Method and apparatus for multiple mode control of voltage from a fuel cell system

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. Thus, the fuel cell system operates 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 as the load pulls even more current. 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.

1 . A fuel cell system, comprising: 
 a fuel cell stack;    a battery;    a series pass element electrically coupled between at least a portion of the fuel cell stack and a portion of the battery; and    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.    
   
   
       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 first diode having an anode and a cathode, the anode coupled to receive the battery charging current error;    a second diode having an anode and a cathode, the anode coupled to receive the battery voltage error; and    a third diode having an anode and a cathode, the anode coupled to receive the stack current error, where the cathode of each of the first diode, the second diode and the third diode are coupled to one another to form an analog OR circuit.    
   
   
       6 . 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;    a first diode having an anode and a cathode, the anode coupled to the battery charging current error integrator;    a second diode having an anode and a cathode, the anode coupled to the battery voltage error integrator; and    a third diode having an anode and a cathode, the anode coupled to the stack current error integrator, where the cathode of each of the first diode, the second diode and the third diode are coupled to one another to form an analog OR circuit;    a level shifter electrically coupled between the analog 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.    
   
   
       7 . The fuel cell system of  claim 1  wherein the series pass element comprises a field effect transistor.  
   
   
       8 . 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.  
   
   
       9 . 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; and    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.    
   
   
       10 . The fuel cell system of  claim 9  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.    
   
   
       11 . The fuel cell system of  claim 9  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.    
   
   
       12 . The fuel cell system of  claim 9  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;    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.    
   
   
       13 . The fuel cell system of  claim 9  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.    
   
   
       14 . The fuel cell system of  claim 9 , further comprising: 
 a battery charging current sensor;    a battery voltage sensor; and    a stack current sensor.    
   
   
       15 . The fuel cell system of  claim 9 , 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.    
   
   
       16 . 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 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; and    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.    
   
   
       17 . The fuel cell system of  claim 16  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.  
   
   
       18 . The fuel cell system of  claim 16  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.  
   
   
       19 . The fuel cell system of  claim 16 , 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; and    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.    
   
   
       20 . The fuel cell system of  claim 16 , 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; and    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.    
   
   
       21 . The fuel cell system of  claim 16 , 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; and    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.