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The battery packs for plug-in hybrid electrical vehicle (PHEV) applications are relatively small in the charge depleting (CD) mode but fairly large in the charge sustaining (CS) mode for their duties in comparison to the battery packs for hybrid electrical vehicle (HEV) applications. Thus, the heaviest battery thermal load for a PHEV pack is encountered at the end of the CD mode. Because the cells in PHEV battery packs are generally larger than those in the HEV packs in both capacity and size, control of the maximum cell temperature and the maximum differential cell temperature for the cells in a PHEV pack with high packing efficiency is a challenge for the cooling system design. The maximum cell temperatures locate in the areas near the terminal tabs where the current densities are highest.

Lithium-ion (Li-ion) batteries are becoming widely used high-energy sources and a replacement of the Nickel Metal Hydride batteries in electric vehicles (EV), hybrid electric vehicles (HEV) and plug-in hybrid electric vehicles (PHEV). Because of their light weight and high energy density, Li-ion cells can significantly reduce the weight and volume of the battery packs for EVs, HEVs and PHEVs. Some materials in the Li-ion cells have low thermal stabilities and they may become thermally unstable when their working temperature becomes higher than the upper limit of allowed operating temperature range. Thus, the cell working temperature has a significant impact on the life of Li-ion batteries. A proper control of the cell working temperature is crucial to the safety of the battery system and improving the battery life. This paper outlines an approach for the thermal analysis of Li-ion battery cells and modules.