US Patent 7833670 Fuel cell system and method of shutting down the same

The output of a fuel cell stack is kept consumed in an external load through a power converter having a constant-current operating function for keeping an output current of the fuel cell stack at a constant value, until an output voltage of the fuel cell stack reaches an operable lower limit voltage value of the power converter after stopping the supply of oxidant-containing gas to a cathode while maintaining the supply of hydrogen-containing gas to an anode. When the output voltage of the fuel cell stack falls below the operable lower limit voltage value, the output of the fuel cell stack is connected to a short-circuit resistor. Thereafter, a reformer is stopped when the output voltage of the fuel cell stack reaches approximately zero.