A detachable and portable fuel cell power unit which may be used with a vehicle. Said power unit with a unitary housing containing: a fuel reservoir for storing fuel; at least one electrochemical fuel cell stack for delivering electrical power; a fueling port in an outer surface of the housing; and an airflow path extending between a first inlet port on an outer surface of the housing and a first outlet port on an outer surface of the housing, via cathode elements in the at least one fuel cell stack. Said unit includes a control circuit for interfacing with a power controller on the vehicle for determining allowable operating conditions.