength requirements of the battery box body are higher.

Collision protection capability: After using CTP technology to remove the side beams of the battery pack, the battery will directly bear the impact of the collision, so the CTP battery pack needs to have sufficient collision resistance.

Insulation, insulation and heat dissipation requirements: CTP or CTB and CTC structures change the bottom plate profile to a water-cooled plate based on the chassis-bearing structural box. The battery box box not only bears the weight of the battery cells, but also provides thermal management and other functions for the battery. The structure is more compact, the manufacturing process is optimized, and the degree of automation is higher.

Reduced maintainability: The highly integrated design makes it complicated to replace the battery pack. For example, in the CTC structure, the battery cells are filled with resin material, which makes it difficult to replace the battery cells and almost impossible to repair.

5- Impact of battery pack integration on electric vehicle charging infrastructure

Choosing different battery pack integration technologies also implies choosing different compensation methods. CTP tends to be battery replacement, while the more highly integrated CTB/CTC tends to be fast charging.

High integration means that more batteries can be accommodated in the same space, thereby increasing the range of electric vehicles. Users may no longer need to charge frequently for short distances, but may prefer to charge quickly during long journeys. Therefore, the planning of charging infrastructure needs to take these changes into account to ensure that it can meet user needs.

As the integration of battery packs increases, the physical size and structure of the battery packs may change, which may affect the design of the charging interface and the compatibility of the charging equipment.

In addition, the increased integration of battery packs may also affect charging speed and efficiency. More efficient battery management systems and charging technologies may need to be developed and deployed to ensure a fast and safe charging process.

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In the production process of battery trays and energy storage liquid cold boxes for new energy vehicles, necessary and appropriate surface treatment is a key step, such as: using coating, oxidation treatment, etc. to form a protective layer on the metal surface to resist the erosion of corrosive media; Components that require electrical isolation, such as battery cells, water-cooling plates, module walls, etc., need to establish an insulating protective film. Insulation is generally achieved by spraying insulating powder or insulating paint. Choosing the appropriate surface treatment technology can not only improve the performance of the tray/liquid cooling box Durability and safety can also meet the needs of different application scenarios. This article summarizes common surface treatment technologies for reference.

1-Cleaning and polishing

During the production process, impurities such as processing oil, engine oil residue, powder, and dust may accumulate on the surface of the pallet. Not only do these impurities affect the service life of the battery tray, they may also adversely affect the performance and safety of the battery. Through cleaning and polishing, these impurities can be effectively removed to ensure the cleanliness of the pallet surface. Cleaning and grinding can effectively remove surface impurities, burrs, and welding slag, making the surface smooth and flat, thus improving the overall quality of the battery tray/box.

a.chemical cleaning

Alkali cleaning: Alkaline cleaning mainly uses alkaline solutions (such as sodium hydroxide, sodium carbonate, etc.) to remove grease, dirt and other organic matter on the surface of aluminum alloys. Alkaline washing removes grease through saponification, emulsification and penetration and wetting, and at the same time generates water-soluble precipitates, thereby achieving a cleaning effect. Alkaline cleaning is usually used to remove grease, dust and organic contaminants from the surface of aluminum alloys.

Pickling: Pickling uses acidic solutions (such as nitric acid, hydrochloric acid, etc.) to remove oxide scale, rust and other inorganic deposits on the surface of aluminum alloys. Pickling converts the oxides on the metal surface into soluble salts through the reaction of acid with the oxides on the metal surface, thereby removing surface impurities. Pickling is mainly used to remove oxide film, rust and inorganic salt scale on the surface of aluminum alloys. Pickling is often used for the final treatment of metal surfaces to improve their finish and flatness.

b.Mechanical grinding

During production, the grinding process can remove processing allowances, correct shape errors, ensure the smoothness and accuracy of the pallet/box surface, meet assembly requirements, and thus improve overall performance and service life.

The cleaned and polished surface can be treated with coating materials or other materials, which is very important for the subsequent construction of anti-corrosion, sealing, thermal conductivity, insulation, thermal insulation and other coatings, and plays a key role in the firm attachment of these materials to the pallet/box.

2-Coating and protective film establishment

In addition to basic cleaning and polishing, the production of pallets/boxes uses a spraying process for surface treatment to form a protective layer to prevent oxidation and corrosion and to meet the needs of different scenarios such as thermal insulation, insulation and voltage resistance.

a.Thermal insulation

Anti-condensation and thermal insulation of battery trays can be achieved through comprehensive design of thermal insulation systems, use of high-efficiency thermal insulation materials, application of aerogels, battery pack insulation design, and spraying of foam insulation materials.

Bottom surface sprayed with PVC and foam material

b.Insulation withstand voltage

The insulation of the battery pack casing and the liquid cooling components is primarily to prevent current leakage, protect personnel from electric shocks, and ensure the normal operation of the battery system. Insulation is typically achieved through two main methods: powder spraying and film lamination. The mainstream film lamination processes include room temperature lamination, hot pressing, and UV exposure.

Internal spraying of insulation powder and insulation paint

3-Logos and Signage

A nameplate or label is set in a prominent position on the battery tray, generally through laser, mechanical engraving, etc. These logos are usually made of wear-resistant and corrosion-resistant media to ensure that they are not easily erased during the entire service life.

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As the core equipment of the energy storage system, the energy storage converter is an important tool for power conversion, energy management, ensuring grid stability, improving energy efficiency, etc. As the energy storage converter power unit moves toward high integration and high efficiency, the development of frequency and large capacity puts higher and higher requirements on heat dissipation.

1-Changes in cooling requirements

a.Matching the larger DC cabin, the converter capacity continues to increase, and efficient heat dissipation technology ensures the reliability of the equipment.

