Mid 2006 a study group at General Motors developed the concept for the electric vehicle with extended range (EREV),. The electric propulsion system should receive the electrical energy from a rechargeable energy storage system (RESS) and/or an auxiliary power unit (APU) which could either be a hydrogen fuel cell or an internal combustion engine (ICE) driven generator. The study result was the Chevrolet VOLT concept car in the North American Auto Show in Detroit in 2007.The paper describes the requirements, concepts, development and the performance of the battery used as RESS for the ICE type VOLTEC propulsion system version of the Chevrolet Volt.The key requirement for the RESS is to provide energy to drive an electric vehicle with “no compromised performance” for 40 miles. Extended Range Mode allows for this experience to continue beyond 40 miles. Multiple factors helped refine a requirement of at least 8 kWh usable energy, and 115 kW discharge power over the applied battery state of charge range. The Chevrolet Volt vehicle is based on GM's global compact vehicle platform. Aggressive targets for mass, volume, and timing have been considered for impact beyond start of production (4Q2010).A battery cell providing both, very high energy density and high power density at the same time had to be developed and validated applying the latest Li-Ion technology.Integration into the car should allow for good aerodynamics, provide the best crash protection and have low impact on customer useable space. The battery must also be able to perform in all typical automotive atmospheric conditions. An inter-cell thermal system was sized and balanced, to efficiently manage temperatures within the battery and help lengthen battery life.New tests and methods had been developed for battery systems development in the lab, in the vehicle and in models. Data and examples will be shared. Specific vehicle/battery test activities will be introduced. Finally performance results demonstrating the characteristic(s) of the system will be shown.