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

35 May/June 2023 | E-Mobility Engineering Focus | Battery case materials The same test on a plate made from long glass fibre polypropylene and a flame retardant (FR) resin reacted very differently. The flame burned through the aluminium in 2.5 minutes, while the glass fibre plate with FR withstood the temperature for the full 5 minutes with no breakthrough. The work on vehicle electrification technologies extends beyond simply pairing materials to individual components in existing designs. The complete EV battery system and vehicle structure has to be taken into account to identify the right material in the right place. For the case, that means using the properties and strengths of thermoplastics to improve performance, reduce costs and weight, and support mass production. This includes integrating individual batteries into pouch cells placed within a thin-walled housing moulded with a 30% glass fibre-filled, FR polypropylene compound. Using plastic also allows different geometric features such as a double- wall construction and new rib patterns to be used that can reduce the weight of the case while meeting the structural requirements. One alternative that has been used to increase the thermal protection is to use large mica sheets. However these are brittle and are more difficult to source and work with. This is becoming more relevant for battery safety. The spray process is also conformable to follow the contours of the top plate. The adhesion of the material on the different substrates, particularly on how the coating behaves with different composites, is also being explored. Coating technology can be applied on the inner side of the battery lid to protect passengers, in the form of a liquid coating, water or epoxy based, that is sprayed onto the lid of the battery case. This is sprayed on with a 1-4 mm coating thickness, and a flat streaming process has been developed for a fast application rate. The spray head was developed with manufacturing partners to be added to equipment. Thermal conductivity Thermal gap filler materials are used to fill gaps in the battery case, but there are nowmore requirements on their structural properties and to provide high thermal conductivity. The thermal requirements are calculated at the module level but the case filler is calculated at the case level. issuewith bonded aluminiumplates and evenwith steel, which can have challenges with resisting the high temperatures that can occur in a thermal runaway. The key standard here is the UL94 V-0 flammability rating of plastic materials, which is now harmonised with the IEC 60695-11-10 and 60695- 11-20 standards and ISO 9772 and 9773. These determine the tendency of a plastic material to extinguish or spread the flame once the sample has been ignited. The V0 rating stops the burning of the material within 10 seconds. However, version 3 of Regulation 100 in the Automotive Regulatory Guide also specifies a minimumwarning to passengers in a vehicle of the risk of a fire in the case. This is not a new challenge: for every EV fire there are 63 fires in vehicles with IC engines. However, the materials in the case are key to meeting this regulatory standard to give passengers sufficient warning in the unlikely event of a thermal runaway and a fire in the vehicle. 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. A battery pack enclosure or cover moulded using Stamax FR resin, which meets the UL94 V-0 flammability rating (Courtesy of SABIC)