 For the 2011 / 2012 model years , the battery pack stores 16 kWh of energy but it is controlled or buffered via the energy management system to use only 10 @.@ 3 kWh of this capacity to maximize the life of the pack . For this reason the battery pack never fully charges or depletes , as the software only allows the battery to operate within a state of charge ( SOC ) window of 65 % , after which the engine kicks in and maintains the charge near the lower level . The minimum SOC varies depending on operating conditions . When more power is required , such as mountain mode , the lower limit of the SOC will rise to 45 % to ensure there is enough power available . The battery capacity was increased to 16 @.@ 5 kWh for the 2013 model year , the SOC window will be increased to use 10 @.@ 8 kWh of the total battery energy , and the buffer to ensure battery life will not be reduced . These changes will increase the Volt 's all @-@ electric range but charging will take slightly longer . The improved battery performance and durability were achieved through minor changes to the material composition of the battery cell chemistry . 
