FUEL CELL SYSTEM AND METHOD OF CONTROLLING FUEL CELL SYSTEM

A fuel cell system includes a fuel cell in which cells are stacked, a voltage sensor that detects a voltage in unit of one or more of the cells, a control unit that determines an operating point of the fuel cell and causes the fuel cell to operate. The control unit causes the fuel cell to operate at a low efficiency operating point having a lower efficiency than an efficiency of a reference operating point in a warm-up operation. In the warm-up operation, the control unit calculates a total number of the cells in which the voltage detected by the voltage sensor is equal to or less than a predetermined first reference voltage and calculates an exhaust hydrogen concentration based on the total number or the cells.