NOVELTY - The system has a fuel cell (10) with a first, higher voltage bus (11) and a second, lower voltage bus (24), between which a bidirectional DC-DC convertor (23) is connected for performing up- and down-conversions. The second bus includes a battery (30) for storing electrical power at the lower voltage and a compressor (18) receives electrical energy from the first bus to provide reaction gas for the fuel cell.    USE - For use in a motor vehicle.    ADVANTAGE - The system enables the vehicles propulsion capability to be increased in transient situations by operating the vehicle and fuel cell 12 volt loads from the battery rather than the fuel cell, allowing all of the energy from the fuel cell to be used to propel the vehicle, with power from the battery available to be additionally up-converted to provide still further power for a limited amount of time. The system also facilitates the storage of regenerated power from the traction engine during regenerative braking, which can be down-converted and stored in the battery. The system simplifies the start up procedure as sufficient power can be provided to the compressor from the battery so that the fuel cell is supplied with sufficient reaction gas prior to the fuel cell providing electrical energy.    DETAILED DESCRIPTION - The propulsion system has a fuel cell that acts as an electrical power source which provides electrical power to a traction motor and to a first bus and a second bus, the second of which has a relatively lower voltage than the first. An electrically driven compressor that acts as a source of reaction gas for the fuel cell and is coupled to receive electrical energy from the first bus, with a battery coupled to the second bus storing electrical energy at the lower voltage.    A bidirectional DC-DC convertor that includes a transformer and shared electromagnetic voltage conversion elements for performing up-conversion and down-conversion is coupled to convert electrical energy from the first bus to the second bus and visa-versa. A controller (20) that includes an inverter (21) controls the compressor and the bidirectional convertor.    DESCRIPTION OF DRAWING(S) - A schematic diagram of the electrical propulsion system.    Fuel cell (10)    High voltage bus (11)    Compressor (18)    Controller (20)    Inverter (21)    Bidirectional DC-DC convertor (23)    Low voltage bus (24)    Battery (30)