NOVELTY - The device for regulating humidity in fuel cell, comprises a processor unit (20), detection units (22), and a circuit breaker unit (24), where the processor unit is connected with the circuit breaker unit and has a first signal input connected with a signal output of the detection unit and a first signal output connectable with a control input of a humidification control unit (18). The detection unit is aligned to connect with the fuel cell and to detect the output voltage of the fuel cell. The processor unit is configured to interrupt a connection between the fuel cell and electrical loads.    USE - The device is useful in a fuel cell system of an aircraft for regulating humidity in a fuel cell (all claimed).    ADVANTAGE - The device is capable of optimally and easily regulating the humidity in the fuel cell with high accuracy and less effort.    DETAILED DESCRIPTION - The device for regulating humidity in a fuel cell, comprises a processor unit (20), detection units (22), and a circuit breaker unit (24), where the processor unit is connected with the circuit breaker unit and has a first signal input connected with a signal output of the detection unit and a first signal output connectable with a control input of a humidification control unit (18). The detection unit is aligned to connect with the fuel cell and to detect the output voltage of the fuel cell. The processor unit is configured to interrupt a connection between the fuel cell and electrical loads for a time period T by controlling the circuit breaker unit, to determine a time constant of the fuel cell from a step response of the output voltage of the fuel cell resulting from it and to control the humidification control unit in dependent of a deviation of the determined time constant from a predetermined time constant. The detection units are configured for individual detection of the output voltage of the several fuel cells of a fuel cell stack (14). The detection unit is designed to connect with the interconnected fuel cells. The processor unit is configured to compare the determined time constant with a predetermined capacity and to induce the humidification control unit during dropping below a predetermined time constant by more than a specified tolerance to reduce the humidification of the fuel cell. The processor unit is configured to induce the humidification control unit during reaching the predetermined time constant to increase the humidification of the fuel cell. The device includes buffer storage for bridging the supply of electrical consumer during determining the step response of the output voltage, where the time period T is 150-500 ms. The processor unit is configured to determine a double layer capacitance and/or a membrane resistance of the fuel cell from the step response, to compare with a specified double layer capacitance and/or a membrane resistance and to control the humidification control unit in dependent of the deviation. INDEPENDENT CLAIMS are included for:    (1) a fuel cell system;    (2) a method for regulating humidity in a fuel cell; and    (3) an aircraft.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of a device for regulating humidity in a fuel cell.    Fuel cell stack (14)    Humidification control unit (18)    Processor unit (20)    Detection units (22)    Circuit breaker unit. (24)