Fuel cell system multiple stage voltage control method and apparatus

• US 6,573,682 B1
• Filed: 12/14/2001
• Issued: 06/03/2003
• Est. Priority Date: 12/14/2001
• Status: Expired due to Term

• Current Assignee: Daimler AG (Mercedes-Benz Group AG)
• Original Assignee: Ballard Power Systems Corporation (Superior Plus Corporation)
• Inventors: Pearson, Martin T.
• Primary Examiner(s): Riley, Shawn
• Assistant Examiner(s): LUK, LAWRENCE W
• Application Number:
  US10/017,461
• Publication Number:
  US 20030111977A1
• Time in Patent Office: 536 Days
• Field of Search: 320/101, 320/103, 320/110, 429/12, 429/17, 429/19, 429/21, 429/23, 429/13, 429/14, 429/15
• US Class Current: 320/101
• CPC Class Codes: H01M 16/006 of fuel cells with rechargeable batteries
• CPC Class Codes: H01M 8/04007 related to heat exchange
• CPC Class Codes: H01M 8/04119 with simultaneous supply or evacuation of electrolyte; Humidifying or dehumidifying
• CPC Class Codes: H01M 8/04373 of auxiliary devices, e.g. reformers, compressors, burners
• CPC Class Codes: H01M 8/04567 of auxiliary devices, e.g. batteries, capacitors
• CPC Class Codes: H01M 8/04589 of fuel cell stacks
• CPC Class Codes: H01M 8/04597 of auxiliary devices, e.g. batteries, capacitors
• CPC Class Codes: H01M 8/04753 of fuel cell reactants
• CPC Class Codes: H01M 8/04798 of fuel cell reactants
• CPC Class Codes: H01M 8/04888 of auxiliary devices, e.g. batteries, capacitors
• CPC Class Codes: H01M 8/0491 of fuel cell stacks
• CPC Class Codes: H01M 8/04917 of auxiliary devices, e.g. batteries, capacitors
• CPC Class Codes: H01M 8/04992 characterised by the implementation of mathematical or computational algorithms, e.g. feedback control loops, fuzzy logic, neural networks or artificial intelligence
• CPC Class Codes: H02J 1/082 Plural DC voltage, e.g. DC supply voltage with at least two different DC voltage levels
• CPC Class Codes: H02J 2300/30 The power source being a fuel cell
• CPC Class Codes: H02J 7/34 Parallel operation in networks using both storage and other dc sources, e.g. providing buffering H02J7/14 takes precedence
• CPC Class Codes: Y02E 60/10 Energy storage using batteries
• CPC Class Codes: Y02E 60/50 Fuel cells

Abstract

A fuel cell system determines each of a battery charging current error, a battery voltage error, and a stack current error. The fuel cell system regulates current through a series pass element in response to a greater of the determined errors, operating in three modes: battery voltage limiting mode, stack current limiting mode and battery charging current limiting mode. Additionally, there can be a fourth “saturation” mode where the stack voltage V_S drops below the battery voltage V_B. A voltage difference across the series pass element is compared to a desired condition such as a saturation level, and a partial pressure of a reactant flow to the fuel cell stack adjusted based on the determined amount of deviation limiting the energy dissipated by the series pass element. Individual fuel cell systems can be combined in series and/or parallel to produce a combined fuel cell system having a desired output voltage and current.

Claims

1. A fuel cell system, comprising:

a fuel cell stack having a number of fuel cells;

a battery having a number of battery cells electrically couplable in parallel across the fuel cell stack;

a series pass element electrically coupled between at least a portion of the fuel cell stack and a portion of the battery;

a regulating circuit for regulating current through the series pass element in response to a greater of a battery charging current error, a battery voltage error and a stack current error;

a reactant delivery system for delivering reactant to the fuel cells, the reactant delivery system including at least a first control element adjustable to control a partial pressure in a flow of a reactant to at least some of the fuel cells; and

a control circuit coupled to receive signals corresponding to a voltage on an input side and an output side of the series pass element and configured to determine a deviation of a voltage difference across the series pass element from a desired operational condition based on the received signals, the control circuit further coupled to control the at least first control element based on the determined deviation.

