Patent ID: 7371476

Claim:
A fuel cell system comprising: a fuel cell stack supplied with fuel gas and oxidant gas to generate an electric power which is supplied through a diode to a load installed on a fuel cell powered vehicle; a secondary battery by which an electric power is charged and discharged; an electric power converter connected to the fuel cell stack through the diode to convert an electric power, charged in the secondary battery, and supply a converted electric power to the load; and a controller that controls the electric power converter under two control modes during a start-up of the fuel cell stack, the two control modes including: a first control mode to set a target voltage level of an electric power to be supplied to the load from the secondary battery equal to or greater than an open voltage level of the fuel cell stack; and a second control mode to set a target electric power level of an electric power to be supplied to the load from the secondary battery equal to an electric power level of an electric power being supplied to the load from the secondary battery, to perform a reduction of the target electric power level at a given rate, and to interrupt the reduction of the target electric power level when one of the following conditions is satisfied: (1) an output electric current level of the fuel cell stack increases beyond a first threshold; and (2) an output voltage level of the fuel cell stack drops below a second threshold, the first and second thresholds being predetermined based on a voltage-current characteristic during the start-up of the fuel cell stack to a completion of the start-up.