As the capacity of energy storage cells becomes larger and larger, the capacity of energy storage systems is also expanding simultaneously. At the beginning of 2023, the standard 20-foot single-cell battery capacity on the market was only 3.35MWh. In the second half of the year, many battery cell companies have launched 310+Ah energy storage products, and the capacity of the 20-foot single-cell battery has also been expanded to 5MWh. However, less than half a year after the 5MWh model was updated, some leading energy storage systems released 6MWh and 8MWh systems. According to general experience, the energy storage converter is configured at 1.2 times the load capacity. The single unit capacity of a 5MWh energy storage system must be greater than 2.5MW. High power requires more efficient cooling technology to ensure stable operation of the equipment under sustained high loads.

Iterative evolution of energy storage system integration topology scheme

b.The application of DC high-voltage technology requires devices to have higher withstand voltage levels and insulation strength, and the heat dissipation of power devices is severe.

In order to match the large-capacity energy storage system, DC high-voltage technology has become a technical trend. Through the increase of voltage level, energy saving, efficiency and performance improvement can be achieved. The 1500V voltage upgrade originated from photovoltaics, and now photovoltaics are involved in energy storage. However, the high-voltage evolution of energy storage PCS still has a long way to go, and some manufacturers have begun to optimize and push it to 2000V. The application of DC high-voltage technology forces the power electronic devices in energy storage converters to have higher withstand voltage levels and higher insulation strength to adapt to high-voltage working environments. In high-voltage environments, the heat dissipation design of power devices becomes more important. The pn junction temperature of power devices generally cannot exceed 125°C, and the temperature of the package shell does not exceed 85°C.

c.Networked energy storage systems require complex control algorithms, circuit designs and high power density energy storage converters

Unlike the essential characteristics of current sources in grid-forming energy storage systems, grid-forming energy storage systems are essentially voltage sources that can internally set voltage parameters to output stable voltage and frequency. Therefore, it is required that grid-forming converters simulate the characteristics of synchronous generators, providing support for voltage and frequency to enhance the stability of the power system. This control strategy necessitates that converters possess higher power density and more complex control algorithms, as well as higher-performance power devices and more intricate circuit designs to implement the control strategy. Effectively managing the heat generated by high power density and complex control strategies, while reducing the size and cost of the cooling system without compromising performance, has become a new challenge in thermal design.

2- Comparison of common cooling solutions

The cooling solution for energy storage inverters has undergone significant iterative evolution in recent years, mainly reflected in the transition of cooling technology from traditional air cooling to liquid cooling technology.

a.Air cooling solution

Air cooling is the temperature control form used in the early stage of energy storage converters. It uses air as the medium and dissipates heat through fans and radiators. The air cooling solution improves heat dissipation efficiency by continuously reducing energy consumption, optimizing structure, and improving heat dissipation materials. At the 2.5MW power level, air cooling can still meet the requirements.

b.Liquid Cooling Solution

As the power density and energy density of energy storage systems continue to increase, liquid-cooled PCS uses coolant with high thermal conductivity as the medium. The coolant is driven by a water pump to circulate in the cold plate and is not affected by factors such as altitude and air pressure. The liquid cooling system has a more efficient heat dissipation efficiency than the air cooling system. The liquid cooling solution has a higher matching degree and has begun to be explored and popularized in the past one or two years.

In addition to the full liquid cooling energy storage solution, some manufacturers have launched energy storage direct cooling machines, which use phase change direct cooling and no water circulation. Direct cooling solutions are also entering the energy storage field.

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The battery pack is a key component of new energy vehicles, energy storage cabinets and containers. It is an energy source through the shell envelope, providing power for electric vehicles and providing consumption capacity for energy storage cabinets and containers. In combination with actual engineering needs, this article summarizes the key points of profile design for battery packs by analyzing the requirements of mechanical strength, safety, thermal management and lightweight of battery packs.

1-Battery pack housing design requirements

a.Mechanical strength, vibration resistance and impact resistance. After the test, there should be no mechanical damage, deformation or loosening of the fastening, and the locking mechanism should not be damaged.

b.Sealing: The sealing of the battery pack directly affects the working safety of the battery system. It is usually required to reach IP67 protection level to ensure that the battery pack is sealed and waterproof.

c.The design of the battery pack shell needs to take thermal management performance into consideration and ensure that the battery operates within an appropriate range through appropriate thermal management design.

d.For installation and fixation, the shell should have space for nameplate and safety signs, and reserve sufficient space and fixed foundation for the installation of acquisition lines, various sensor elements, etc.

e.e. All connectors, terminals, and electrical contacts of non-polar basic insulation should meet the corresponding protection level requirements when combined.

f.Lightweighting: Lightweighting of the shell is of great significance to improving the energy density of the battery pack. Aluminum alloy is light in weight and high in quality, making it the most feasible choice at present. The lightweighting level can be improved through appropriate extreme design in combination with actual applications.

g.Durability: The design life of the battery pack shell shall not be less than the service life of the overall product. No obvious plastic deformation should occur during the use cycle. The protection level and insulation performance should not be reduced. The structure should be easy to maintain, including the layout of nameplates and safety signs, and protection of connectors.

Figure 1 Typical aluminum alloy welded battery pack shell

2-Typical aluminum alloy battery pack shell solution

Commonly used aluminum alloy materials for battery pack shells include 6061-T6, 6005A-T6 and 6063-T6, etc. These materials have different yield strengths and tensile strengths to meet different structural requirements. The strength of these materials is: 6061-T6＞6005A-T6＞6063-T6.

At present, the battery pack shell forming solutions include aluminum profile welding, aluminum alloy casting, cast aluminum plus profile aluminum, stamped aluminum plate welding, etc. The aluminum profile welding solution has become the mainstream choice due to its flexibility and processing convenience. As shown in Figure 1, the shell is mainly composed of an aluminum alloy profile frame and an aluminum alloy profile bottom plate, which are welded using 6 series aluminum alloy extruded profiles. The aluminum alloy casting solution is regarded as the future development direction due to its simplified process and cost reduction potential.

