Introduction
The power battery system is the core driving force of electric vehicles. It consists of a battery module, electrical system, thermal management system, battery management system, housing and other parts. The main function of the shell is to load and protect the battery module, which needs to meet mechanical requirements such as strength, stiffness and collision safety.
It is generally believed that if the weight of pure electric vehicles is reduced by 10%, the range can be increased by about 6%. The battery pack system accounts for more than 20% of the weight of the whole vehicle, and the cost accounts for 30%-60% of the whole vehicle. New energy vehicles need to be lighter than traditional cars. In the power battery system, the battery case accounts for about 20-30% of the total weight of the system, which is the main structural component. Therefore, under the premise of ensuring the functional safety of the battery system and the overall safety of the vehicle, the light weight of the battery case has become one of the main improvement targets of the battery system. Lightweight battery case technology includes new materials, new processes and new designs (shell and thermal management system integration, body integrated design).
The following is a brief introduction to the lightweight progress of battery systems of some well-known host factories and battery suppliers.
New materials:
Steel plates, aluminium plates, extruded aluminium, die-cast aluminium, glass fibre composites, SMC complexes, and carbon fibre composites are all used.
Steel shell:
Nissan Leaf adopts a steel shell, and the main process is steel plate stamping and spot-welding connection. Steel shell can provide high strength and stiffness, and the process is simple. It is the most traditional and mature process in the field of body manufacturing.
Aluminium shell:
Aluminium plate + die-cast aluminium
The battery tray box of cast aluminium is relatively integrated, avoiding welding, sealing, leakage and corrosion of steel or extruded profiles. Hybrid Cadillac CT6 and Audi Q7 e-Tron both use aluminium alloy shells. The lower shell of the battery of the two models is made of die-cast aluminium alloy, and the top case (cover plate) is made of aluminium plate stamping parts. The aluminium alloy die-casting shell adopts a primary moulding process, which is simple and can provide good strength, stiffness and sealing performance. The aluminium alloy top case mainly plays a sealing role, and aluminium plate stamping parts are used to reduce weight. Due to the tonnage of die-casting machine equipment, the size of the aluminium die-casting shell is small, which is generally used in the power battery system of hybrid models.
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Aluminium plate + extruded aluminium
The aluminium alloy frame and aluminium plate structure battery case structure are flexible, with obvious weight loss and mature process. The extruded aluminium frame can provide high stiffness and high strength, and aluminium plate stamping parts are sealed. The battery cases of the Tesla Model S, NIO ES8, Volkswagen MEB and other projects all adopt aluminium alloy frames and aluminium plate structures.
Ningde Times CATL applied aviation-grade “7 series aluminium” to the box under the battery for the first time. 7 series aluminium is often used to make aircraft landing gears, which are light, strong and safe. The application of 7 series aluminium also has many risks, especially stress corrosion. To this end, they have improved through hundreds of experiments and related processes. The power battery equipped with the box under the “7 series aluminium” has the following advantages: the energy of the on-board power battery system is increased by 50%; the weight of the whole vehicle can be reduced by 250 kg on the existing basis, increasing the standard operating mileage of the model to more than 600 kilometres. The following picture is the product roadmap of the Ningde era.
Mixed materials:
Steel and aluminium mixing
The top cover of the Tesla Model3 battery is 0. an 8mm steel plate, the floor is 3. 2mm aluminium plate.
Plastic + aluminium plate
BYD, the world’s largest electric vehicle manufacturer, uses upper plate plastic and lower plate aluminium as battery cases to increase the energy density of the battery pack and increase battery life. Taking the Qin Pro EV500 as an example, compared with the previous generation of Qin EV models, the battery pack has been reduced by 157kg, and the energy density of the system has increased to 160.9Wh/kg. According to BYD’s official data, the working range of Qin Pro EV500 is 420 kilometres, and the maximum range is 500 kilometres.
