The power battery pack is one of the core components of electric vehicles. The weight of the battery pack accounts for about 20-30% of the total weight of the vehicle, and it is also one of the components with the highest cost in vehicle production. As we all know, the battery pack has very strict safety requirements, and it also determines the performance of the vehicle in terms of power and cruising range. In order to improve performance, the weight of the overall body is also very important. Among them, the lightweight battery pack has also become a key issue.
1. Why should lightweight battery pack?
Research data shows that a 10% reduction in the weight of traditional fuel vehicles can increase the economy by 6-8%; while a 10% reduction in the self-weight of an electric vehicle under constant speed driving conditions can increase the driving range of the vehicle by about 10%. The vehicle power battery pack can increase the mileage of the vehicle through the improvement of the specific energy of the single unit, the lightweight battery pack, and the improvement of the battery manufacturing process.
The specific energy of a single cell is limited by the internal material composition and external dimensions of the cell, and it is difficult to make breakthroughs in the short term. The lightweight battery pack design can reduce weight and increase the driving range of the vehicle. Therefore, the lightweight battery pack and module compact design are undoubtedly a feasible way to increase the driving range.
2. What are the methods to lightweight battery pack?
Lightweight battery pack design mainly has multiple approaches such as system design level and detailed design level. The structural design of the battery pack needs to meet various performance requirements such as mechanical safety, sealing insulation and fire prevention. The structural strength, stiffness, crashworthiness, reliability and other factors of the box body, as well as the internal module layout, will all affect the performance of the battery pack.
The ideal power battery pack structure design should meet the performance requirements of electric vehicles such as car battery voltage, safety to lightweight battery pack. Among them, the more effective lightweight battery pack methods are as follows.
① Optimization of the layout of the battery pack
In the limited space of the battery pack, a certain number of battery cells form a battery module through specific mechanical and electrical connections. According to the spatial shape and loading characteristics of the vehicle battery pack, the battery modules are arranged in series and parallel to form a power battery system, and the arrangement and structural form of the modules in the battery pack are quite different. The layout of the power battery pack is usually determined by the space characteristics of the vehicle, and factors such as the driving mode of the vehicle, the position of the center of gravity of the vehicle, and the ground clearance need to be considered.
According to the needs of vehicle manufacturers, power battery companies in the world have developed vehicle power battery packs with different module arrangements, battery pack box shapes and mounting lug positions. After continuous research and development, the commonly used structural layout forms of battery packs include suspension under the body, integrated body structure, and distributed standard boxes.
The increasing demand for the driving range of electric vehicles and the improvement of vehicle forward design technology have prompted the coordinated development of body design and lightweight battery pack. Strive for a lightweight battery pack, with a compact body and better battery pack performance, and gradually increase the number of integrated power battery packs with platform-based and modular body structures.
② Optimization of battery modules
The lightweight battery pack design at the system design level starts with the selection of cell parameters and cell size. There are matching design problems between lithium-ion power cells and power battery systems under different chemical systems and size parameters, which usually need to be calculated and determined in the conceptual design stage of the battery system. Then by optimizing the internal layout of the battery pack box and reducing the design level, the lightweight battery pack is realized.
For example, the Cell To Pack technology (CTP) proposed by CATL. Compared Tesla 4680 battery vs CATL CTP battery, in this CTP design, the monomer and the battery management system are directly fixed in the battery pack shell, the battery cells are built in the upper and lower shells, and the inside of the shell is filled with thermal conductive glue. In addition, there is a built-in pressure or temperature sensor between the side wall of the battery cell and the battery cell shell. The two sensors work together to check for bad battery cells and detect safety accidents such as thermal runaway of the battery cell in advance.
Since this design form does not use a module structure, the volume utilization rate of the battery pack is increased by 15-20%, realizing a lightweight battery pack. The battery cells are assembled separately, which reduces the difficulty of assembly and improves production efficiency by about 50%. More importantly, the timely detection and replacement of faulty cells can be realized, and the strengthening scheme of the cell shell can reduce the protection level of the battery pack shell.
③ New grouping method
Large modular design Increasing the size and capacity of a single cell results in a reduction in the mass of structural parts shared by each single cell. For example, the CATL large module design structure eliminates the battery box in the prior art through the large module design, and directly installs the battery module on the vehicle through the support sleeve and the installation beam through the fixing piece. Realize the lightweight battery pack while improving the connection strength of the battery pack on the vehicle. Integrated design
Reduce intermediate levels such as battery pack modules, optimize the size of single cells, and improve the utilization of box space. For example, the blade battery battery pack design scheme proposed by BYD, through the design of a flat and large-sized battery cell, is arranged in an array arrangement inside the battery pack box. This design enables lightweight battery pack, which increases the specific energy of the battery pack by about 50%, and reduces the production cost by about 30%.
④ Lightweight material application
At present, the energy density of lithium-ion single cells is increasing slowly, and the weight reduction design of the cabinet is a very effective way for lightweight battery pack, and the application of lightweight materials has a very obvious effect on the weight of lightweight battery pack. At present, there are two types of lightweight materials that are relatively mature in application: aluminum-magnesium alloys and composite materials. Aluminum, magnesium, and titanium alloys are lightweight materials with low density in the current metal material system.
Aluminum alloy is light in weight, recyclable, and good in oxidation resistance, and is currently a commonly used material for lightweight battery pack. The remarkable features of composite materials are light weight, good insulation and easy molding and processing. Auto parts made of various composite materials are replacing some metal parts, such as engine covers, oil pans, battery pack upper case covers, etc.
Composite materials are subject to factors such as raw materials and production costs. At present, composite materials that are widely used in battery packs include glass fiber reinforced plastics (SMC) and modified resins. The battery pack upper case cover made of SMC is about 38% lighter than the traditional metal material upper cover, and the application of carbon fiber composite material (CFRP) is also gradually increasing, and the lightweight battery pack effect of composite material is obvious.
Some companies try to apply composite materials to the lower floor of electric vehicles, but the stiffness characteristics of composite materials are poor, and it is necessary to thicken the size or use a sandwich structure to improve the bending resistance of the structure. The lower box of the battery pack is designed as a sandwich structure and a metal or honeycomb aluminum structure is added to the middle layer, which has many advantages such as light weight, high strength, and good crashworthiness.
⑤ Limit design
Limit design refers to the performance optimization in the detailed design stage of the product or the design improvement of the product in the later stage. The limit design needs to know the critical value of the design, not only to meet the performance requirements, but also to meet the parts processing and product assembly process requirements.
Limit design usually uses computer-aided design (CAE) to explore product performance critical values and production process parameters, and accurately locates through CAE simulation analysis technology. For example, the design of the load-bearing part of the battery pack box is strengthened, and the non-load-bearing part is made of thin-walled materials. The thickness of different positions of the box is changed to achieve structural performance that meets the design requirements and is as lightweight battery pack as possible.