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

38 electrical isolation and mechanical issues of drive loads, crashes and impacts to consider. Leak tightness is another important factor, and this is very much a benefit for composite materials such as glass fibre as the case can be a single component without seams and potential leakage points. EMI considerations need to include a metallic or carbon fibre layer for shielding, so glass fibre can be used with a foil and stack layers via a cost- effective press-moulded process. With composites, the top and bottom cover can be a flat sheet formed by compression moulding. For the side walls a lot of designs rely on extrusion and pultrusion; for composites the top and bottom covers are most suited to the glass fibre, as elongation and strength are key mechanical considerations. For even more stiffness, carbon fibre can be more appropriate but that has a price premium, although there are applications for carbon fibre bottom plates for ultra-high end hypercars. Thermoset materials are used for cost considerations, and use compression moulding with a dry textile and resin applied on top. This has a short cycle time. Prepreg glass fibre is typically more expensive. Continuous fibre is needed for the underbody covers, while for the top cover shorter fibres can be used with sheet moulding if complex shapes are to be produced that cannot be easily draped from continuous fibre textiles. This is not as easy on the mechanical design and has thermal complications. For example internal pressure from thermal issues in the box can put a mechanical load on the structure, so it needs a thicker wall. Depending on the load cases, the typical wall thickness of a cover starts at around 2 mm and as heavy a textile as possible is used to minimise the number of layers, leading to applications with over 3 mmwall thickness, which is only two layers of fabric. However, the thicker the weight by optimising the design at the module level rather than by the choice of material, and allows the use of composites to increase the overall energy density. For example, using a modular approach allows six aluminium modules to be replaced with one large module made from a non-metallic material. The only metal in the module is used for the cooling plate. The platform builds in layers of protection for all the potential problems, whether it is water build-up or channelling vent gases, which is very much down to the choice of material. If there is a catastrophic cell failure the materials need to channel the gases away from the other cells, to minimise temperature increases in other cells. De-bondable technology For sustainable and reusable battery cases there is also work on de-bondable adhesives for the cells or modules. At the module level it could help to de- bond a complete module, but that is still challenging as it needs to have a certain design to make it repairable, so it needs a frame in the case. De-bonding on demand uses an external factor such as current, heat, chemical soaking or even mechanical force to separate the modules or cells from the case. Still under development, an additional layer can be triggered to remove the adhesion. It is not yet decided which is the right way to go, as the trigger needs to be designed into the case, and it could use printed layers that can provide heating for de- bonding, for example. The need to disassemble cases is driving more interest in sealant gaskets, so there is an identified need for foam- cast gaskets to rework the case. These are also under development. Testing Glass fibre top covers, bottom covers and impact protection plates can provide a more cost-effective material for battery cases. The most challenging factor is TRP, as the combustion needs to be contained in the box. Then there are EMI, thermal and A modular battery pack makes recycling easier (Courtesy of Ionetic) May/June 2023 | E-Mobility Engineering