This paper outlines the development of a battery system test regime for plug-in hybrid electric vehicles. The test regime is focused on a specific vehicle, the plug-in hybrid Electric Sprinter Van jointly developed by DaimlerChrysler and the Electric Power Research Institute. This paper describes an ongoing process to validate the cycle life performance of advanced batteries subjected to a PHEV duty cycle. Plug-in hybrid electric vehicles share many component and powertrain architecture commonalities with power assist hybrid electric vehicles-but with the ability to recharge a larger energy storage system from off-board electrical power. The PHEV has unique battery requirements that often require a compromise between the high energy battery systems required by battery electric vehicles (BEV) and the high power energy storage systems used in power assist hybrid electric vehicles (HEVs). The duty cycle is generally a combination of deep and shallow discharge behavior found respectively in BEVs and HEVs and is dependent on both vehicle requirements and the energy management strategy of the hybrid powertrain.