This paper addresses one of the most significant problems associated with ambient-temperature batteries in electric vehicles (EVs): maintaining optimum, uniform temperatures within a battery pack. Issues such as temperature dependence of capacity, effects of temperature extremes, and temperature variation within a pack are discussed. Calculations based on a battery/vehicle model are then given that demonstrate the effects of temperature on driving range. Battery thermal management (TM) is next discussed and test results comparing circulating-air and circulating-liquid TM systems with non-managed systems in EVs are presented. These data show the importance of incorporating a TM system into an EV to achieve optimal battery and vehicle performance and battery life.