2. The fuel cell system of claim 1 wherein the regulating circuit comprises:

a battery charging current error integrator having a first input coupled to receive a battery charging current signal and a second input coupled to receive a battery charging current limit signal;

a battery voltage error integrator having a first input coupled to receive a battery voltage signal and a second input coupled to receive a battery voltage limit signal; and

a stack current error integrator having a first input coupled to receive a stack current signal and a second input coupled to receive a stack current limit signal.

3. The fuel cell system of claim 1 wherein the regulating circuit comprises:

a charge pump; and

a level shifter coupled between the charge pump and the series pass element.

4. The fuel cell system of claim 1 wherein the regulating circuit comprises:

an OR circuit.

5. The fuel cell system of claim 1 wherein the regulating circuit comprises:

a battery charging current error integrator having a first input coupled to receive a battery charging current signal and a second input coupled to receive a battery charging current limit signal;

a battery voltage error integrator having a first input coupled to receive a battery voltage signal and a second input coupled to receive a battery voltage limit signal;

a stack current error integrator having a first input coupled to receive a stack current signal and a second input coupled to receive a stack current limit signal;

an OR circuit having an input side and an output side, the input side coupled to the battery charging current error integrator, the battery voltage error integrator, and the stack current error integrator;

a level shifter electrically coupled between the OR circuit and the series pass element; and

a charge pump coupled to supply a charge to the series pass element via the level shifter.

6. The fuel cell system of claim 1 wherein the series pass element comprises a field effect transistor.

7. The fuel cell system of claim 1 wherein at least a portion of the battery is electrically coupled in parallel with at least a portion of the fuel cell stack.

8. The fuel cell system of claim 1 wherein the control circuit comprises a first comparator coupled to receive a first and a second voltage, and a second comparator coupled to receive the voltage difference from the first comparator and value corresponding to the desired operational condition.

9. The fuel cell system of claim 1 wherein the control circuit comprises a first comparator coupled to receive a first and a second voltage, and a second comparator coupled to receive the voltage difference from the first comparator and value corresponding to the desired operational condition, wherein the desired operational condition is between approximately 75 percent and 95 percent of a saturation level for the series pass element.

10. A fuel cell system, comprising:

a number of fuel cells forming a fuel cell stack;

a number of battery cells forming a battery;

a series pass element;

a blocking diode electrically coupled between the fuel cell stack and the series pass element;

a regulating circuit for regulating current through the series pass element in proportion to at least a greater of a difference between a battery charging current and a battery charging current limit, a difference between a battery voltage and a battery voltage limit, and a difference between a stack current and a stack current limit;

a reactant delivery system for delivering reactant to the fuel cells, the reactant delivery system including at least a first flow regulator adjustable to control a partial pressure in a flow of a reactant to at least some of the fuel cells; and

a control circuit coupled to receive signals corresponding to a voltage across the series pass element and to provide a control signal to at least the first control element mathematically related to a voltage difference across the series pass element.

11. The fuel cell system of claim 10 wherein the regulating circuit comprises:

a battery current integrator having a first input, a second input and an output, the first input coupled to receive a battery current value and the second input coupled to receive a battery current limit value;

a battery voltage integrator having a first input, a second input and an output, the first input coupled to receive a battery voltage value and the second input coupled to receive a battery voltage limit value;

a stack current integrator having a first input, a second input and an output, the first input coupled to receive a stack current value and the second input coupled to receive a stack current limit value; and

an OR circuit coupled to the output of each of the battery current integrator, the battery voltage integrator and the stack current integrator to select the greater of a value on each of the respective outputs.

12. The fuel cell system of claim 10 wherein the regulating circuit comprises:

a level shifter electrically coupled between the OR circuit and the series pass element; and

a charge pump coupled to provide current to the series pass element through the level shifter.