3- Profile section design

a. Section size and complexity: The section size of the profile is measured by the circumscribed circle. The larger the circumscribed circle, the greater the extrusion pressure required. The section of the profile is usually composed of multiple cavities to improve the structural rigidity and strength. Usually, the frame, middle partition, bottom plate, beam, etc. adopt different section designs to adapt to different structural and functional requirements.

Figure 2 Typical aluminum alloy profile section

b. Aluminum profile wall thickness: The minimum wall thickness of a specific aluminum profile is related to the profile circumscribed circle radius, shape and alloy composition. For example, when the wall thickness of 6063 aluminum alloy is 1mm, the wall thickness of 6061 aluminum alloy should be about 1.5mm. The extrusion difficulty of the same section is: 6061-T6＞6005A-T6＞6063-T6. In the design of battery pack profiles, the frame profile is usually made of 6061-T6 aluminum alloy material, and its typical section is composed of multiple cavities, and the thinnest wall thickness is about 2mm; the bottom plate profile is also composed of multiple cavities, and the material is generally 6061-T6, 6065A-T6, and the thinnest wall thickness is also about 2mm; in addition, in the design of the bottom plate load-bearing tray and bottom plate liquid cooling integration, the bottom plate generally adopts a double-sided structure, the bottom plate thickness is generally 10mm, and the wall thickness and the inner wall of the cavity are about 2mm.

c. Tolerance of profile cross-sectional dimensions: The tolerance of cross-sectional dimensions should be determined based on the processing allowance of the aluminum profile, the conditions of use, the difficulty of profile extrusion, and the shape of the profile. For some aluminum profiles that are difficult to extrude, the shape can be changed or the process allowance and dimensional tolerance can be increased to reduce the difficulty of extrusion and extrude aluminum profile products that are close to the requirements, and then they can be reshaped or processed to meet the use requirements.

In addition, when designing the profile section, it is necessary to consider the specific requirements of different welding processes for joints, grooves, wall thickness, etc.

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Battery trays, also known as battery boxes or PACK boxes, are increasingly valued as a very important component in the development of new energy vehicles. The design of battery trays needs to balance the relationship between factors such as weight, safety, cost, and material performance. Aluminum alloys are widely used in automotive lightweight engineering because of their low density and high specific strength, which can ensure rigidity while ensuring vehicle body performance.

1-Battery tray welding location and method selection

Aluminum battery trays are made of extruded aluminum profiles, and the various components are combined into a whole by welding to form a complete frame structure. Similar structures are also widely used in energy storage pack boxes.

The welding parts of the battery tray usually include the bottom plate splicing, the connection between the bottom plate and the side, the connection between the side frame, the horizontal and vertical beams, the welding of liquid cooling system components, and the welding of accessories such as brackets and hanging ears. When selecting welding methods, different welding methods will be selected according to different material and structural requirements, see the table below:

2-Analysis of the influence of welding thermal deformation

Welding is a local heating processing method. Since the heat source is concentrated at the weld, the temperature distribution on the weld is uneven, which eventually leads to welding deformation and welding stress inside the welded structure. Welding thermal deformation is the phenomenon that the shape and size of the welded parts change due to uneven heat input and heat output during the welding process. Combined with actual engineering project experience, the parts that are prone to welding thermal deformation and the influencing factors are summarized:

a. Long straight welding area

In actual production, the bottom plate of the battery tray is generally made of 2 to 4 aluminum alloy profiles spliced together by stir friction welding. The welds are long, and there are also long welds between the bottom plate and the side plate, and between the bottom plate and the spacing beam. Long welds are prone to local overheating in the welding area due to concentrated heat input, resulting in thermal deformation.

Battery tray frame welding

b. Multi-component joints

It is caused by local high temperature heating and subsequent cooling during the welding process at the multi-component weld. During the welding process, the weldment is subjected to uneven heat input, resulting in a significant temperature difference between the weld area and the surrounding parent material, which causes thermal expansion and contraction effects, causing deformation of the welded parts. The electrical installation end of the energy storage pack box is usually equipped with a water nozzle, a wiring harness bracket, a beam, etc., and the welds are dense and very easy to deform.

In the weld-intensive area, the front side of the pallet is warped and deformed

c.Cold plate channel side wall

In the battery tray with integrated design of liquid cooling plate, parts with smaller structural rigidity, such as thin plates and pipe structures, cannot resist thermal deformation well during welding and are prone to deformation. For example, the side wall of the liquid cooling plate flow channel is very thin, generally only about 2mm. When welding beams, wiring harness brackets and other parts on the module mounting surface, it is easy to cause cracks and deformation wrinkles on the side wall of the flow channel, affecting the overall performance.

Thermal crack defects on the liquid cooling channel wall caused by beam welding

3-Welding thermal deformation control method

a. Segment welding, double-sided welding

For parts with relatively low strength requirements, segmented welding is adopted, and the welding process is broken down into multiple small sections. The welds are arranged symmetrically, and the welds are arranged symmetrically near the neutral axis in the construction section, so that the deformations caused by the welds may offset each other. At the same time, the length and number of welds are minimized, and excessive concentration or crossing of welds is avoided, which can reduce the welding temperature gradient and thus reduce welding deformation. For parts with high strength requirements such as the bottom plate, bottom plate and side frame, double-sided welding is adopted to increase strength while reducing bending deformation caused by large parts and long welds.