Greely Emigrant EV450 and GAC Trump chi GE3 530 and other models use the top case SMC lightweight material and the lower shell high-strength aluminium for packaging. The energy density of the latter battery system is 160Wh/kg, which is already at the mainstream level.
Carbon fibre composite material:
The carbon fibre reinforced compound (CFRP) battery case of NIO ES6 is 40% lighter than the traditional aluminium or steel battery case, is highly rigid, and has a thermal conductivity 200 times lower than aluminium.
Other materials:
The battery case can be considered to use hot-form steel as the battery case. In case, it is necessary to avoid the intrusion of the battery pack and avoid the risks of fire and explosion. However, it has not been widely used in automobiles.
Battery cases can also be made of foam aluminium and other materials, but they have not been widely used in automobiles.
New process
The manufacturing process of the battery system includes the packaging of the battery monomer, the layout of the battery module, the design of the thermal management electrical system, etc. It also includes the connection process of the battery monomer and the battery box body.
The Tesla Model 3 changed the original 18650 to 21700, and the energy density of the battery increased by about 20% (250→300Wh/kg), increasing the size of the monomer, thus making the whole package light.
New design
Through CAD/CAE/CAM integration technology, the battery case is analysed and optimized to realize the streamlining, integration and lightweight of components, which has become the main design method in battery case development. The lightweight design methods of the battery box mainly include topology optimization, shape optimization, shape optimization and size optimization. In the early design process of the box, that is, the conceptual design stage, topology and free size optimization methods are generally used. In the later stage of structural design, for specific technical requirements, more dimensional optimization, shape and free shape optimization technologies are used to meet specific design requirements.
Tesla integrated all the electronic control units into the package, and the AC charger and DC-DC converter were set into smaller and lighter modules. The high integration greatly reduced the total wiring length of Model 3.
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- The battery tray is lightweight, aluminium or high-strength steel:
In the power battery, the tray accounts for 20-30% of the weight of the battery system, which is actually the main structural component. Therefore, under the premise of ensuring the safety of battery function, the lightweight of the tray has become one of the main improvement goals of battery structures.
From the perspective of the comprehensive material evaluation, aluminium alloy material can first meet the structural needs of vehicle parts, including battery systems, and is still the preferred material to replace some steel structures.
However, high-strength steel plates themselves are also taking the path of lightweight technology. Therefore, aluminium alloy materials and lightweight high-strength steel plates have always shown a state of glue on the road of material selection.
- Adhesive aluminium and steel:
Due to the trend of energy conservation, environmental protection and lightweight products, aluminium is generally the main solution for enterprises to achieve lightweight. However, lightweight is not the only consideration for car companies when choosing materials, and so is the cost.
There is no doubt that aluminium lightweight is the most obvious, so it will be more and more widely used in the future. Although the cost of aluminium alloy is relatively high, its excellent machinability, low density (the density of aluminium alloy is 2.7g/cm), corrosion resistance, high recyclability and recycling, etc., have obvious advantages, and it is still an important symbol of the lightweight process of electrification of new energy vehicles.
Duck Global Consulting has conducted research and forecasts on the average amount of aluminium used in North America. They found that the application of aluminium in vehicles has shown an increasing trend year by year since 1996 and has been on the rise since 2012. The aluminium content of vehicle has reached 400 pounds/vehicle (about 181kg/vehicle) in 2015, exceeded 450 pounds/vehicle (about 204kg/vehicle) by 2020, and exceeded 550 pounds/vehicle (about 249kg/vehicle) by 2028.
Of course, due to cost factors, aluminium alloys are used differently in various models.
Early Tesla should have been a radical in lightweight applications. In the beginning, the Model S accounted for a large proportion of aluminium materials from the body to the battery system structure. Because Model S was positioned as a consumer group at that time, targeting luxury customers.
The following figure shows the proportion of various metal materials in the application of well-known vehicle products in the world. The yellow part represents the application status of aluminium. The Tesla Model S is the model with the highest aluminium content.
