E-Mobility Engineering 019 | In conversation: Stephen Lambert l WAE EVR l Battery case materials focus l Quality control insight l Clipper Automotive Clipper Cab digest l Optimising battery chemistries insight l Powertrain testing focus

Focus | Battery case materials 39 easily meets the impact requirements. An epoxy resin with flame retardants in the resin is typically used. Phenolic resins are interesting as they do not burn as easily as other resins, and are working well in testing, but the choice depends on customer requirements The technology is evolving rapidly, and internal testing for impact and temperature performance is key. This testing showed thicker composite materials than expected are required for the battery pack, and although that can take up more space compared to metal it is still lighter. Initial tests have focused on a single battery cell in a box, but this is now moving to testing a pressure-tight box with 25 large cells in the 21700 form factor. This is similar to the UL2596 standard test, although that uses the smaller 18650 cells. The testing needs to include temperature testing on the surface at the same time as testing the mechanical loads. A cell fire moves through the box to produce a short-lived high local temperature in different locations and lower-temperature burning throughout the case for a longer time. A case design needs to cope with both requirements, and have the mechanical strength to handle pressure build-up and particles from cell explosion. A bigger panel for a larger case with bigger cells also bends more under pressure, and current tests are showing how a material performs with bigger panels. The next step of a test system is then a full-scale case test. There are lot of differences in the type of textile and the wall thickness of the whole case, as the resin system has an influence on the performance, as does the fibre length. The test systems are now focusing on impact protection. There is a lot of difference in the requirements of the OEMs, and there are generally two typical scenarios – a small object at high speed, and a larger object with lower velocity hitting the bottom plate. material limits the draping properties of the material for creating certain geometric shapes. Battery cases used as part of the chassis is an opportunity for composites, as designers need to close off the case with the top and bottom covers. This is good for torsional stiffness of car bodies. Then there is the side impact load case, which needs to transfer the load across the side rails. Thermal propagation is the factor that often drives the design, so the covering plate needs to be rigid and thick enough to withstand the thermal propagation. Plastics are now competing with aluminium, which will not be able to contain a battery fire in the latest test regimes. Also, steel gets hot very quickly so it needs additional insulation, meaning that the final design is not so thin and not so cheap. That’s the big advantage of composites in isolating the thermal characteristics of the case. With compression moulding there are limits on geometries from the draping of the material, so there are trade-offs. Injection moulding on the thermoplastic side or sheet moulding compounds on the thermoset side have more flexibility in design, but both need more costly tools and bigger machines. The interesting thing is that no one design is the way forward. A big portion of the market can be addressed with continuous fibre thermoset. The bottom panel is mostly flat, and Composite carbon battery enclosures offer high stiffness but come with an extra cost (Courtesy of SGL Carbon) May/June 2023 | E-Mobility Engineering