b.Optimizing welding sequence

Control welding deformation, use joints with lower rigidity, avoid two-way and three-way intersecting welds, and avoid high stress areas. Optimize the welding sequence, weld the weaker rigidity areas first, and the better rigidity areas last, such as welding the fillet welds first, then the short welds, and finally the long welds; weld the transverse welds first, then the longitudinal welds. A reasonable welding sequence can effectively control welding deformation, thereby controlling the weld dimensions.

c. Welding parameter adjustment

Control welding parameters and processes, and reasonably set welding speed, number of welding layers and thickness of each weld. For thicker welds, use multi-layer and multi-channel welding methods, and the thickness of each weld layer should not exceed 4mm. Multi-layer welding can reduce structural microstructure and improve joint performance. Accurately control welding parameters and reasonably select parameters such as welding current, voltage, electrode model and welding speed to ensure consistent shape and size of the molten pool, thereby avoiding errors caused by improper parameter selection.

d.Welding skills improvement

Improve the welder's operating skills (use mechanical processing for large components or nodes with strict requirements) to ensure consistency and standardization of actions during welding and reduce dimensional problems caused by human factors.

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Aluminum alloy is the most widely used non-ferrous metal structural material in industry, especially in scenarios where the thermal conductivity of materials is of great concern, and in situations where efficient heat conduction is required, such as electronic equipment heat dissipation, electric vehicle three-power heat dissipation, and battery energy storage systems. In the fields of heat dissipation and aerospace, it is usually used to manufacture efficient heat transfer equipment such as radiators, heat conduction plates, and electronic components.

Thermal conductivity, also called thermal conductivity, is a parameter index that characterizes the thermal conductivity of materials. It indicates the heat conduction per unit time, unit area, and negative temperature gradient. The unit is W/m·K or W/m·℃. Aluminum alloy is an alloy material composed of aluminum and other metals. Its thermal conductivity is very excellent, and the thermal conductivity coefficient is usually between 140-200W/(m·K). As the metal with the highest content in the earth's crust, aluminum has a relatively low thermal conductivity coefficient. It is favored because of its high height, low density and low price.

1-Thermal conductivity principle of aluminum alloy materials

When there is a temperature difference between adjacent areas of a material, heat will flow from the high-temperature area to the low-temperature area through the contact part, resulting in heat conduction. There are a large number of free electrons in metal materials. Free electrons can move quickly in the metal and can quickly transfer heat. Lattice vibration is another way of metal heat transfer, but it takes a back seat compared to the free electron transfer method.

Comparison of heat conduction methods between metals and non-metals

2-Factors affecting the thermal conductivity of aluminum alloys

a.Alloying is one of the main factors affecting thermal conductivity. Alloying elements exist in the form of solid solution atoms, precipitated phases and intermediate phases. These forms will bring crystal defects, such as vacancies, dislocations and lattice distortion. These defects will increase the probability of electron scattering, resulting in a reduction in the number of free electrons, thereby reducing the Thermal conductivity of alloys. Different alloying elements produce different degrees of lattice distortion on the Al matrix, and have different effects on thermal conductivity. This difference is the result of multiple factors such as the valence of alloy elements, atomic volume differences, extranuclear electron arrangement, and solidification reaction type.

b.Heat treatment is a very important step in the processing of aluminum alloys. By changing the microstructure and phase transformation of aluminum alloys, its thermal conductivity can be significantly affected. Solid solution treatment is to heat the aluminum alloy to a certain temperature to fully dissolve the solute atoms in the matrix, and then quickly cool it to obtain a uniform solid solution. This treatment improves the material's mechanical properties but usually reduces its thermal conductivity. Aging treatment is through appropriate cold deformation and reheating after solid solution treatment, which can optimize the microstructure of the alloy and improve its overall performance. The aging treatment takes into account the mechanical properties and thermal conductivity of the alloy, so that the alloy maintains high strength while also having good thermal conductivity. Annealing improves the microstructure of the alloy by maintaining it at a lower temperature to precipitate and redistribute the second phase in the alloy. Annealing treatment can improve the plasticity and toughness of aluminum alloys, but the effect on thermal conductivity varies depending on the specific situation.

Schematic diagram of crystal structure changes during the aging process of Al-Cu alloy

c.Other factors influence, impurities and second phase particles: Impurities and second phase particles (such as oxides, carbides, etc.) in aluminum alloys can scatter hot carriers (electrons and phonons), thereby reducing thermal conductivity. The higher the impurity content, the coarser the second phase particles and generally the lower the thermal conductivity. The grain size of aluminum alloys also affects thermal conductivity. Generally speaking, the smaller the grain size, the more grain boundaries there are and the lower the thermal conductivity. In addition, the processing method of aluminum alloy (such as rolling, extrusion, forging, etc.) will affect its microstructure and residual stress state, thereby affecting the thermal conductivity. Work hardening and residual stresses reduce thermal conductivity.

In summary, aluminum alloy is an ideal choice for high thermal conductivity materials. Factors such as the type of alloy elements in aluminum alloys and their forms, heat treatment methods, impurities, grain size, and molding methods will all affect the thermal conductivity of aluminum alloy materials. Comprehensive considerations should be taken when designing material composition and process planning.

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The air tightness of the battery pack is a key factor in ensuring the quality and safety of the battery pack. It is related to the safety, reliability and service life of the battery pack. The air tightness test of the battery pack should be carried out not only during the production process, but also during battery maintenance and inspection.

1-Battery Pack Airtightness Requirements

In actual production, the air tightness of the battery pack must meet the following requirements:

Sealing performance, the battery pack shell, interface and connectors must have good sealing performance to prevent dust, water vapor and other external impurities from entering the battery pack, which can be achieved through welding, sealants, waterproof materials, etc.

