Abstract: A hybrid propulsion system for powering vehicles such as class 8 DOT classified semi-tractor trucks under normal load conditions and at highway speeds comprises an internal combustion engine, an AC generator powered by the engine, DC battery packs, an AC/DC controller, and an AC electric motor driving the drive train of the vehicle. The AC generator and the DC battery packs provide input to the AC/DC controller, which, in turn, converts the DC input from the DC battery packs via a DC circuit to AC so that the output from the AC/DC controller to the electric motor is AC for powering the vehicle. The DC battery packs may comprise thin plate flooded lead acid cells and may be connected in series, in parallel or a combination thereof. Vehicles may be retrofitted to incorporate the propulsion system.