Fuel cell voltage control

An auxiliary load ( 148 ) for a fuel cell stack ( 102 ) is alternatively connected and disconnected from the fuel cell external circuit ( 177, 178 ) by a switch ( 200 ) in response to a switch control ( 201 ), repetitively, during startup and shutdown. The switch may be an insulated gate bipolar transistor ( 208 ) which is turned on and off by hunting between an upper limit voltage ( 207 ) and a lower limit voltage ( 208 ), which may be performed by compare circuits ( 205, 206 ), by the controller ( 202 ), or by commercially available voltage responsive hysteresis switches. Schedules of duty cycle as a function of cell stack voltage for startup ( 212 ) and shutdown ( 213 ) control a pulse width modulator ( 215 ) which operates the switch. Controls ( 229, 231 ) may limit the modulation so that the auxiliary load does not overheat, in response to temperature ( 221 ) of the load or a voltage/power model ( 235 ). The auxiliary load may comprise a heater in a water accumulator ( 247 ), an air intake ( 257 ) or an enthalpy recovery device ( 262 ).