Waterproof performance, to prevent moisture from entering the battery, causing short circuits, corrosion and other problems. According to the national standard GB38031-2020 "Safety Requirements for Power Batteries for Electric Vehicles", the sealing performance of batteries and their components should meet the IP67 standard. Most new energy vehicles have higher sealing performance requirements for batteries and their components, and must meet the IP68 standard, that is, the battery pack can prevent water from entering within the specified water depth and submersion time.

Traditional air tightness testing methods include pressure method and immersion method (water test). The immersion method is to immerse the liquid cooling plate in water and observe whether bubbles are generated to judge the sealing.

Liquid Cooling Plate Water Channel Air Tightness Test Tank

Although the IP68 standard is more stringent, in actual applications, the pressure drop method is often used as the main detection method to meet the IP68 requirements by setting appropriate airtightness detection standards. The pressure drop method determines the airtightness of the battery pack by measuring the pressure change inside the battery pack. When performing airtightness testing, multiple parameters need to be paid attention to, such as inflation pressure, inflation time, pressure stabilization time and leakage rate.

                                    Differential pressure basic principle diagram                                             Direct pressure basic principle diagram

2-Analysis of Liquid Cooling Plate Leakage Problem

With the continuous upgrading of market demand for power battery vehicles, battery energy storage systerms, etc., higher energy density and power density battery packs are widely used. Because of the thermal characteristics of batteries, to ensure the stable operation of core equipment such as batteries and improve energy utilization efficiency, liquid cooling technology is one of the mainstream technical routes for energy storage thermal management, and the air tightness test of the liquid cooling system has become a key link.

Liquid cooling plate leakage is a serious problem: the leakage will hinder the normal flow of the coolant, affect the heat dissipation effect of the liquid cooling plate, and reduce the performance of the equipment; the leakage may also cause aging and damage of system components, reducing the reliability of the system; the leakage may also corrode electronic components and circuits, increasing the risk of equipment failure and fire.

Why does the leakage problem still occur after rigorous air tightness testing during the production and manufacturing process of the liquid cooling plate?

Liquid Cooling System Airtightness Test Process

Liquid seepage may be caused by a variety of factors:

l Tiny cracks and defects. Landscape air tightness testing may detect large leakage channels, but tiny cracks and defects may still exist. These tiny cracks may expand under liquid pressure or high temperature environment, causing liquid seepage.

l Coolant surface tension and wettability differences: When the surface tension of the coolant is low, it is easier to penetrate through tiny gaps. If the surface tension design of the liquid cold plate is unreasonable or the coolant is not properly selected, the liquid seepage problem may be aggravated.

Wettability differences: Different coolants have different wettability on solid surfaces. If the material surface roughness of the liquid cold plate is high or there are microstructural defects, the coolant may penetrate more easily.

l Installation or process problems: If the installation process of the liquid cold plate is not fine enough, or there are defects in the welding, connection and other processes, it may also lead to poor sealing and increase the possibility of liquid seepage.

l Environmental conditions: Changes in temperature, especially in high-pressure environments, may affect the permeability of the coolant. Although these environmental factors may not be considered during air tightness testing, in actual operation, temperature fluctuations may cause seal failure.

l Material aging or fatigue: If the material of the liquid cold plate is used for too long, it may age or fatigue, causing its sealing performance to deteriorate, thereby increasing the risk of liquid leakage.

3-Preventive Measures for Liquid Cooling Plate Leakage

l  Improve the design of liquid cooling plate: By optimizing the structure and design of the liquid cooling plate, reduce small cracks and defects, and improve its sealing performance. For example, when welding the module installation beam on the flow channel surface, take anti-leakage measures to avoid coolant leakage.

l Improve the manufacturing process level: In the production process of the liquid cooling plate, high-quality welding processes and materials are used to ensure that the coolant is not easy to penetrate. At the same time, during the assembly process, strictly follow the operating procedures to avoid looseness or incorrect installation.

l Optimize the combination of detection methods to ensure detection efficiency while improving detection accuracy and reducing missed detection rate. The immersion method and pressure drop method are used for air tightness detection, which is simple to operate, economical, and efficient, and is suitable for large-scale routine detection needs. However, the detection accuracy of the two methods is low. The detection accuracy of the pressure drop method is generally a leakage rate of 1×10-4Pa·m³/s, and the accuracy of the detection results is easily interfered by factors such as temperature, humidity, cleanliness, and pressure. Use detection equipment with higher detection accuracy and better effect to increase the detection accuracy to 1×10-6Pa·m³/s, thereby improving the detection effect.

In addition to the preventive measures for the liquid cooling plate itself, it is also necessary to adopt appropriate response strategies in multiple aspects such as coolant selection, seal selection and equipment working environment.

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In heat dissipation design, adopting effective cost-reduction methods can improve the reliability and efficiency of the overall system while reducing unnecessary costs.

1-Derating design  reduces costs

Derating design is a design method that intentionally reduces the electrical, thermal and mechanical stresses that components or products are subjected to during operation. In actual production and use scenarios, the stability of electronic equipment can be improved by reducing the stress borne by components.

Schematic diagram of heat dissipation paths for 2D and 3D packaging

l Reduce working stress: During product design and operation, the working stress of components can be reduced by reducing the workload, controlling the operating frequency, limiting the current and voltage, etc.

l Reduce environmental stress: Reduce environmental stress by selecting appropriate component types, layouts, and packaging forms, such as selecting components with a large temperature margin, or using packaging forms with good sealing to reduce the effects of temperature, humidity, and pressure on components.

l Reliability engineering application: reasonable redundant design, fault detection and isolation, etc., further reduce the failure risk of components.

By reducing the stress on components during operation, their power consumption and heat generation can be reduced. When power devices operate under stress conditions lower than their rated stress, their power consumption and heat generation can be reduced, which helps to improve the energy efficiency and reliability of the system. In the long run, derating design effectively increases the life of components, reduces failure rates, reduces maintenance workload, and thus reduces costs.

