Fuel cell system relative humidity control

Operating strategy for a fuel cell system controls the hydration level of the membranes in the fuel cells and achieves a desired operational performance. The membrane hydration level is managed by controlling the relative humidity of the cathode gas flowing through the cathode flow path. Targeted relative humidity set points for the cathode gas entering and exiting the cathode flow path are established based on the water vapor in the cathode gas. Temperature set points for the cathode gas to achieve the targeted relative humidity are established. The coolant supply subsystem is operated to cause the required cathode gas temperatures to be achieved.