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A control strategy results in a relative humidity profile that is substantially the same or constant regardless of the operational power level of the fuel cell stack. The strategy maintains the relative humidity profile within a range that enables high current density operation of the fuel cell stac

A control strategy results in a relative humidity profile that is substantially the same or constant regardless of the operational power level of the fuel cell stack. The strategy maintains the relative humidity profile within a range that enables high current density operation of the fuel cell stack. The profile is achieved by adjusting a coolant flow rate through the fuel cell stack to maintain a temperature change across the coolant flow path from inlet to outlet substantially constant regardless of the operational power level of the fuel cell stack.

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1. A method of operating a fuel cell system including a fuel cell stack and a coolant supply subsystem, the fuel cell stack having cathode and coolant flow paths therethrough, the method comprising: (a) operating the fuel cell stack to produce electricity;(b) monitoring a power level of the fuel cel

1. A method of operating a fuel cell system including a fuel cell stack and a coolant supply subsystem, the fuel cell stack having cathode and coolant flow paths therethrough, the method comprising: (a) operating the fuel cell stack to produce electricity;(b) monitoring a power level of the fuel cell stack;(c) determining a coolant flow through the coolant flow path to substantially achieve a predetermined coolant temperature change through the coolant flow path based on said power level; and(d) adjusting operation of the coolant supply subsystem to substantially achieve said determined coolant flow. 2. The method of claim 1, wherein (c) includes determining said coolant flow with a model that provides said coolant flow. 3. The method of claim 2, wherein (c) includes determining said coolant flow with a model that provides said coolant flow based on a prediction of an outlet relative humidity for a cathode gas flowing through the cathode flow path using a relative humidity of said cathode gas flowing into the cathode flow path, a pressure drop across the cathode flow path, said power level of the fuel cell stack, and said predetermined coolant temperature change. 4. The method of claim 1, wherein (c) includes accessing a look-up table that provides said coolant flow. 5. The method of claim 4, wherein (c) includes accessing a look-up table that provides said coolant flow based on an outlet relative humidity for a cathode gas flowing through the cathode flow path and taking into account a relative humidity of said cathode gas flowing into the cathode flow path, a pressure drop across the cathode flow path, said power level of the fuel cell stack, and said predetermined coolant temperature change. 6. The method of claim 1, wherein (d) includes providing said coolant flow as pulses of flow through the coolant flow path when said power level is below a predetermined threshold. 7. The method of claim 1, wherein (d) includes providing said coolant flow as a continuous stream through the coolant flow path when said power level exceeds a predetermined threshold. 8. The method of claim 1, further comprising maintaining a cathode gas relative humidity in the cathode flow path greater than about 40% and less than 100% during nominal operation.