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A fuel cell system includes a fuel cell stack, a battery, and a control system. A power circuit couples the fuel cell system selectively between the fuel cell stack and the battery. The power circuit includes a battery supply switch responsive to a voltage across the fuel cell stack, and a stack sup

A fuel cell system includes a fuel cell stack, a battery, and a control system. A power circuit couples the fuel cell system selectively between the fuel cell stack and the battery. The power circuit includes a battery supply switch responsive to a voltage across the fuel cell stack, and a stack supply switch responsive to an operating state of the fuel cell system. The battery supply switch uncouples the battery from an on-board power supply when a voltage across the fuel cell stack exceeds a first threshold voltage and couples the battery to the on-board power supply when the fuel cell stack voltage is less than a second threshold voltage. The stack supply switch couples power the fuel cell stack to the on-board power supply when in a running state. A diode-OR circuit couples the source with the highest potential to the on-board power supply.

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1. A fuel cell system, comprising:a fuel cell stack; a battery; and a fuel cell control system, including a power supply switch responsive to a voltage across the fuel cell stack to couple power from the fuel cell stack to the fuel cell control system at a first time while the voltage across the fue

1. A fuel cell system, comprising:a fuel cell stack; a battery; and a fuel cell control system, including a power supply switch responsive to a voltage across the fuel cell stack to couple power from the fuel cell stack to the fuel cell control system at a first time while the voltage across the fuel cell stack is above a fuel cell stack threshold voltage and to couple power from the battery to the fuel cell control system at a second time while the voltage across the fuel cell stack is below the fuel cell stack threshold voltage. 2. The fuel cell system of claim 1 wherein the battery is rechargeable and the fuel cell stack is selectively couplable to provide power to recharge the battery.3. The fuel cell system of claim 1 wherein the power supply switch is further responsive to an operating state of the fuel cell system.4. A fuel cell system, comprising:a fuel cell stack; a battery; and a fuel cell control system, including a power supply switch responsive to a voltage across the fuel cell stack to couple power from the fuel cell stack to the fuel cell control system at a first time when the voltage across the fuel cell stack rises above a first fuel cell stack threshold voltage and to couple power from the battery to the fuel cell control system at a second time when the voltage across the fuel cell stack falls below a second fuel cell stack threshold voltage. 5. The fuel cell system of claim 4, further including a second power supply switch responsive to an operating state of the fuel cell system.6. A fuel cell control system for controlling operation of a fuel cell system having a fuel cell stack, the fuel cell control system comprising:a microcontroller; at least one sensor positioned to measure an operating parameter of the fuel cell system and coupled to provide signals to the microcontroller corresponding to the measured operating parameter; at least one actuator coupled to receive signals from the microcontroller in response to the measured operating parameter; and a power switching circuit configured to selectively couple power from the fuel cell stack to the microcontroller at a first time, and to selectively couple power from a battery to the microcontroller at a second time wherein the power switching circuit comprises a voltage responsive switching circuit having an output coupled to the microcontroller, a stack supply switch responsive to an operating state of the fuel cell system to provide an electrical path between the fuel cell stack and the voltage responsive switching circuit and a battery supply switch responsive to a voltage across the fuel cell stack to provide an electrical path between the battery and the voltage responsive switching circuit. 7. The fuel cell control system of claim 6 wherein the battery supply switch is responsive to the voltage across the fuel cell stack in a number of the operating states of the fuel cell system.8. A fuel cell control system for controlling operation of a fuel cell system having a fuel cell stack, the fuel cell control system comprising:a microcontroller; at least one sensor positioned to measure an operating parameter of the fuel cell system and coupled to provide signals to the microcontroller corresponding to the measured operating parameter; at least one actuator coupled to receive signals from the microcontroller in response to the measured operating parameter; and a power switching circuit configured to selectively couple power from the fuel cell stack to the microcontroller at a first time, and to selectively couple power from a battery to the microcontroller at a second time wherein the power switching circuit comprises a diode-OR circuit, a stack supply switch responsive to an operating state of the fuel cell system to provide an electrical path between the fuel cell stack and the diode-OR circuit and a battery supply switch responsive to a voltage across the fuel cell stack to provide an electrical path between the battery and the diode-OR circuit. 