E-Mobility Engineering | November/December 2023 45 Battery Show North America 2023 | Report enough static dissipation to mitigate that build-up.” That a fluid in direct contact with high-energy cells has a flashpoint (the minimum temperature at which there is enough vapour for a spark to ignite it) and an auto-ignition temperature is an uncomfortable fact, but it is to be expected because these fluids are hydrocarbon-based. Corrigan said that even in tests that have pushed cells into thermal runaway, the fluid, in conjunction with a wellengineered system, has proved resilient, even though cell temperatures reach well above the 300 oC or so, which coincides with these fluids’ auto-ignition temperatures. Part of the reason for this, he said, is that the fluid circulates in a sealed, oxygen-free environment, and module/pack venting systems ensure that gases generated in a thermal runaway do not pass through the coolant. Naturally, developing thermal management fluids for diverse and demanding applications requires a lot of testing. Corrigan said, “I put together our formulated packages in-house, and we have extensive analytical testing capabilities. Things like flash point tests are routine, as are viscosity, density, heat capacity and thermal conductivity, for example. “For testing with live battery cells up to module and pack level, Lubrizol partners with Exoes. We’ve designed an immersion module with them and have actually tested both prismatic and pouch cells using it, and we’re now working on a cylindrical cell test programme,” he said. EFI Polymers showed a low-density potting material for encapsulating cells in batteries, which is designed to slow and stop a thermal runaway, according to Brian Hanson. Known informally as low density (LD) material, EFI’s material contains glass microspheres, and lasts four or five times longer than a conventional foam in stopping thermal propagation. It is also much lighter than conventional high-density potting materials, which are usually about 40% heavier. Typically used around cylindrical cells inside a module, the potting can be dispensed using most conventional, off-the-shelf meter mix equipment. “We had customers who were using expanding foams that claimed to extinguish fires, but we don’t really believe there’s a polymeric that can stop a fire once it starts,” Hanson said. “So this is more to slow the spread of a thermal runaway down and make sure a fire never breaks out at all. Some OEM automotive companies were asking for something exactly like this, so we formulated it.” The material is a two-part polyurethane. The glass microspheres, which look like a powder to the naked eye, are suspended in one of the parts. Lubrizol promoted its new range of Evogen single-phase dielectric thermal management fluids for immersion cooling of EV batteries and other applications in the powertrain, such as inverters and motors, and discussed their fundamentals with us. With the move to higher power and energy densities, immersing all the battery’s cells in a dielectric fluid circulated by a pump is attracting more interest from vehicle and battery OEMs, particularly for high-performance applications with demanding C-rates for both charging and discharging, Shawn Jarecki said, “Higher C-rates and safety is a really big issue, particularly around thermal runaway. In abuse testing, we’ve shown that our fluids can prevent propagation of thermal runaway from one cell to others in the pack.” Thomas Corrigan explained that these fluids have to be carefully formulated for each application to optimise key parameters such as thermal conductivity, heat capacity, viscosity, dielectric strength, flashpoint and autoignition temperature. Emphasising that formulating these fluids is a delicate balancing act, he pointed to Lubrizol’s anti-static chemistry. “If you’re too dielectric, you can actually get static build-up and have arcing issues, which can lead to shock and fire,” he said. “We’ve found the sweet spot of electrical conductivity where we’re still insulating but have PACT’s ThermoShield ink-on-paper material Immersion cooling from Lubrizol
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