NOVELTY - The fuel cell system comprises a fuel cell, a measurement unit that measures an electrical characteristic of the fuel cell, and a control unit that: receives a signal to discontinue an operation of the fuel cell; performs a scavenging process, in response to the signal, for causing water present in an interior of the fuel cell to be discharged out of the fuel cell; lowers a voltage of the fuel cell after the scavenging process; and estimates a water content using the electrical characteristic during a period in which the voltage drops.    USE - The fuel cell system is useful as a power source for driving, and in a fuel cell automobile and an electric automobile.    ADVANTAGE - The fuel cell system exhibits degraded startability when the fuel cells are restarted below freezing, and reduced fuel consumption while driving.    DETAILED DESCRIPTION - The fuel cell system comprises a fuel cell, a measurement unit that measures an electrical characteristic of the fuel cell, and a control unit that: receives a signal to discontinue an operation of the fuel cell; performs a scavenging process, in response to the signal, for causing water present in an interior of the fuel cell to be discharged out of the fuel cell; lowers a voltage of the fuel cell after the scavenging process; and estimates a water content using the electrical characteristic during a period in which the voltage drops, where the water content is an amount of liquid water present in the interior of the fuel cell. The control unit further performs an additional scavenging process when the water content exceeds a tolerance, and does not perform the additional scavenging process when the water content is equal to or smaller than the tolerance. The measurement unit measures, as the electrical characteristic, a current that is generated when the voltage of the fuel cell is decreased. The control unit executes a first process. In the first process, the control unit: calculates an amount of electricity from an integrated value of the current, calculates an amount of substance of reaction gas, present in the interior of the fuel cell using the amount of electricity corrected by the control unit, on the basis of the amount of electricity; calculates a volume of gas present in the interior of the fuel cell from the amount of substance of the reaction gas; calculates a volume of liquid water present in the interior of the fuel cell from the volume of the gas; and estimates a first water content as the water content from the volume of the liquid water. The reaction gas is present in the interior of the fuel cell before the voltage drops. The control unit acquires a first information relating to an operating state of the fuel cell, and corrects a composition ratio of air present in the interior of the fuel cell in accordance with the first information. In the first process, the control unit calculates the volume of gas present in the interior of the fuel cell from the amount of substance of the reaction gas using the corrected composition ratio. The first information includes an information of the current generated from the fuel cell and an information of a gas inflow amount to the fuel cell. The control unit corrects an amount of electricity generated as a result of reduction of a catalyst contained in the fuel cell, in accordance with a difference between the first resistance and a second water content estimated according to a second process. The second process is a process of estimating the second water content, before the scavenging process, from a resistance of a constituent member of the fuel cell that includes an electrolyte. The control unit corrects a volume of a gas flow channel in accordance with a pressure loss in the gas flow channel. In the first process, the control unit calculates the volume of gas present in the interior of the fuel cell from the amount of substance of the reaction gas using the corrected volume of the gas flow channel. The gas flow channel is provided in the interior of the fuel cell. The control unit acquires a second information relating to an operating state of the fuel cell and estimates the water content before the scavenging process on the basis of the second information. When performing the scavenging process, the control unit modifies the content of scavenging in accordance with the water content that is estimated before the scavenging process and with the resistance of a constituent member of the fuel cell that contains the electrolyte. The second information includes an information of a temperature of the fuel cell and an information of a gas inflow amount to the fuel cell. During discontinuation of the fuel cell, the control unit makes a second voltage drop rate twice or more a first voltage drop rate. The first voltage drop rate is a rate of voltage drop that occurs when in the interior of the fuel cell the fuel gas permeates an electrolyte membrane. The second voltage drop rate is a rate of voltage drop at a time when the voltage of the fuel cell is lowered. The resistance of the constituent member is an impedance of the fuel cell. When the operation of the fuel cell is discontinued, if the control unit has not performed the scavenging process, the control unit does not estimate the water content using a voltage drop. An INDEPENDENT CLAIM is included for a method for controlling a fuel cell system.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic view of a fuel cell system.    Fuel gas supply/discharge system (200)    Hydrogen tank (210)    Flow rate adjustment unit (220)    Circulation compressor (240)    Oxidizing gas supply flow channel. (331)