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

space in the case. This is different from retrofitting a silica system to address condensation problems, as most of the silica approaches use a cartridge system or silica bag, which need fixing and can be heavy. The important question is howmuch space is available for the swelling of the material, and that is also part of the design decision. To manage a coolant leak the material would be on the bottom of the case. For condensation it would be on the bottom but next to the cold plate. To manage humidity the material can be placed on the bottom, side or top. Conclusion 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. That opens up more modular design opportunities for recycling the cases Other materials are also vital though. New flame-retardant additives for resins and coating materials for composites can boost a case’s thermal performance, but can require new application technologies such as spray heads. Managing the moisture in the case for avoiding corrosion is also vital, whether from airborne humidity or leaks in the cooling system. New absorbent polymers in non-woven materials are providing more options for vehicle developers, for new platforms as well as retrofit. Acknowledgements The author would like to thank Lucas Krahl at Henkel, James Eaton at Ionetic, Christoph Ebel and Christian Schludi at SGL Carbon, and Dr Ulrich Schneider and Patrick Richly at Freudenberg Performance Materials for their help with researching this article. 42 Focus | Battery case materials cycles from +80 to -40 oC and +80% humidity to zero did not see any ageing, indicating a suitable lifetime. The material is also flat, rather than packets of silica that need space in the case, and so can be layered in the centre of the material using an organic superabsorbent polymer (SAP) that coats the dry layer. In general, a SAP polymer provides 400 g/g absorption, and there are many SAP producers on the market. The one used for battery cases is a high absorption version for water-glycol mixtures, and at 50% relative humidity in the air the absorber shows a mass change of 30-50%. If there is a lot of condensate, 200 g can be absorbed just from humidity, so the layer can swell from 2-3 mm to 10 mm. It is also tested to the UL94 flame-retardant standard. The advantage of the SAP-based material is that decreasing the humidity immediately decreases the weight of the material, and with no hysteresis. This is a key difference from silica gel, which does have hysteresis. This lack of hysteresis and the fast drying allows the material to be used for passive climate management, adapting to the humidity in the case. The case is an open systemwith venting, and the change in humidity comes from air exchange in pressure differences, for example in a tunnel, going past a truck or with the temperature change between night and day. The question is howmuch free air is present in the case, and that leads to discussions with the designers about the conditions. These determine the amount of SAP used in the material and the positioning of the material. That in turn leads to the design of the layer in the pad with the SAP, but there is a limit on the amount of SAP that can be added. That is because pure SAP slows down the absorbing process and re-drying, so the layered non-woven structure drives the performance. Development work is continuing on increasing the amount of SAP that can be held in the non-woven layers, as well as reducing the weight of the fibres, which could be different types. There is also work on speeding up the absorption. A key advantage with this approach is that it can be used as a design element or as a change to implement a retrofit. For a retrofit, it does not need additional Dynamic sorption in a drying chamber at a constant temperature, raising the humidity 140% by weight absorption at 90% humidity (Courtesy of Freudenberg) May/June 2023 | E-Mobility Engineering