Patent ID: 8129062

Claim:
A method of operating a fuel cell system, the method comprising the steps: (a) generating electricity by a fuel cell using fuel gas and oxidizing gas; (b) allowing cooling water to flow in the fuel cell and thereby cooling the fuel cell in step (a); (c) transferring moisture from exhaust gas to supply gas using a first humidifier, the exhaust gas being discharged from the fuel cell, the supply gas being supplied to the fuel cell; (d) transferring moisture from the cooling water to the supply gas humidified with step (c) using a second humidifier; (e) at least one of the steps of detecting humidity of the supply gas humidified with step (d), wherein at least one step comprises detecting temperature of the cooling water, detecting the flow rate of the supply gas and detecting the amount of electricity generated by the fuel cell; and (f) controlling the humidity of the supply gas humidified with step (d) based on a value detected by step (e).