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Ideal operation temperatures for Li-ion batteries fall in a narrow range from 20°C to 40°C. If the cell operation temperatures are too high, active materials in the cells may become thermally unstable. If the temperatures are too low, the resistance to lithium-ion transport in the cells may become very high, limiting the electrochemical reactions. Good battery thermal management is crucial to both the battery performance and life. Characteristics of various battery thermal management systems are reviewed. Analyses show that the advantages of direct and indirect air cooling systems are their simplicity and capability of cooling the cells in a battery pack at ambient temperatures up to 40°C. However, the disadvantages are their poor control of the cell-to-cell differential temperatures in the pack and their capability to dissipate high cell generations.

Battery packs for plug-in hybrid electrical vehicle (PHEV) applications can be characterized as high-capacity and high-power packs. For PHEV battery packs, their power and electrical-energy capacities are determined by the range of the electrical-energy-driven operation and the required vehicle drive power. PHEV packs often employ high-power lithium-ion (Li-ion) pouch cells with large cell capacity in order to achieve high packing efficiency. Lithium-ion battery packs for PHEV applications generally have a 96SnP configuration, where S is for cells in series, P is for cells in parallel, and n = 1, 2 or 3. Two PHEV battery packs with 355V nominal voltage and 25-kWh nominal energy capacity are studied. The first pack is assembled with 96 70Ah high-power Li-ion pouch cells in 96S1P configuration. The second pack is assembled with 192 35Ah high-power Li-ion pouch cells in 96S2P configuration.