An electric storage battery system carried on a hybrid energy off-highway vehicle including wheels for supporting and moving the vehicle, an electrical power generator, and traction motors for driving the wheels, with electrical power generated on the vehicle being stored at selected times in the electric storage battery system and discharged from the electric storage battery system for transmission to the traction motors to propel the vehicle, with the vehicle and battery system being exposed to a range of environmental conditions is provided. The storage battery system includes at least one battery for storing and releasing electrical power, wherein the at least one battery generates an internal battery operating temperature that is independent of and exceeds the highest environmental temperature of the vehicle and the at least one battery.