One plug-in hybrid EV built in China is already using a thermoplastic polypropylene compound instead of aluminium for its battery case cover, providing savings in weight. Other EVs now in production around world are using several thermoplastic materials for components such as cell carriers and housings, battery modules and battery enclosures.
In a modular case, most of the materials are set in the battery platform. These include the plastic carriers, the adhesives and the busbars, all with a UL94 rating of V-0. The battery case casing is part of the vehicle integration, so each vehicle designer comes with different needs.
The choice of materials used for a battery case has to cover a wide range of performance issues. Replacing steel or bonded aluminium with thermoplastics or glass fibre composites is offering lighter cases and more options for increasing the energy density by using larger components that can be more easily assembled.
One perception is that plastics are not suitable for battery packs as they cannot prevent thermal runaway and fires. However in testing, an aluminium plate was exposed for 5 minutes to a flame with a temperature of 1100 ºC. The same test on a plate made from long glass fibre polypropylene and a flame retardant (FR) resin reacted very differently.
Using plastics for a battery case can reduce the number of components required by using larger components, which also makes the assembly process more efficient. That also opens up new design approaches for modular cases.
One issue with using thermoplastics has been the structural considerations. Designers are increasingly looking at using the battery cells as part of the structure of a vehicle, which means there can be less structural pressure on the design of the case as the cells and modules take more of the stress.