Patent ID: 7736814

Claim:
A fuel-cell system, wherein fuel electrode nitrogen concentration, which indicates concentration of nitrogen in a fuel electrode of a fuel cell, is estimated on the basis of temperature of a fuel-cell stack at the time when operation of the fuel cell is stopped, pressures in the fuel electrode before and after hydrogen pressurization at the time when the operation of the fuel cell is restarted, and a standing time between when the operation of the fuel cell is stopped and when the operation of the fuel cell is restarted, the system further comprising: a first map which illustrates the relation when the fuel electrode nitrogen concentration is in a first low nitrogen concentration state, a second map which illustrates the relation when the fuel electrode nitrogen concentration is in a second high nitrogen concentration state, wherein when the standing time is shorter than a time period in which the pressure in the fuel electrode reaches a peak of negative pressure, the first map is used, but when, on the other hand, the standing time is longer than the time period in which the pressure in the fuel electrode reaches the peak of the negative pressure, the second map is used, and the estimated value of the fuel electrode nitrogen concentration is obtained after determining which map is used.