13. The fuel cell system of claim 10 wherein the regulating circuit comprises:

a battery current integrator having a first input, a second input and an output, the first input coupled to receive a battery current value and the second input coupled to receive a battery current limit value;

a battery voltage integrator having a first input, a second input and an output, the first input coupled to receive a battery voltage value and the second input coupled to receive a battery voltage limit value;

a stack current integrator having a first input, a second input and an output, the first input coupled to receive a stack current value and the second input coupled to receive a stack current limit value;

an OR circuit coupled to the output of each of the battery current integrator, the battery voltage integrator and the stack current integrator;

a level shifter coupled to the OR circuit to receive the greater of the value on each of the outputs; and

a charge pump coupled to the series pass element through the level shifter.

14. The fuel cell system of claim 10 wherein the regulating circuit comprises a microprocessor programmed to regulate the current through the series pass element by:

integrating a difference between a battery current and a battery current limit;

integrating a difference between a battery voltage and a battery voltage limit;

integrating a difference between a stack current and a stack current limit;

selecting a greater of the integrated differences; and

applying a control signal to the series pass element proportional to the greater of the integrated differences.

15. The fuel cell system of claim 10, further comprising:

a battery charging current sensor;

a battery voltage sensor; and

a stack current sensor.

16. The fuel cell system of claim 10, further comprising:

a battery charging current sensor;

a stack current sensor;

battery voltage sensor;

a battery temperature sensor; and

a temperature compensation circuit coupled to the battery temperature sensor to produce a battery voltage limit that is temperature compensated.

17. A circuit for a fuel cell system having a fuel cell stack and a battery, the circuit comprising:

a series pass element electrically coupleable between at least a portion of the fuel cell stack and a portion of the battery;

a regulating circuit for regulating current through the series pass element in response to a greater of a battery charging current error, a battery voltage error and a stack current error; and

a control circuit coupled to receive signals corresponding to a voltage on an input side and an output side of the series pass element and configured to determine a deviation of a voltage difference across the series pass element from a desired operational condition based on the received signals and to produce a control signal based on the determined deviation.

18. The circuit of claim 17 wherein the regulating circuit comprises:

a battery charging current error integrator having a first input coupled to receive a battery charging current signal proportional to a battery charging current, a second input coupled to receive a battery charging current limit signal proportional to a battery charging current limit, and an output to supply a battery current error signal proportional to a difference between the battery charging current and the battery charging current limit;

a battery voltage error integrator having a first input coupled to receive a battery voltage signal proportional to a battery voltage, a second input coupled to receive a battery voltage limit signal proportional to a battery voltage limit, and an output to supply a battery voltage error signal proportional to a difference between the battery voltage and the battery voltage limit;

a stack current error integrator having a first input coupled to receive a stack current signal proportional to a stack current, a second input coupled to receive a stack current limit signal proportional to a stack current limit, and an output to supply a stack current error signal proportional to a difference between the stack current and the stack current limit; and

an OR circuit coupled to the output of each of the error integrators to select a greater one of the error signals from the error integrators.

19. The circuit of claim 17 wherein the series pass element comprises a transistor having a first terminal, a second terminal and a control terminal, the first and the second terminals coupleable between the fuel cell stack and the battery, and wherein the regulating circuit comprises:

a level shifter coupled to receive the greater of the battery charging current error, the battery voltage error and the stack current error; and

a charge pump coupled to the control terminal of the transistor by way of the level shifter.

20. A circuit for a fuel cell system, comprising:

a series pass element;

a blocking diode electrically coupled in series with the series pass element;

a regulating circuit coupled to the series pass element to regulate a current through the series pass element in proportion to at least a greater of a difference between a battery charging current and a battery charging current limit, a difference between a battery voltage and a battery voltage limit, and a difference between a stack current and a stack current limit; and

a control circuit coupled to receive signals corresponding to a voltage across the battery and to provide a control signal mathematically related to a difference between the voltage across the battery and a defined desired voltage across the battery.

21. The circuit of claim 20, further comprising:

a battery charging current sensor;

a battery voltage sensor; and

a stack current sensor.