2-Optimize layout

The working efficiency of the radiator can be significantly improved through reasonable arrangement of thermal components, and a reasonable component layout strategy can achieve a balance between product performance and cost.

l Distribute heat dissipation components: Disperse components that generate large amounts of heat to reduce the heat load per unit area.

l Location that is conducive to heat dissipation: Place the heating element in a location that is conducive to heat dissipation, such as near a vent or the edge of the device.

l Staggered arrangement: During layout, stagger the heating components with other general components, and try to make the heating components principle temperature-sensitive components to reduce their impact on the heat-sensitive components.

l Improve airflow: By changing the direction design and component layout, the airflow path is optimized, the flow rate is increased, and the heat transfer coefficient is improved.

Spacing recommendations between components

3-Choice of cooling method

As the performance of electronic components improves and the degree of integration increases, the power density continues to increase, resulting in a significant increase in the heat generated by the electronic components during operation. When choosing a heat dissipation method for electronic components, the temperature control requirements mainly include the following aspects:

l Temperature range: Different components have different temperature tolerance ranges. For example, high-performance chips such as CPUs have operating temperature requirements between 85-100°C, while some low-power devices can tolerate higher temperatures, so the cooling system must ensure Components operate within a safe temperature range.

l Temperature control accuracy: In some scenarios with strict temperature control requirements, it is necessary to adopt a heat dissipation solution that can accurately control the temperature to avoid component performance degradation or even damage caused by excessively high or low temperatures.

l Ambient temperature: The heat dissipation effect of electronic equipment not only depends on the heat dissipation capacity of the device itself, but is also affected by the surrounding ambient temperature. The heat dissipation design needs to consider changes in ambient temperature and try to keep the device within a suitable temperature range through heat dissipation means.

l Power consumption and reliability: Some low-power electronic components can use natural heat dissipation when they generate low heat. For high-power consumption equipment, it is necessary to wait for the heat dissipation technology of universities to ensure that it maintains normal performance and prolongs operation under high load. service life.

l Sealing and density: In sealed and high-density assembled devices, if the heat generation is not high, you can rely on natural heat dissipation. When components are densely packed and generate large amounts of heat, more effective heat dissipation technologies such as forced heat dissipation or liquid cooling are needed. Liquid cooling and heat pipe technology are used in scenarios with high power consumption and large heat generation, such as high-power electronic components such as traveling wave tubes, magnetrons, and power amplifier tubes, servers and high-power consumption equipment, and three-electric systems of new energy vehicles. Its unique application advantages.

                    Charging pile air cooling module                                                  Charging pile liquid cooling module

When choosing a cooling method for electronic components, it is necessary to comprehensively consider factors such as heat generation and heat flux, ambient temperature and operating temperature, space constraints and thermal isolation requirements, and cost and feasibility. By using appropriate cooling technology and cooling devices to ensure that components operate at a suitable temperature, the cost of system replacement and maintenance can be effectively reduced. In addition, reusing historical projects is also an effective strategy to reduce development and manufacturing costs and improve reliability.

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Energy storage immersion liquid cooling technology is an advanced battery cooling method that uses the efficient thermal conductivity of liquid to achieve rapid, direct and sufficient cooling of the battery, ensuring that the battery operates in a safe and efficient environment. Its basic principle is to completely immerse the energy storage battery in an insulating, non-toxic liquid with heat dissipation capabilities. This technology uses direct contact between the liquid and the battery for heat exchange, thereby quickly absorbing the heat generated by the battery during charging and discharging, and bringing it to the external circulation system for cooling.

Schematic diagram of the principle of a single-phase immersion liquid-cooled energy storage system

As a key component for carrying the battery pack and ensuring that the battery cells work in a suitable environment, the immersion liquid-cooled energy storage Pack box mainly undertakes functions such as carrying the battery pack and coolant, safety protection, and heat conduction. Therefore, in the design of the box structure, it is necessary to comprehensively consider multiple aspects such as airtightness, cooling efficiency, safety, material selection, and processing technology to ensure efficient, safe, and reliable operation of the system. The box structure design is the basis of the entire liquid cooling system.

1-Unified load

The lower box of the immersed liquid-cooled energy storage pack is composed of a bottom plate and side plates. The bottom plate serves as a basic support, and the side plates are fixed around the bottom plate, which together form the main frame of the box. The size of the box should be adjusted taking into account the overall needs and load conditions of the liquid cooling system. In the design of larger-sized boxes, internal partitions or support structures can be reasonably set up to divide the large space into multiple small spaces. force area to improve the uniform load-bearing capacity. In the internal structure, the local load-bearing capacity can be improved by adding support ribs and reinforcing ribs, and a load-sharing structure can also be set up inside the box to balance the load at each corner.

At the same time, in order to reduce the impact of plastic deformation on uniform load-bearing, the processing surfaces of different heights can be designed as the same plane, which can reduce the number of machine tool adjustments and avoid deformation caused by height differences; the width or height of the box can also be increased to disperse the load and reduce deformation.

In addition, the integrated design of the liquid cooling channel and the bottom plate of the box is completed by stir friction welding or laser welding. This design can effectively improve the overall structural strength.

Schematic diagram of the lower box structure of a single-phase immersion liquid-cooled energy storage pack

2-Heat exchange design

Thermal conductivity is an important part of immersion liquid cooling energy storage technology. The design goal is to ensure that the battery can effectively dissipate heat in a high temperature environment, thereby maintaining its performance and safety.

The material of the box should have high thermal conductivity. Commonly used materials are aluminum alloy, copper, and aluminum-based composite materials. The box design also needs to consider the impact of ambient temperature changes. The appropriate thickness of the insulation layer can ensure that the temperature inside the box is within a relatively constant range, thereby improving the overall efficiency of the system.

