Fuel cell system

1. A fuel cell humidity determining method comprising:

determining an absolute voltage difference between an average cell voltage for a plurality of individual cells of a fuel cell stack and a minimum cell voltage value for a voltage output by the individual cells;

comparing the absolute voltage difference with a first preset value;

selectively changing a flow rate of an oxidizer gas to change humidity in the fuel cell based on a comparison between the absolute voltage difference and the first preset value, the changing the flow rate of the oxidizer gas based on the comparison between the absolute voltage difference and the first preset value being prohibited when the absolute voltage difference is less than the first preset value so that the flow rate of the oxidizer gas is maintained, and the changing the flow rate of the oxidizer gas based on the comparison between the absolute voltage difference and the first preset value occurring when the absolute voltage difference greater than or equal the first preset value;

determining a humidity state in the fuel cell stack based on a change in the absolute voltage difference due to the change in the humidity in the fuel cell after the flow rate of the oxidizer gas has been changed, such that a change in the humidity state indicates whether the fuel cell is becoming excessively humid or insufficiently humid depending on the change in the flow rate of the oxidizer gas, wherein in the excessive humidity state, the flow rate of the oxidizer gas is increased and liquid in the fuel cell is dispersed; and

at least one of the following:

comparing the absolute voltage difference with a second preset value, which is greater than or equal to the first preset value, after the flow rate of the oxidizer gas has been increased, and determining the humidity state in the fuel cell stack being in an insufficiently humid state when the absolute voltage difference is greater than or equal to the second preset value; and

comparing the absolute voltage difference with the second preset value after the flow rate of the oxidizer gas has been increased, and determining the humidity state in the fuel cell stack being either in an excessive humid state or in an insufficient oxygen state when the absolute voltage difference is less than the second preset value.

2. The fuel cell humidity determining method according to claim 1 , wherein

the changing the flow rate of the oxidizer gas based on the comparison between the absolute voltage difference and the first preset value entails reducing the flow rate of the oxidizer gas; and further comprising

comparing the absolute voltage difference with a third preset value, which is less than or equal to the first preset value, after the flow rate of the oxidizer gas has been reduced, and determining the humidity state in the fuel cell stack being in an excessively humid state when the absolute voltage difference is greater than or equal to the third preset value.

3. The fuel cell humidity determining method according to claim 2 , further comprising

determining insufficient oxygen in the fuel cell stack when the absolute voltage difference is greater than or equal to a fourth preset value, which is greater than or equal to the third preset value.

4. The fuel cell humidity determining method according to claim 1 , wherein

the changing the flow rate of the oxidizer gas based on the comparison between the absolute voltage difference and the first preset value entails reducing the flow rate of the oxidizer gas; and further comprising

comparing the absolute voltage difference with a third preset value, which is less than or equal to the first preset value, after the flow rate of the oxidizer gas has been reduced, and determining the humidity state in the fuel cell stack being in an insufficient humid state when the absolute voltage difference is less than the third preset value.

5. The fuel cell humidity determining method according to claim 1 , further comprising

measuring a temperature of the fuel cell stack before the changing of the flow rate of the oxidizer gas;

increasing the flow rate of the oxidizer gas when the temperature is less than a preset temperature; and

reducing the flow rate of the oxidizer gas when the temperature is greater than or equal to the preset temperature.

6. The fuel cell humidity determining method according to claim 1 , wherein

the changing the flow rate of the oxidizer gas based on the comparison between the absolute voltage difference and the first preset value entails changing the flow rate of the oxidizer gas only when the absolute voltage difference that is obtained as a result of the comparing of the absolute voltage difference with the first preset value is less than the first preset value for at least two times that the comparing is performed.

7. The fuel cell humidity determining method according to claim 1 , wherein

the determining of the humidity state of the fuel cell stack based on the change in the absolute voltage difference entails measuring the absolute voltage difference at at least two instances after the flow rate of the oxidizer gas has been changed, and further determining the humidity state of the fuel cell stack when the absolute voltage differences are identical at least twice.

8. The fuel cell humidity determining method according to claim 1 , wherein

the change of the flow rate of the oxidizer gas decreases as a load current drawn from the fuel cell stack increases.

9. The fuel cell humidity determining method according to claim 1 , wherein

the determining of the absolute voltage difference is accomplished by calculating the average cell voltage for the individual cells of the fuel cell stack, and calculating the minimum cell voltage value for the voltage output by the individual cells.

10. The fuel cell humidity determining method according to claim 9 , wherein

the determining of the average cell voltage is determined by dividing a voltage of the fuel cell stack by a total number of the cells forming the fuel cell stack.

11. The fuel cell humidity determining method according to claim 9 , wherein

the determining of the minimum cell voltage value is obtained using a formula

F=C −( D− 1)× E

where F is the minimum value of cell voltage calculated from a voltage C of a cell block in which the fuel cell stack is divided into an arbitrary number of the individual cells with an average cell voltage E, and a number of cells D that form the cell block.

12. The fuel cell humidity determining method according to claim 11 , wherein

the cell block is composed of one of the individual cells.

13. The fuel cell humidity determining method according to claim 11 , wherein

the cell block is composed of a plurality of the individual cells.

14. A fuel cell system comprising:

an oxidizer gas flow rate adjuster configured to adjust a flow rate of an oxidizer gas of a fuel cell stack; and

a controller operatively coupled to the oxidizer gas flow rate adjuster to control the oxidizer gas flow rate adjuster, the controller including a voltage comparator configured to compare an absolute voltage difference between an average cell voltage for a plurality of individual cells of a fuel cell stack and a minimum cell voltage value for a voltage output by the individual cells with a first preset value,

the controller being configured to selectively instruct the oxidizer gas flow rate adjuster to change the flow rate of the oxidizer gas to change humidity in the fuel cell based on a comparison between the absolute voltage difference and the first preset value,

the controller being further configured to determine a humidity state in the fuel cell stack based on a change in the absolute voltage difference due to the change in the humidity in the fuel cell after the flow rate of the oxidizer gas has been changed, such that a change in the humidity state indicates whether the fuel cell is becoming excessively humid or insufficiently humid depending on the change in the flow rate of the oxidizer gas, wherein in the excessive humidity state, the flow rate of the oxidizer gas is increased and liquid in the fuel cell is dispersed; and

the controller being further configured to perform at least one of the following operations:

compare the absolute voltage difference with a second preset value, which is greater than or equal to the first preset value, after the flow rate of the oxidizer gas has been increased, and determine the humidity state in the fuel cell stack being in an insufficiently humid state when the absolute voltage difference is greater than or equal to the second preset value; and

compare the absolute voltage difference with the second preset value after the flow rate of the oxidizer gas has been increased, and determine the humidity state in the fuel cell stack being either in an excessive humid state or in an insufficient oxygen state when the absolute voltage difference is less than the second preset value.

15. The fuel cell system according to claim 14 , wherein

the controller further includes

an average cell voltage calculator configured to calculate the average cell voltage for the plurality of individual cells of the fuel cell stack;

a minimum cell voltage calculator configured to calculate the minimum cell voltage value for the voltage output by the individual cell; and

a voltage difference calculator configured to calculate the absolute voltage difference between the average cell voltage and the minimum cell voltage value.