22. The circuit of claim 20 wherein the regulating circuit comprises:

a battery current integrator having a first input, a second input and an output, the first input coupled to receive a battery current value and the second input coupled to receive a battery current limit value;

a battery voltage integrator having a first input, a second input and an output, the first input coupled to receive a battery voltage value and the second input coupled to receive a battery voltage limit value;

a stack current integrator having a first input, a second input and an output, the first input coupled to receive a stack current value and the second input coupled to receive a stack current limit value; and

an OR circuit coupled to the output of each of the battery current integrator, the battery voltage integrator and the stack current integrator to select the greater of a value on each of the outputs.

23. The circuit of claim 20 wherein the regulating circuit comprises:

a battery current integrator having a first input, a second input and an output, the first input coupled to receive a battery current value and the second input coupled to receive a battery current limit value;

a battery voltage integrator having a first input, a second input and an output, the first input coupled to receive a battery voltage value and the second input coupled to receive a battery voltage limit value;

a stack current integrator having a first input, a second input and an output, the first input coupled to receive a stack current value and the second input coupled to receive a stack current limit value;

an OR circuit coupled to the output of each of the battery current integrator, the battery voltage integrator and the stack current integrator to select the greater of a value on each of the outputs;

a level shifter coupled to the OR circuit to receive the greater of the value on each of the outputs; and

a charge pump coupled to the series pass element through the level shifter.

24. The circuit of claim 20 wherein the series pass element comprises a field effect transistor.

25. A circuit for a fuel cell system, comprising:

a battery charging current sensor;

a battery charging current error integrator having a first input coupled to the battery charging current sensor to receive a battery charging current signal proportional to a battery charging current, a second input coupled to receive a battery charging current limit signal proportional to a battery charging current limit, and an output to supply a battery current error signal proportional to a difference between the battery charging current and the battery charging current limit;

a battery voltage sensor;

a battery voltage error integrator having a first input coupled to the battery voltage sensor to receive a battery voltage signal proportional to a battery voltage, a second input coupled to receive a battery voltage limit signal proportional to a battery voltage limit, and an output to supply a battery voltage error signal proportional to a difference between the battery voltage and the battery voltage limit;

a stack current sensor;

a stack current error integrator having a first input coupled to the stack current sensor to receive a stack current signal proportional to a stack current, a second input coupled to receive a stack current limit signal proportional to a stack current limit, and an output to supply a stack current error signal proportional to a difference between the stack current and the stack current limit;

an OR circuit coupled to the output of each of the battery current error integrator, the battery voltage error integrator and the stack current error integrator;

a series pass element having a pair of terminals for selectively providing a current path and a control terminal coupled to the OR circuit for regulating current through the current path in proportion to a greater of the battery current error signal, the battery voltage error signal and the stack current error signal; and

a control circuit coupled to receive signals corresponding to a voltage on an input side and an output side of the series pass element and configured to determine a deviation of a voltage difference across the series pass element from a desired operational condition based on the received signals and to produce a control signal based on the determined deviation.

26. The circuit of claim 25 wherein the regulating circuit comprises a number of discrete integrators.

27. The circuit of claim 25 wherein the regulating circuit comprises a microprocessor.

28. The circuit of claim 25, further comprising:

a temperature compensation circuit coupled to the battery temperature sensor to produce a battery voltage limit that is compensated for temperature.

29. A circuit for a fuel cell system, comprising:

means for determining a greater of a difference between a battery charging current and a battery charging current limit, a difference between a battery voltage and a battery voltage limit, and a difference between a stack current and a stack current limit;

series pass regulating means for regulating a flow of stack current through a blocking diode in proportion to the determined greater difference;

means for determining a difference between a voltage difference across the series pass regulating means and a desired a desired operational condition of the series pass regulating means; and

means for controlling a partial pressure of at least one reactant flow in proportion to the determined difference between the voltage difference across the series pass regulating means and the desired operational condition of the series pass regulating means.

30. The circuit of claim 29, comprising:

integrating means for determining the difference between the battery charging current and the battery charging current limit;

integrating means for determining the difference between the battery voltage and the battery voltage limit; and

integrating means for determining the difference between the stack current and the stack current limit.