However, the most important material selected for other Volkswagen models is high-strength steel, which enjoys a cost advantage. For example, Nissan Leaf, Volkswagen Golf and Toyota Prius prefer to work on high-strength steel plates and special-shaped steel.
It can be seen that although the trend of lightweight development and application of aluminium alloy is clear and clear, cost factors still restrict its strides. This is in turn conducive to low-cost high-strength steel, which is reflected in the resurgence of application.
Tesla is not all technology crazy. Considering the cost factor, adjusting the amount of aluminium is also a reasonable technical behaviour. In the design of Model 3, the design idea has changed from the early stage of “radical” and “luxury”. The body structure is made of steel and aluminium mixed metal, which reduces the proportion of aluminium applications. Even the designers of the well-known Volkswagen MEB platform have indicated that they should prefer low-cost steel plates, and said that new energy vehicles are not just “rich fashion”.
In fact, one material cannot completely replace another. Any material, whether from the perspective of cost and performance, has its own strengths and is developed in parallel. It can only be said that a material can better meet the needs of technology or market development in a certain way.
The application of aluminium materials in the new energy is still the demand for lightweight and energy conservation. At present, taking the battery system of 40KWh as an example, if the steel structure is adopted, the cost can be controlled within 1,000 yuan; if the aluminium profile welding shell structure is adopted, it is between3, 000 and 5,000 yuan. In terms of cost ratio, aluminium alloy is still 3 to 5 times that of steel plate.
The cost factor of aluminium in the promotion and application of new energy is still a roadblock. However, this does not hinder the progress and development of technology.
- But what we need to be clear about is what the design differences caused by the differences in steel and aluminium characteristics at this stage
- The structural design of the battery tray needs to be “teaches according to their aptitude”
Steel and aluminium have great differences in strength, fatigue resistance, elastic modulus, tensile resistance, compressive resistance, shear resistance, bending resistance and other characteristic parameters. Using metal alloy technology, it has indeed achieved a very significant improvement in some aspects, such as strength characteristics, compared with pure aluminium. However, the strengthening of a single feature does not represent the transfer and complete change of essential characteristics. Especially in vehicle engineering, under dynamic and static loads, the differences in characteristics are more obvious.
Therefore, in structural design, although the functions are exactly the same parts, the aluminium alloy structure cannot be equated with steel structure design.
For a long time, domestic new energy vehicles have not been designed positively. The body structure or platform is transitioned from the fuel vehicle. The body structure has not been modified and designed with much adaptiveness. At this time, the battery tray and body are fixed in position and form, and can only take advantage of the situation.
However, with the amplification and popularization of the new energy market, more and more attention has been paid to the functional safety of battery systems, which cannot meet new functional needs.
For the new energy products produced in the early stage, in the process of customer use, the product boom cracking, IP failure, internal module structure failure, electrical performance failure and other failures. The unreasonable design of pallet hanging ear position structure is one of the main reasons directly or indirectly.
The density of the battery body is very high. As a battery tray or case carrying the battery module, it has always been under heavy load. The fatigue performance of aluminium is only half that of steel, and the elastic modulus is only one-third that of steel.
If the pallet hoist is overloaded, or the force difference between different hoses is large and uneven, the dynamic performance will be worse in the face of complex road conditions of the vehicle. Aluminium is more prone to fatigue in a state of high vibration and high-stress concentration, leading to cracking and deformation.
Therefore, it is not surprising that the pallet has failures such as cracking in the hanging ear position and the inner frame beam structure, and even the falling off of the fixed point of the module.
The aluminium hoisting points of the tray should be large and evenly arranged, as shown in the Audi e-Tron aluminium pallet case shown in the figure below.
Not only that, it is not easy to integrate the battery module and the tray. It can stand the test of vibration experiments and is also the best way to test the design results. In the experiment, the cracking of the inner frame and pallet welding and the cracking of the inner frame support beam are often encountered.
Preliminary analysis of the cause of cracking:
From the analysis of material characteristics, the stress at the fault point exceeds the stress or concentration that the material itself can bear.
