ISSUE 011 Autumn 2021 Candela C-7 hydrofoil speedboat dossier l In conversation: Robert Hoevers l Battery recycling focus l Vehicle dynamics insight l ZeroAvia hydrogen-electric aircraft digest l Motor materials

Using materials with different coefficients of thermal expansion (CTE) can also create gaps as internal and ambient temperatures change. Any gaps between the housing and the cover are critical, so seals must be able to cope with that movement without allowing contaminants to enter. Sealing battery modules and packs is therefore a critical design consideration. Joints inside a pack can be complicated and require a variety of sealing properties, including resistance to electrolytes in the cells and chemicals in water-based coolants as well as the new dielectric fluids used in immersive cooling systems. Sealants must also withstand a range of temperatures from -40 C up to those associated with catastrophic pack failures, as well as meeting UL94-V0 flammability requirements. They must also exhibit a combination of mechanical properties including strength, elongation and cohesive robustness under tensile loads. Further challenges are presented by substrates with which it is inherently difficult to form adhesive bonds. For example, non-polar materials have low surface energy, which is a measure of how attracted a material’s molecules are to each other and to other materials’ molecules. Materials such as polypropylene, polyethylene, polystyrene and ethylene vinyl acetate all have low surface energy. Their opposites are polar substrates, materials with high surface energies such as most common structural metals along with plastics such as polyethylene terephthalate, polycarbonate, polyvinyl chloride and acrylonitrile butadiene styrene, and forming adhesive bonds with those is relatively easy. The challenge lies in forming impermeable and structurally strong bonds between materials in these two different categories. EV electronic components must be protected from environmental stresses such as temperature fluctuations, vibration and the ingress of dirt and liquids, as must sensors such as cameras and radars. This is generally achieved by gluing down the lids of housings or by full potting of the components, in which they are fully embedded in the sealant that then cures around them. With EVs encompassing battery electric, hybrid and fuel cell vehicles, there are broad similarities in the fundamental sealing requirements as well as a few differences. In one respect – the need to handle high pressures over a wide temperature range – the demands that battery EVs place on sealants are not as severe as those imposed by IC-engined vehicles. The extreme case is a diesel engine fuel injection system, which commonly operates at pressures in excess of 2000-2500 bar in road vehicles, vehicles. Flame resistance is important to mitigate the consequences of battery thermal runaway. Conductivity is important because static electric charges can build up between the rotating and static elements of rotary seals, so a supplementary lip made of a conductive material can be added to earth the charge continuously to prevent sparking from high-voltage discharges. Further, internally generated currents in induction motors that pass through bearings can damage them, and such currents can also be a source of EMI, so shielding for radial shaft seals is a requirement for EVs. Solutions include the use of electrically conductive non- woven components, sometimes in combination with low-friction materials or even gas-lubricated mechanical seals, one specialist sealing company points out. Batteries are also becoming larger, which can make it difficult to access their internals for repairs such as replacing modules or even individual cells, and their chemistries can create conditions in which corrosion becomes a problem. Skateboard challenge The rise of purpose-designed EVs as opposed to those derived from IC- engined vehicles has led to a growing adoption of the ‘skateboard’ format. This is usually associated with a long, broad and shallow battery mounted under the floor that takes up much of the available space there – the vertical space under the floor being very limited – and a lightweight construction. All these factors present sealing challenges. Large, lightweight battery housings are prone to much greater mechanical movement than the comparatively rigid ones of earlier EVs, and that creates gaps that must be kept sealed. Parking a car with one wheel up on a kerb can generate a twisting force that opens a static gap, while forces acting while the vehicle is in motion can open and close gaps dynamically. Numerous items in an EV powertrain subject their seals to a wide range of temperatures, pressures and chemical conditions (Courtesy of Freudenberg Sealing Technologies) Autumn 2021 | E-Mobility Engineering 65 Focus | Sealing