31. The circuit of claim 29 wherein the means for determining a difference between a voltage difference across the series pass regulating means and a desired a desired operational condition of the series pass regulating means includes first comparator means for comparing a first voltage on an input side of the series pass regulating means and a second voltage on an output side of the series pass regulating means, and second comparator means for comparing the voltage difference across the series pass regulating means with a value corresponding to a desired percentage of a saturation value for the series pass regulating means.

32. A method of operating a fuel cell system, comprising:

supplying current at a number of output terminals from at least one of a fuel cell stack and a battery electrically coupled in parallel with the fuel cell stack;

in a first stage, regulating a current through a series pass element in proportion to at least a greater of a difference between a battery charging current and a battery charging current limit, a difference between a battery voltage and a battery voltage limit, and a difference between the stack current and the stack current limit; and

in a second stage, adjusting a partial pressure of a reactant flow to at least a portion of the fuel cell stack to maintain a series pass element at a desired saturation level.

33. The method of claim 32 wherein the first stage and the second stage occur during a same time.

34. The method of claim 32, further comprising:

determining a first voltage on an input side of the series pass element;

determining a second voltage on an output side of the series pass element;

determining a difference in voltage across the series pass element from the first and the second voltages; and

determining an amount of deviation of the difference in voltage across the series pass element from a value corresponding to the desired saturation level, and wherein adjusting a partial pressure of a reactant flow to at least a portion of the fuel cell stack to maintain the series pass element at the desired saturation level includes adjusting the partial pressure of the reactant flow based on the determined amount of deviation.

35. A method of operating in a fuel cell system, the method comprising:

determining a battery charging current error;

determining a battery voltage error;

determining a stack current error;

regulating current through a series pass element in response to a greater of the battery charging current error, the battery voltage error and the stack current error;

determining a voltage difference across the series pass element;

determining an amount of deviation of the determined voltage difference from a desired operational condition of the series pass element; and

for at least one reactant flow to at least a portion of the fuel cell stack, adjusting a partial pressure of the reactant flow based on the determined amount of deviation.

36. The method of claim 35 wherein,

determining a battery charging current error includes integrating a difference between a battery charging current and a battery charging current limit over time;

determining a battery voltage error includes integrating a difference between a battery voltage and a battery voltage limit over time; and

determining a stack current error includes integrating a difference between a stack current and a stack current limit over time.

37. The method of claim 35, further comprising:

selecting the greater of the battery charging current error, the battery voltage error and the stack current error;

level shifting the selected one of the battery charging current error, the battery voltage error and the stack current error; and

applying the level shifted selected one of the battery charging current error, the battery voltage error and the stack current error to a control terminal of the series pass element.

38. The method of claim 35, further comprising:

determining a temperature proximate a battery;

determining a battery voltage limit based at least in part on the determined temperature; and

integrating a difference between a battery voltage and the determined battery voltage limit over time to determine the battery voltage error.

39. The method of claim 35, further comprising:

selectively coupling charge from a charge pump to a control terminal of the series pass element in response to the greater of the battery charging current error, the battery voltage error and the stack current error.

40. The method of claim 35, further comprising:

selectively coupling charge from a charge pump to a control terminal of the series pass element in response to the battery charging current error at a first time, the battery voltage error at a second time and the stack current error at a third time.

41. The method of claim 35, further comprising:

determining a first voltage on an input side of the series pass element; and

determining a second voltage on an output side of the series pass element.

42. The method of claim 35, further comprising:

determining a first voltage on an input side of the series pass element; and

determining a second voltage on an output side of the series pass element, and wherein determining a voltage difference across the series pass element includes determining the difference between the first and the second voltages.

43. The method of claim 35 wherein determining an amount of deviation of the determined voltage difference from a desired operational condition of the series pass element includes determining a difference between the determined voltage difference and a value corresponding to a percentage of a saturation level of the series pass element, where the percentage is between approximately 75 percent and approximately 95 percent.

44. The method of claim 35 wherein adjusting a partial pressure of the reactant flow based on the determined amount of deviation includes adjusting a partial pressure of a flow of fuel to at least a portion of the fuel cell stack and adjusting a partial pressure of a flow of oxidant to at least the same portion of the fuel cell stack.