From a process perspective, the burning damage caused by the welding of the material changes or weakens the parameter characteristics of the material.
From the perspective of structure, whether the cracked support beam is the same as the inner frame structure. The overall structure is more conducive to stress dispersion and uniform stress and uniform vibration frequency.
Audi’s battery tray design is a good case. The yellow arrow is the state of force. The internal pressure is reasonably released through a uniform frame. At the same time, it corresponds to the hanging ear hole of the external frame, so that the internal and external structures are integrated. At the same time, it can also resist damage from external collisions.
Pallet design soul: aluminium outer frame beam strength design
The interior and exterior of the tray structure design mentioned above are integrated, and the outer frame design is also very important.
From the perspective of material characteristics and parameters, the yield strength and tensile strength of aluminium are lower than that of steel.
The yield strength and tensile strength of aluminium and its alloys are 30-500 N / square mm and 79-570 N / square mm respectively. The yield strength and tensile strength of steel are within the range of 250-1000 N / square mm and 400-1250 N / square mm respectively.
The outer frame A on both sides is the first bearer of the Z-directional vector of the battery system; the front and rear outer frame B mainly carry vector loads from X-directional. Therefore, when it comes to the position or structural design of the pallet hanging ear, this factor must be considered.
At the same time, the elastic modulus of aluminium is worse than that of steel, which is also very important, which is related to the fatigue or life of the material of the structure.
Automotive aluminium alloy applications mainly include 5××× series (Al-Mg series), 6××× series (Al-Mg-Si series) and so on. It is understood that the aluminium pallet mainly uses 6 series aluminium profiles (the application of materials needs to be further analysed and explored).
Several structural types commonly used in aluminium battery trays
Aluminium battery trays, because of their lightweight and low melting point, usually take several forms: die-cast aluminium tray, extruded aluminium alloy frame and aluminium plate welding tray (shell), moulded top cover.
The structural characteristics of die-cast aluminium pallets are more manifested in primary die-casting moulding, which reduces the material burnout and strength problems caused by pallet structure welding, and the overall strength characteristics are better.
The frame structure of the tray of this structure is not obvious, but the overall strength can meet the battery-bearing requirements. It is common in small-energy battery system structures. The Audi A3 die-casting tray is shown in the figure below.
The extruded aluminium splicing frame structure is more common and also a relatively flexible structure. Through the welding and processing of different aluminium profiles, the needs of various energy sizes can be met. At the same time, it is easy to modify the design and adjust the materials used.
From the perspective of cost, compared with the die-cast aluminium tray, the extruded aluminium splicing frame structure has a certain advantage. Of course, depending on the quantity of mass production, it is not certain whether this cost advantage exists or not.
The frame structure is a structural form of the tray, which was described in detail in the previous article “3+6”. The frame structure is more conducive to lightweight and more conducive to the strong assurance of different structures.
The structure form of the aluminium battery tray also follows the design form of the frame structure: the outer frame body mainly completes the bearing function of the whole battery system; the inner frame body mainly completes the bearing function of modules, water-cooled plates and other submodules; in the middle protective surface of the inner and outer frame body, it mainly completes the isolation and protection of the battery pack from the outside world, such as gravel. Impact, waterproof, thermal insulation, etc.
The following figure shows the frame structure of Audi. Each layer of structure carries different functions.
Conclusion:
- Aluminium battery cases are currently the mainstream of electric vehicles;
- Mixed battery cases are the trend. Different materials are used in different parts to achieve the optimal solution of performance and cost.
- Integrated design is the trend.
As an important material for lightweight of vehicles, aluminium must be based on the global market and pay long-term attention to its sustainable development. At the same time, we should also correctly view the cost factors and technological progress of steel and aluminium in-vehicle applications.
The correct application of aluminium in design requires a deeper understanding of the material characteristics. Especially for the heavy-duty battery tray application, we also need to constantly explore, know it well, and constantly accumulate application experience, so that we can be proficient in lightweight applications and make continuous progress.
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