EV battery case – definition and material analysis

As demand for electric vehicles continues to grow, automakers are investing heavily in developing battery technology to offer longer range and faster charging times.

However, in these developments, the importance of EV battery case cannot be ignored. In this article, we’ll explore what EV battery case is and what materials are currently available.

What is EV battery case?

The battery box is a pure incremental component in new energy vehicles, and the value of a single vehicle is about 3,000 yuan. The battery box is mainly composed of an upper cover and a lower case, which is the “skeleton” of the power battery module, and is used to protect the battery PACK against external impact, dustproof and waterproof.

In the lithium ion battery structure, EV battery case accounts for about 20-30% of the total weight of the system and 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 lightweight of EV battery case has become the main improvement goal of the battery system. 

The lightweight technology of EV battery case includes new materials, new processes and new designs (integration of the case and thermal management system, integrated design of the body).

EV battery case material

Steel plates, aluminum plates, extruded aluminum, die-cast aluminum, glass fiber composites, SMC composites, and carbon fiber composites are all used.

Steel shell
Nissan Leaf adopts steel shell, the main process is steel stamping and spot welding connection. The steel shell can provide high strength and rigidity, and the process is simple, which is the most traditional and mature process in the field of body manufacturing.

 Aluminum case
◾ Aluminum plate + die-casting aluminum
The cast aluminum battery tray box has good integrity, which avoids the problems of welding, sealing, water leakage and corrosion of steel or extruded profiles. The hybrid versions of the Cadillac CT6 and Audi Q7 e-tron both use aluminum alloy casings. The lower battery case of the two models is made of die-cast aluminum alloy, and the upper case (cover plate) is made of stamped aluminum plate.

The aluminum alloy die-casting lower shell adopts a one-time molding process, which is simple and can provide better strength, rigidity and sealing performance.

The aluminum alloy upper shell is mainly used for sealing, and the aluminum plate stamping parts are used to reduce the weight. Limited by the tonnage of die-casting machine equipment, aluminum die-casting shells are relatively small in size, and are generally used in power battery systems for hybrid vehicles.

◾ Aluminum plate + extruded aluminum
The aluminum alloy frame and aluminum plate structure battery shell have flexible structural design, obvious weight reduction and mature technology. The extruded aluminum frame can provide high rigidity and high strength, and the aluminum plate stamping parts are sealed. Tesla Model S, Weilai ES8, Volkswagen MEB and other projects all use aluminum alloy frame and aluminum plate structure.

Mixed materials
◾ Plastic + aluminum plate
BYD, the world’s largest electric vehicle manufacturer, uses plastic on the upper plate and aluminum on the lower plate as the battery case to increase the energy density of the battery pack and increase the battery life.

Taking its Qin Pro EV500 as an example, compared with the previous generation Qin EV model, the weight of the battery pack is reduced by 157kg, and the system energy density is increased to 160 Wh/kg. According to BYD’s official data, the Qin Pro EV500 has a cruising range of 420 kilometers and a maximum cruising range of 500 kilometers.

Models such as Geely Emgrand EV450 and GAC Trumpchi GE3 530 are packaged with SMC lightweight materials for the upper shell and high-strength aluminum for the lower shell. The energy density of the latter battery system is 160Wh/kg, which is already at the mainstream level.

Carbon fiber composites
The carbon fiber reinforced composite (CFRP) battery casing of the NIO ES6 is 40% lighter than conventional aluminum or steel battery casings, has high rigidity, and has a thermal conductivity 200 times lower than aluminum.

Other materials
EV battery case can be made of hot-formed steel. In the collision, it is necessary to avoid the intrusion of the battery pack and avoid risks such as fire and explosion. However, there is no large-scale application in automobiles at present.
Besides, EV battery case can also be made of foamed aluminum and other materials, but it has not been widely used in automobiles yet.

Choose aluminum or high-strength steel?

Judging from the evaluation of comprehensive material indicators, aluminum alloy materials can firstly meet the structural requirements of vehicle components including battery systems, and are still the preferred material to replace some steel structures.

However, the high-strength steel plate itself is also taking the road of lightweight technology. Therefore, aluminum alloy materials and lightweight high-strength steel plates have always been in a state of stalemate on the road of material selection.

Undoubtedly, the lightweight effect of aluminum is the most obvious, so it will be more and more widely used in the future. Although the cost of aluminum alloy is relatively high, its excellent machinability, low density (the density of aluminum alloy is 2.7g/cm), corrosion resistance, high recyclability and recycling have obvious advantages. An important indicator of the quantification process.

Of course, subject to cost factors, the application of aluminum alloys on various models is not the same.

The Tesla Model S is the vehicle with the highest aluminum content. From the body of the Model S to the structure of the battery system, aluminum materials account for a large proportion. Because the consumer group positioning of Model S at that time was aimed at luxury customers.

However, the most important material used in other popular models is high-strength steel, which enjoys a cost advantage. For example, Nissan Leaf, Volkswagen Golf, and Toyota Prius are more inclined to work on high-strength steel plates and special-shaped steel.

In fact, it is impossible for one material to completely replace another material. Any material, no matter from the perspective of cost or performance, has its own strengths and develops in parallel. It can only be said that a material, in a certain aspect, can better meet the needs of technology or market development.

Summary

The market demand for battery casings can be expected in the future. As a heavy asset manufacturing industry, battery boxes are less economical for car companies to self-produce on a large scale, and battery boxes are mainly supplied by third parties.

Nowadays, most battery factories in the industry provide EV battery cell to auto companies, and auto companies complete battery packs by themselves, so most of the battery boxes are directly purchased by auto companies.

Relying on the huge consumer market, power battery shell companies are expected to take the lead in market development and technology research and development, forming the core technical advantages of aluminum alloy lightweight auto parts, and accumulating experience for the application of aluminum alloys in body parts.