65 Thermal runaway prevention | Focus E-Mobility Engineering | May/June 2024 expert says. The primary enemy is heat – heat generated within the cell, as well as that carried by the gases produced by the failing cell. “The thermal management system is solely responsible for dissipating the heat conducted/convected from the cell. Dissipation of the heat will mitigate the temperature rise in neighbouring cells and reduce the chance that they enter thermal runaway. “The gas is more difficult to deal with, as it is hard to limit the interaction of the gas with the rest of the battery pack. This is where battery pack engineering comes into play. The solutions vary substantially due to the shapes, sizes and chemistries of the various types of battery cells used,” he says. “Compared to cell-to-pack (C2P) architecture, a module-based pack design means there are additional components in the battery pack that intercept particle ejection in particular, so the pack lid is not directly affected. This ensures the influence of the thermal runaway on the pack housing is reduced,” notes another expert. Proper battery management, and vehicle energy and thermal management are critical to ensuring each cell in a pack is operated within safe limits. It is also critical to avoid high thermal stresses that could lead to excessive heat that cannot be removed if the thermal management system is not operating properly immediately after the vehicle has been driven hard or charged rapidly, another expert cautions. Immersion cooling Immersion cooling, a technology borrowed from the IT domain and used in data centres, provides excellent thermal management and mitigation of thermal runaway propagation. “The increased surface area for heat transfer, and the direct contact between the cells and coolant ensures maximum heat transfer,” an expert says. “The drawbacks are the lack of familiarity with immersion cooling and the weight of the fluid. Water-glycol indirect cooling has been used for decades and OEMs are very familiar with the technology, but immersion is new. “Additionally, the expectation is that the mass of fluid added will outweigh the water-glycol currently used, as well as the weight of the aluminium cold plate and thermal interface material. A comprehensive study and demonstration have yet to be done, but the sentiment still exists in the industry.” The main drawbacks to conventional, indirect cold-plate cooling are the limited surface area for heat transfer, and the amount of material between the hot Pack factors It is critical that good assumptions are made in the initial pack design for the volume and performance requirements of components, from cell to pack level. “Far too often, we work on projects where ambition has met reality and it is necessary to ‘ re ght’ to address unforeseen issues,” another expert says. “There is still a skills gap in the industry for how to properly design and integrate batteries into a vehicle, and certainly, this will improve with time as OEMs and engineers gain experience.” There must be a plan for what occurs during thermal runaway and the pack design must reflect that, a further The separator is an ideal, fail-safe device in the heart of a cell, which shuts it down in an emergency Immersion cooling can provide tight control of cell temperatures and rapidly extract heat from cells in incipient runaway, possibly preventing its progression (Image courtesy of Mahle)
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