Delphi Automotive began commercial production of a valve-regulated lead acid (VRLA) battery in 1996 that was specifically designed to deliver the power and energy demands required of electric vehicles such as the EV1. Field results support the need for thermal control of the battery pack in order to maintain capacity, state-of-charge balance, and overall performance. The influences of manufacturing variance, self discharge rate, monoblock age, and charge acceptance are quantified and discussed.