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. The battery temperatures are managed with a direct liquid cooling system for the 96S1P pack and with an indirect liquid cooling system for the 96S2P pack. Procedures are discussed for the cooling system design for both direct liquid cooling and indirect liquid cooling packs. A design criterion is proposed for obtaining a uniform coolant flow distribution in the battery pack. Thermal behavior of the cells in both battery packs are simulated using 3D finite element models under 4C continuous discharge from a fully charged state to 90% depth of discharge, simulating the worst condition for the PHEV battery pack utilization.