45. The method of claim 35, further comprising:

holding a pressure of the at least one reactant flow approximately constant while adjusting the partial pressure of the at least one reactant flow.

46. A method of operating in a fuel cell system, the method comprising:

determining a difference between a battery charging current and a battery charging current limit;

determining a difference between a battery voltage and a battery voltage limit;

determining a difference between a-stack current and a stack current limit;

regulating a current through a series pass element in proportion to at least a greater of the difference between the battery charging current and the battery charging current limit, the difference between the battery voltage and the battery voltage limit, and the difference between the stack current and the stack current limit;

determining a voltage difference across the series pass element;

determining an amount of deviation of the determined voltage difference from a desired operational condition of the series pass element; and

for at least one reactant flow to at least a portion of the fuel cell stack, adjusting a partial pressure of the reactant flow based on the determined amount of deviation.

47. The method of claim 46, further comprising:

selecting the greater of the battery charging current error, the battery voltage error and the stack current error;

level shifting the selected one of the battery charging current error, the battery voltage error and the stack current error; and

applying the level shifted selected one of the battery charging current error, the battery voltage error and the stack current error to a control terminal of the series pass element.

48. The method of claim 46, further comprising:

determining a temperature proximate a battery;

determining the battery voltage limit based at least in part on the determined temperature.

49. The method of claim 46, further comprising:

selectively coupling charge from a charge pump to a control terminal of the series pass element in proportion to the greater of the battery charging current error, the battery voltage error and the stack current error.

50. The method of claim 46, further comprising:

selectively coupling charge from a charge pump to a control terminal of the series pass element in proportion to the battery charging current error at a first time, the battery voltage error at a second time and the stack current error at a third time.

51. A fuel cell system, comprising:

a voltage bus;

a first fuel cell stack electrically couplable across the voltage bus;

a first battery electrically couplable across the voltage bus;

a first series pass element electrically coupled in series on the voltage bus between at least a portion of the first fuel cell stack and a portion of the first battery;

a first regulating circuit for regulating current through the first series pass element in response to a greater of a battery charging current error, a battery voltage error and a stack current error;

a first reactant delivery system for delivering reactant to the first fuel cell stack, the reactant delivery system including at least a first control element adjustable to control a partial pressure in a flow of a reactant to at least some of the fuel cells of the first fuel cell stack;

a first control circuit coupled to receive signals corresponding to a voltage on an input side and a voltage on an output side of the first series pass element and configured to determine a deviation of a voltage difference across the first series pass element from a desired operational condition based on the received signals, the first control circuit further coupled to control the at least first control element based on the determined deviation;

a second fuel cell stack electrically couplable across the voltage bus;

a second battery electrically couplable across the voltage bus;

a second series pass element electrically coupled in series on the voltage bus between at least a portion of the second fuel cell stack and a portion of the second battery;

a second regulating circuit for regulating current through the second series pass element in response to a greater of a battery charging current error, a battery voltage error and a stack current error;

a second reactant delivery system for delivering reactant to the second fuel cell stack, the reactant delivery system including at least a second control element adjustable to control a partial pressure in a flow of a reactant to at least some of the fuel cells of the second fuel cell stack; and

a second control circuit coupled to receive signals corresponding to a voltage on an input side and a voltage on an output side of the second series pass element and configured to determine a deviation of a voltage difference across the second series pass element from a desired operational condition based on the received signals, the second control circuit further coupled to control the at least second control element based on the determined deviation.

52. The fuel cell system of claim 51 wherein the second fuel cell stack, the second battery and the second series pass element are electrical coupled in series with the first fuel cell stack, the first battery and the first series pass element.

53. The fuel cell system of claim 51 wherein the second fuel cell stack, the second battery and the second series pass element are electrical coupled in parallel with the first fuel cell stack, the first battery and the first series pass element.

