An electrical charging strategy and system for a high voltage electrical energy storage system able to supply electrical energy to a hybrid vehicle is disclosed. The system charges the electrical energy storage system so state-of-charge at the end of a trip is substantially unchanged. The strategy and system employs opportunity charging to achieve maximum energy efficiency of the hybrid system, thus minimizing fuel consumption and maximizing fuel economy. The charging system operation is controlled, based upon: the state-of-charge of the electrical energy storage system, and, the operating efficiency of the internal combustion engine. Battery life is likewise extended through use of this strategy.