9. A fuel cell control system for controlling operation of a fuel cell system having a fuel cell stack, the fuel cell control system comprising:a microcontroller; at least one sensor positioned to measure an operating parameter of the fuel cell system and coupled to provide signals to the microcontroller corresponding to the measured operating parameter; at least one actuator coupled to receive signals from the microcontroller in response to the measured operating parameter; and a power supply switch configured to selectively switch power from the fuel cell stack to at least one of the microcontroller, the sensor and the actuator when a voltage across the fuel cell stack is above a first fuel cell stack threshold voltage and to selectively switch power from a battery to at least one of the microcontroller, the sensor and the actuator when the voltage across the fuel cell stack is below a second fuel cell stack threshold voltage, the second fuel cell stack threshold voltage different than the first fuel cell stack threshold voltage. 10. The fuel cell control system of claim 9 wherein the second fuel cell stack threshold voltage is less than the first fuel cell stack threshold voltage.11. The fuel cell control system of claim 9 wherein the power supply switch comprises:a diode-OR circuit having a first input, a second input, and an output, the first input couplable to the fuel cell stack, the output coupled to at least one of the microcontroller, the sensor and the actuator; a battery supply transistor coupled between the battery and the second input of the diode-OR circuit. 12. A fuel cell control system for controlling operation of a fuel cell system having a fuel cell stack, the fuel cell control system comprising:a microcontroller; at least one sensor positioned to measure an operating parameter of the fuel cell system and coupled to provide signals to the microcontroller corresponding to the measured operating parameter; at least one actuator coupled to receive signals from the microcontroller in response to the measured operating parameter; and a power supply switch configured to selectively switch power from the fuel cell stack to the microcontroller in a first operating state when a voltage across the fuel cell stack is above a first fuel cell stack threshold voltage and to selectively switch power from a battery to the microcontroller in the first operating state when the voltage across the fuel cell stack is below a second fuel cell stack voltage. 13. The fuel cell control system of claim 12 wherein the power supply switch comprises:a diode-OR circuit having a first input, a second input, and an output, the output coupled to at least one of the microcontroller, the sensor and the actuator; a battery supply transistor coupled between the battery and the first input of the diode-OR circuit; and a fuel cell stack transistor coupled between the fuel cell stack and the second input of the diode-OR circuit. 14. A controller-readable media carrying instructions for causing a controller to control operation of a fuel cell system having a fuel cell stack and a battery, by:coupling power from the fuel cell stack to the controller when a voltage across the fuel cell stack moves above a first threshold voltage; and coupling power from the battery to the controller when the voltage across the fuel cell stack drops below a second threshold voltage wherein the second threshold voltage is lower than the first threshold voltage. 15. A method of operating a fuel cell system, the method comprising:providing power from a fuel cell stack to a fuel cell control system at a first time, wherein the first time corresponds to a time when a voltage across the fuel cell stack is above a first threshold voltage; and providing power from a battery to the fuel cell control system at a second time, wherein the second time corresponds to a time when the voltage across the fuel cell stack is below a second threshold voltage, different from the first threshold voltage. 16. The method of claim 15 wherein the first time corresponds to a time when the fuel cell system is in a first operating state and the second time corresponds to a time when the fuel cell system is in a second operating state.17. The method of claim 15 wherein the first time corresponds to a time when the fuel cell system is in a first operating state and a voltage across the fuel cell stack is above a first threshold voltage, and the second time corresponds to a time when the fuel cell system is in the first operating state and the voltage across the fuel cell stack is below a second threshold voltage.18. A method of operating a power switching circuit in a fuel cell system to couple power to an on-board power supply selectively from a fuel cell stack and a battery, the method comprising:changing a state of a stack supply switch to couple the fuel cell stack to the on-board power supply when entering a running operational state of the fuel cell system from a starting operational state of the fuel cell system; setting a state of a battery supply switch to uncouple the battery from the on-board power supply if a voltage across the fuel cell stack is above an upper fuel cell stack threshold voltage; setting the state of the battery supply switch to couple power from the battery to the on-board power supply if the voltage across the fuel cell stack falls below a lower fuel cell stack threshold voltage; and changing the state of the battery supply switch to couple the battery to the on-board power supply when entering a stopping operational state of the fuel cell system from the running operational state. 19. The method of claim 18 wherein changing a state of the stack supply switch includes coupling a signal from the controller to a switching input terminal of the stack supply switch.20. The method of claim 18 wherein the battery supply switch is a transistor and setting a state of the battery supply switch includes coupling a signal from a comparison circuit to a switching input terminal of the transistor.21. The method of claim 18, further comprising:coupling the battery supply switch to a first input of a diode-OR circuit; and coupling the stack supply switch to a second input of the diode-OR circuit. 22. The method of claim 18, further comprising:determining that the voltage across the fuel cell stack exceeds a third threshold voltage before entering a running state.