The structural design of the box directly affects its thermal conductivity. Reasonable flow channel layout ensures that the liquid flows smoothly inside the box and maximizes the contact area, which is the main strategy to improve the thermal conductivity of the box. Multiple flow channels can be set inside the box to increase the coolant circulation path, thereby improving the heat dissipation effect.

               Solution 1:  single item + plate replacement                    Solution 2: single item + box replacement

The liquid cooling system includes a cooling medium, a heat-conducting structure, a liquid cooling pipeline and a supporting structure.

In the first scheme, the same or different types of coolants can be selected to be filled into the liquid cooling plate flow channel cavity and the box cavity respectively, and the two cavities are sealed and not connected to each other. In the box cavity, the coolant immerses the battery module, fully contacts it, cools it without flowing, and absorbs the heat on the battery surface by using the good thermal conductivity of the liquid to reduce the temperature rise. In the liquid cooling plate, the coolant is divided into multiple flow channels in the water inlet manifold and enters the cold plate in parallel, and then converges and flows out in the water outlet manifold, which is mainly responsible for taking out the heat and achieving heat dissipation.

In the second scheme, the coolant with low temperature flows in from the bottom or side, and the coolant with high temperature flows out from the top. The coolant circulates in the battery pack, which can effectively and evenly distribute the heat, improve the overall cooling efficiency, and maintain the consistency of the temperature of the battery cell or battery pack.

In order to further improve the cooling effect, a variety of optimization measures can be taken, such as optimizing the liquid flow and circulation method, selecting a coolant with high heat capacity, and improving the temperature distribution of the liquid. These measures reduce heat buildup and energy loss, ensuring the battery operates in an efficiently cooled state.

3-Sealed design

For liquid cooling pack boxes, the fully sealed design is carried out by adopting advanced sealing materials and structures. The sealing design should not only consider air tightness, but also the sealing of liquid media to ensure that the battery cells are leak-free in all directions.

The design should select the appropriate sealing form and shape according to the specific application requirements, and also consider factors such as the leakage freedom, wear resistance, medium and temperature compatibility, and low friction of the seal, and select the appropriate seal type and material according to the detailed specifications.

In addition, the choice of welding process also has a great influence on the sealing performance. For different materials and thicknesses, choosing the appropriate welding method can effectively improve the quality of the weld to ensure the overall strength and sealing of the system.

Finished picture of the lower box of the single-phase immersion liquid-cooled energy storage pack

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Application Scenarios

Working condition: high heat flux density

Layout Location: Single Sided

Typical Applications：Customed

Features：Excellent Cooling Effect

Thermal Design

 Use simulation software to analyze the heat dissipation performance of heat sinks and cold plates

Application Scenarios

Process: Aluminum profile splicing welding

Layout Location: Bottom Liquid Cooling

Typical Applications：Customed

Features：Light Weight ,Excellent Cooling Effect

In a liquid cooling system, the Cold Plate is directly laid at the bottom of the battery or inserted into the gaps between the batteries. After circulating the coolant, it is cooled through a heat exchanger and then circulated back into the system.

The integration of electric vehicle structures can achieve lightweighting, improve space utilization through shared parts, lighten the entire vehicle, enhance endurance, and reduce costs. Based on years of experience in the new energy vehicle industry,Walmate is able to better provide multi in one powertrain components for vehicle manufacturers

New energy vehicles refer to vehicles that use unconventional vehicle fuels as power sources (or use conventional vehicle fuels and new onboard power devices), integrate advanced technologies in vehicle power control and driving, and form advanced technical principles with new technologies and structures.

Structure Design

Stress, load-bearing, and deformation analysis of cold plates and Heatsinks

Application Scenarios

Working condition: high heat flux density

Layout Location: Single Sided

Typical Applications：Customed

Features：Excellent Cooling Effect

Application Scenarios

Process: Aluminum profile splicing welding

Layout Location: Bottom Liquid Cooling

Typical Applications：Customed

Features：Light Weight ,Excellent Cooling Effect

Clean energy alternative fuel oil：

Climate, environment, resources, energy are closely related to national economy and people's livelihood. Dealing with these related issues well determines whether human society can sustainably develop. Under the pressure of energy shortage and environmental pollution, the two technological routes of energy storage battery vehicles and hydrogen fuel cells have become the main directions for the development of the new energy vehicle industry.

Automotive Light Weighting：

Considering 75 percent of energy consumption is related to vehicle weight, lightweight is an important technological means for new energy vehicles to save energy, reduce consumption, and increase range,

lightweight design is one of the current key drivers to reduce the energy consumption of vehicles. The use of lightweight new materials, structural optimization, and process improvement are the key paths to achieve automotive light weighting.

Thermal Management：

For energy storage electric vehicles, thermal management will become a crucial technology to achieve fast charging and improve range. For hydrogen fuel cell vehicles, water and heat management are key core technologies in the development of fuel cell power systems, which have a decisive impact on the performance, safety, and lifespan of the vehicle's power system.

Generally, external air is used as a heat sink to transfer the heat generated by chip operation to the heat sink through different media and interfaces, and dissipate the heat.

The IGBT power module in the inverter plays a very important role in this process. During the energy conversion process, IBGT will generate a large amount of heat, but when the temperature of IGBT exceeds 150 ℃, IGBT cannot function. Walmate provides various IGBT heat dissipation solutions such as air cooling, single liquid cooling, and phase change.

Product Testing

We offer customized testing procedures to meet our customers’ requirements

Application Scenarios

Working condition: high heat flux density

Layout Location: Single Sided

Typical Applications：Customed

Features：Excellent Cooling Effect

Application Scenarios

Process: Aluminum profile splicing welding

Layout Location: Bottom Liquid Cooling

Typical Applications：Customed

Features：Light Weight ,Excellent Cooling Effect

Under the requirements of higher system energy density, taking into account factors such as heat dissipation and lightweight cost reduction, Walmate continuously optimizes the production process of battery trays to ensure system safety and reliability while improving cost-effectiveness.

