Patent ID: 8679690

Claim:
A fuel cell system comprising: a fuel cell stack configured to generate power by causing hydrogen and oxygen to electrochemically react with each other; a fuel gas supply device configured to supply a fuel gas that contains the hydrogen, to the fuel cell stack; an oxidizing agent gas supply device configured to supply an oxidizing agent gas that contains the oxygen, to the fuel cell stack; a voltage detection section configured to detect a voltage generated by the fuel cell stack; an oxygen concentration transient reduction section configured to reduce transiently an oxygen concentration in a cathode electrode of the fuel cell stack by increasing a load current to be extracted from the fuel cell stack in a response time shorter than a response time taken to supply the oxidizing agent gas from the oxidizing agent gas supply device to the cathode electrode; a voltage variation detection section configured to detect a voltage variation when the oxygen concentration is transiently reduced by the oxygen concentration transient reduction section; and a voltage stabilization maintenance determination section configured to determine whether a present oxygen utilization ratio is appropriate for maintaining the voltage of the fuel cell stack stable, based on an output of the voltage variation detection section.