54. The fuel cell system of claim 51, further comprising:

a third fuel cell stack electrically couplable across the voltage bus;

a third battery electrically couplable across the voltage bus;

a third series pass element electrically coupled in series on the voltage bus between at least a portion of the third fuel cell stack and a portion of the third battery;

a third regulating circuit for regulating current through the third series pass element in response to a greater of a battery charging current error, a battery voltage error and a stack current error;

a third reactant delivery system for delivering reactant to the third fuel cell stack, the reactant delivery system including at least a third control element adjustable to control a partial pressure in a flow of a reactant to at least some of the fuel cells of the third fuel cell stack; and

a third control circuit coupled to receive signals corresponding to a voltage on an input side and a voltage on an output side of the third series pass element and configured to determine a deviation of a voltage difference across the third series pass element from a desired operational condition based on the received signals, the third control circuit further coupled to control the at least third control element based on the determined deviation.

55. The fuel cell system of claim 51, further comprising:

a third fuel cell stack electrically couplable across the voltage bus;

a third battery electrically couplable across the voltage bus;

a third series pass element electrically coupled in series on the voltage bus between at least a portion of the third fuel cell stack and a portion of the third battery;

a third regulating circuit for regulating current through the third series pass element in response to a greater of a battery charging current error, a battery voltage error and a stack current error, wherein the second fuel cell stack, the second battery and the second series pass element are electrical coupled in series with the first fuel cell stack, the first battery and the first series pass element and wherein the third fuel cell stack, the third battery and the third series pass element are electrical coupled in series with the first and the second fuel cell stack, the first and the second battery and the first and the second series pass element;

a third reactant delivery system for delivering reactant to the third fuel cell stack, the reactant delivery system including at least a third control element adjustable to control a partial pressure in a flow of a reactant to at least some of the fuel cells of the third fuel cell stack; and

a third control circuit coupled to receive signals corresponding to a voltage on an input side and a voltage on an output side of the third series pass element and configured to determine a deviation of a voltage difference across the third series pass element from a desired operational condition based on the received signals, the third control circuit further coupled to control the at least third control element based on the determined deviation.

56. The fuel cell system of claim 51, further comprising:

a third fuel cell stack electrically couplable across the voltage bus;

a third battery electrically couplable across the voltage bus;

a third series pass element electrically coupled in series on the voltage bus between at least a portion of the third fuel cell stack and a portion of the third battery;

a third regulating circuit for regulating current through the third series pass element in response to a greater of a battery charging current error, a battery voltage error and a stack current error, wherein the second fuel cell stack, the second battery and the second series pass element are electrical coupled in series with the first fuel cell stack, the first battery and the first series pass element and wherein the third fuel cell stack, the third battery and the third series pass element are electrical coupled in parallel with the first and the second fuel cell stack, the first and the second battery and the first and the second series pass element;

a third reactant delivery system for delivering reactant to the third fuel cell stack, the reactant delivery system including at least a third control element adjustable to control a partial pressure in a flow of a reactant to at least some of the fuel cells of the third fuel cell stack; and

a third control circuit coupled to receive signals corresponding to a voltage on an input side and a voltage on an output side of the third series pass element and configured to determine a deviation of a voltage difference across the third series pass element from a desired operational condition based on the received signals, the third control circuit further coupled to control the at least third control element based on the determined deviation.

57. A fuel cell system combination, comprising:

a voltage bus;

a first fuel cell system having a first fuel cell stack and a first battery electrically coupled in parallel across the voltage bus; and

a second fuel cell system having a second fuel cell stack and a second battery electrically coupled in parallel across the voltage bus.

58. The fuel cell system of claim 57 wherein the first fuel cell stack has a first fuel cell polarization curve and the first battery has a first battery polarization curve, the first battery polarization approximately matching the first fuel cell polarization curve, and wherein the second fuel cell stack has a second fuel cell polarization curve and the second battery has a second battery polarization curve, the second battery polarization approximately matching the second fuel cell polarization curve.

59. The fuel cell system of claim 57, further comprising:

means for approximately matching a polarization curve of the first fuel cell stack and a polarization curve of the first battery; and

means for approximately matching a polarization curve of the second fuel cell stack and a polarization curve of the second battery.