The purpose of a heatsink is to acquire higher heat transfer surface area within a given volume by optimizing the structural shape, thereby improving the heat transfer efficiency from its surface to the surrounding fluid. Through surface treatment and other methods, the effective heat transfer area is increased, achieving enhanced cooling and temperature control.

In applications where volumetric power density and heat flux density requirements are not high, rectangular fin heatsinks are favored by engineers due to their simple structure, reasonable manufacturing cost, and good heat dissipation performance.

Comparision of different heat transfer methods

1- Heatsink Fin Design

The heatsink primarily extends its heat-dissipating surface through the fins, focusing on parameters like fin height, shape, spacing, and the thickness of the substrate.

Plate fin heat sink dimensions

According to the formula, the extended surface area of the heatsink can be calculated as:

The area of a single fin: Af = 2L(h+t/2),
The area between fins: Ab = Lh,
Total heat dissipation area: At = nAf + (n±1) Ab  (n is the number of fins).

Fin sectional view

The primary function of the fins is to increase surface area and improve heat transfer efficiency. The spacing, thickness, and height of the fins are critical factors in determining the number of fins, their distribution, and the expanded surface area.

When the surface area of the heatsink increases, the contact area with the air also increases, making it easier for heat to be dissipated. Engineers can further enhance the expansion area of the heatsink by optimizing the shape of the fins, such as using corrugated or serrated designs.

Although a larger surface area generally improves cooling performance, it is not always true that bigger is better. Whether using natural convection or forced cooling, the spacing between the heatsink fins is a crucial factor that determines the heat transfer coefficient of the air flowing over its surface.

The impact of Fin spacing and height on heat dissipation efficiency

In natural convection, the walls of the heatsink will generate natural convection due to temperature changes on the surface, causing airflow in the boundary layer near the fin walls. If the fin spacing is too small, it will hinder the natural convection process. In forced cooling scenarios, the thickness of the fin boundary layer will be compressed, allowing for narrower fin spacing. However, due to manufacturing methods and the driving force of the power components, the spacing cannot be too small. Therefore, balancing the thickness and height of the fins in actual design is very important.

2-Heatsink Substrate Design

The thickness of the substrate is a significant factor affecting the efficiency of the heatsink. When the heatsink substrate is thin, the thermal resistance to the fins farther from the heat source is larger, leading to uneven temperature distribution across the heatsink and weaker thermal shock resistance.

Increasing the substrate thickness can improve the temperature uniformity and enhance the heat shock resistance of the heatsink, but excessively thick substrates can cause heat accumulation, which in turn reduces thermal conductivity.

Heatsink working principle diagrammatic sketch

As shown in the figure above:

When the area of the heat source is smaller than the area of the base plate, heat must diffuse from the center to the edges, creating a diffusion thermal resistance. The position of the heat source also affects the diffusion thermal resistance. If the heat source is closer to the edge of the heatsink, heat can be conducted away more easily through the edges, thus reducing diffusion thermal resistance.

Note: Diffusion thermal resistance refers to the resistance encountered during the process of heat spreading from the center of the heat source to the edges in heatsink design. This phenomenon usually occurs when there is a significant difference between the area of the heat source and the area of the base plate, requiring heat to spread from a smaller area to a larger area.

3-Connection Process of Fins and Substrate

The connection process between heatsink fins and the substrate typically involves various methods to ensure good thermal conduction and mechanical stability between the two, mainly divided into two categories: integrated molding and non-integrated molding.

In integrated molding heatsinks, the fins and the heatsink substrate are formed as one piece, eliminating contact thermal resistance. The main processes include:

l Aluminum Die Casting: Aluminum ingots are melted into liquid form and then filled into metal molds under high pressure, allowing for the creation of complex fin shapes directly through die casting.

l Aluminum Extrusion: Aluminum material is heated and placed into an extrusion cylinder, where pressure is applied to force it through specific die holes, producing the desired cross-sectional shape and dimensions. Further processing includes cutting and machining.

l Cold Forging: This method allows for the creation of fine, dense fins with a high thermal conductivity material, but the costs are relatively high, and it has better capability for complex shapes compared to aluminum extrusion.

l The material for finned heatsinks can be copper, which has a high thermal conductivity. The fins can be made very thin, and they are directly carved from the substrate using cutting tools. However, when the fins are tall and long, they are susceptible to deformation due to stress.

In non-integrated molding, the fins and the heatsink substrate are processed separately and then joined together through welding, riveting, or bonding processes. The main processes include:

l Welding: The fins and substrate are connected together using solder; this can involve high-temperature brazing or low-temperature soldering. Welding has good thermal conductivity; however, using solder for aluminum substrates and fins requires nickel plating and can be costly, making it unsuitable for large heatsinks. Brazing does not require nickel plating, but the welding costs remain high.

l Riveting: The fins are inserted into grooves in the substrate, which are then pressed together with a mold to tightly hold the fins in place. The advantage of riveting is good thermal conductivity, but there is a risk of gaps and loosening after repeated use. Improving the riveting process can enhance reliability, but costs will also increase, so riveted heatsinks are often used in applications where reliability requirements are not high.

l Bonding: This generally involves using thermally conductive epoxy resin to tightly bond the fins and substrate together, facilitating heat transfer. While bonding uses thermally conductive epoxy resin, its thermal conductivity is significantly lower than that of welding, making it suitable for heatsinks with higher fins and small spacing. It can be used in scenarios where thermal performance requirements are not high.

We will regularly update you on technologies and information related to thermal design and lightweighting, sharing them for your reference.

Thank you for your attention to Walmate.