Please substitute the new Abstract submitted herewith for the original Abstract: A method increases the safety and/or the reliability of the operation of a fuel cell stack. The method determines that the fuel cell stack is in a space with a reduced air exchange rate. The method determines consumption information with respect to an oxygen consumption of the fuel cell stack within an interval of time. The method determines, on the basis of the air exchange rate, inflow information with respect to an amount of oxygen which was supplied to the space within the interval of time. The method determines an estimated value for an oxygen content of air in the space on the basis of the consumption information and on the basis of the inflow information.

When the fuel cell stack is operated in a state in which the air stoichiometric ratio is smaller than the predetermined value, the controller calculates the amount of retained water that has been retained in the cathode flow path of the fuel battery cell per fixed time in such a way that the calculated amount includes an extra amount therein, integrates the amount of retained water per fixed time that has been calculated in such a way that the calculated amount includes the extra amount therein, and executes air blow in the cathode flow path of the fuel battery cell when the integrated value of the amount of retained water becomes equal to or larger than the threshold.

A system and method of controlling water imbalance in an electrochemical cell is provided. The method includes determining a present water imbalance in the electrochemical cell by summing a water.sub.in and a water.sub.created less a water.sub.out. Water.sub.in represents an amount of water introduced into the electrochemical cell by an oxidant feed gas; water.sub.created represents an amount of water created by the electrochemical cell from the electrochemical reaction; and water.sub.out represents an amount of water discharged from the electrochemical cell by an oxidant exhaust gas. The method includes tracking a cumulative water imbalance during operation of the electrochemical cell by repeatedly determining the present water imbalance and continuing to sum the results during operation. And, the method also includes adjusting a flow rate of the oxidant feed gas entering the electrochemical cell based on the cumulative water imbalance.

A method for determining a humidity condition in a fuel cell system includes steps of: detecting an amount of water in a container that receives water discharged from a fuel cell stack, and determining a humidity condition in the fuel cell stack, based on the amount of water detected. As a result, the actual humidity condition in the fuel cell stack may be accurately determined even when humidification performance is degraded over operating time of the fuel cell system.

A fuel cell system includes a mass flow rate measuring unit, an oxygen consumption mass flow rate acquiring unit, an exhaust-side air temperature acquiring unit, and an exhaust-side air humidity estimating unit. The mass flow rate measuring unit measures a first mass flow rate of intake-side air and a second mass flow rate of exhaust-side air. The oxygen consumption mass flow rate acquiring unit acquires a mass flow rate of oxygen consumption. The exhaust-side air humidity estimating unit estimate humidity in the exhaust-side air, on the basis of a difference between a flow rate of intake gas in the fuel cell system and a flow rate of exhaust gas from the fuel cell system, and the mass flow rate of the oxygen consumption.

A system and method of controlling water imbalance in an electrochemical cell is provided. The method includes determining a present water imbalance in the electrochemical cell by summing a water.sub.in and a water.sub.created less a water.sub.out. Water.sub.in represents an amount of water introduced into the electrochemical cell by an oxidant feed gas; water.sub.created represents an amount of water created by the electrochemical cell from the electrochemical reaction; and water.sub.out represents an amount of water discharged from the electrochemical cell by an oxidant exhaust gas. The method includes tracking a cumulative water imbalance during operation of the electrochemical cell by repeatedly determining the present water imbalance and continuing to sum the results during operation. And, the method also includes adjusting a flow rate of the oxidant feed gas entering the electrochemical cell based on the cumulative water imbalance.

A method of controlling the operation of a fuel cell system is provided. The method includes diagnosing a water shortage state in a fuel cell stack based on degradation of cooling performance and deterioration of the fuel cell stack and determining a diagnosis level of the fuel cell system based on the diagnosed water shortage state of the fuel cell stack. In addition, a regenerative operation is performed by selecting a regenerative operation mode which corresponds to the determined diagnosis level.

A computer-implemented method estimates a state of health of a humidifier of a fuel cell system for a vehicle. The method includes in response to determining that a predefined condition for vehicle onboard measurements is fulfilled: performing step response measurements, comprising measuring parameters related to dynamic response of water mass transport and heat transport of the humidifier when step increases of the fuel cell system are performed, wherein a step increase is indicative of an instantaneous power increase of the fuel cell system, and fitting the measured parameters in a humidifier aging model and therefrom estimating the state of health of the humidifier.

An electrolysis cell system includes a cathode portion configured to output a cathode exhaust stream, an anode portion configured to output an anode exhaust stream, a sensor configured to detect a concentration in an exhaust stream and to output sensor data, wherein the sensor is either a hydrogen concentration sensor configured to detect a hydrogen concentration in the cathode exhaust stream or a water concentration sensor configured to detect a water concentration of the anode exhaust stream, and a controller. The controller is configured to receive the sensor data from the sensor and, based on the sensor data, control at least one of (a) an air pressure adjustment device to adjust a pressure of air entering the anode portion or (b) a steam pressure adjustment device to adjust a pressure of steam entering the cathode portion.

A fuel cell vehicle on which a fuel cell system including a fuel cell is mounted includes a discharge mechanism configured to discharge moisture, generated by the fuel cell, from the fuel cell system to an outside of the vehicle, a camera configured to capture an image outside the vehicle, and an electronic control unit configured to determine whether predetermined control based on an information obtained from the image and executed or stopped in response to a driving status or drive mode of the vehicle in an on-state of an ignition switch is being executed, and, when it is determined that the predetermined control is being executed, execute a low discharge process in which a discharge flow rate of water vapor that is discharged from the discharge mechanism to the outside of the vehicle is reduced as compared to when it is determined that the predetermined control is stopped.