Automotive Powertrain Technology International Automotive Powertrain Technology International June 2025 Issue: Page 36

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BATTERY COOLING RIGHT: The construction of immersion-cooled packs is more complex than indirect cooling, but in some applications the benefits they offer outweigh the complications wetted, as per a system described later in this article, then the sealing arrangement becomes more complex with a great number of potential failure points. PARTIAL IMMERSION COOLING As the name suggests, partial immersion cooling only immerses a fraction of the cell volume in coolant. Kreisel Electric 5 has patented direct cooling of cylindrical cells with immersion fractions over the range 20% to <100% of the cell’s length. This is not a large cavity with a fractional fill volume, but a liquid-filled ‘jacket’ that extends over the appropriate portion of the cell outer surface. Although perhaps complicated by the requirement to provide two reliable seals against each cell, the reduced bulk and volume of the system should result in a lightweight battery pack. In its patent, Kreisel notes the importance of maintaining individual cell temperature so that the maximum temperature difference does not exceed a certain value. Although it was not dealing with an automotive application, maintaining a large number of individual cells within a tight operating range is something that Electroflight took very seriously in its (non-immersion) cooling system for the battery used in the successful electric air speed record attempt. 6 TWO-PHASE COOLING Two-phase immersion cooling takes advantage of the latent heat of vaporization of a coolant. As a fluid reaches boiling point, its temperature remains constant until the phase change is complete. The condensation of the fluid on a cold surface is also a very effective way to reject heat. Hwang et al state, “In terms of EV BTMS applications there are several drawbacks as compared to the single-phase system. Such disadvantage includes gradual loss of coolant fluid due to inefficiencies of the condenser unit, increased system complexity, higher maintenance cost, increased system volume and risk of coolant vapor contamination to the other electronic components at the undercarriage of the EV.” They note that this list of disadvantages explains why two-phase systems, although theoretically superior, are less preferred than the conventional (single-phase) type owing to their increased complexity and technical risk. HEATING Battery conditioning is undertaken to bring a battery to an efficient working temperature before use. This is important because the battery capacity is much reduced at low temperatures. According to Luo et al, 7 the reduced energy capacity of a lithium-ion battery at excessively low temperatures is “due to the significant reductions in the dielectric conductivity, charge transfer kinetics and solid-state diffusion of lithium ions”. Although it consumes energy to heat the battery, conditioning can be a better use of energy than letting the battery come to temperature through use. It is also a key consideration in battery longevity. If the battery is operated at high charge and discharge rates when very cold, durability will be compromised. Of course, regardless of weather, customers expect to have the full operational performance of their vehicle available, and for durability not to be reduced in order to achieve this. In vehicles with sufficient performance to permanently damage their high-voltage batteries during 36 www.automotivepowertraintechnologyinternational.com / June 2025

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