CONTROLLING PEM FUEL CELL VOLTAGE DURING POWER TRANSITIONS AND IDLING

The controller (185) of a fuel cell stack (151) in a vehicle (150) responds to lower demand to cause a diverter (172) to direct all air from the cathodes (19) except as required to generate sufficient power to limit cell voltage to a safe idle voltage threshold, cause storage in an energy storage system (201) sufficient to limit cell voltage to the idle voltage threshold unless SOC is too high, connects a voltage limiting load (220) to the stack sized to consume all power not consumed by auxiliary BOP, or, for longer idles, air-starves the stack and powers BOP from storage. In response to increase in a demand signal, the controller causes flow of all air to the cathodes. In response to an off signal (223) or a start signal (193) the controller causes a shutdown routine or a startup routine in each of which all generated power is stored to maintain fuel cell voltage below a threshold, or is consumed in the voltage limiting load.