A plug-in hybrid propulsion system includes a fast energy storage device that preserves battery life, where the energy storage elements of the hybrid drive train may be charged with externally supplied electricity as well as energy from the engine or regenerative braking. Electronic switches, passive electronics, an enclosure, controller circuitry, and/or control algorithms are used to manage the flow of power between a fuel powered engine, a battery, a fast energy storage system, traction motors; a charger, ancillary systems, an electrical distribution system, and/or a drive train.