Passive power management and battery charging for hybrid fuel cell/battery system

Abstract

Disclosed herein is a circuit for passively managing power between a fuel cell stack and a battery in a hybrid system. The circuit includes a buck-boost converter circuit, a direct charge circuit; and a network which interconnects them. The network is configured so that in response to a voltage level in the network being lower than or equal to a maximum battery charge voltage, the battery is charged via the direct charge circuit; and in response to another voltage level in the network which is higher than the maximum battery charge voltage, the battery is charged via the buck-boost converter circuit. Also disclosed is a device incorporating the circuit and a method of passively managing power.

What is claimed is: 
     
         1 . A device for passively managing the power between a fuel cell stack and a battery in a hybrid system, the device comprising:
 a) a fuel cell stack having a first voltage and current characteristic;   b) a battery having a second voltage and current characteristic;   c) a network having a bus, the network electrically interconnecting the fuel cell stack, the battery and the bus, the fuel cell stack and the battery being matched to permit passive power sharing between the fuel cell stack and the battery depending on the amount of current being drawn from the hybrid system, so as to share power based on the resulting voltage in the bus.   
     
     
         2 . The device, according to  claim 1 , in which the network electrically interconnects the fuel cell stack, the battery, and the bus in parallel configuration. 
     
     
         3 . The device, according to  claim 1 , in which the network includes a buck-boost converter circuit. 
     
     
         4 . The device, according to  claim 3 , in which the network includes a direct charge circuit. 
     
     
         5 . The device, according to  claim 4 , in which the network includes a mode selector, a current limit circuit, which network interconnects the buck-boost converter, the direct charge circuit, the mode selector, the current limit circuit and the network is configured such that:
 i) in response to the first voltage level in the network being lower than or equal to the maximum battery charge voltage, the mode selector connects the direct charge circuit to the battery so that the battery is charged; and   ii) in response to the second voltage level in the network being higher than the maximum battery charge voltage, the mode selector connects the buck-boost converter circuit to the battery so that the battery is charged.   
     
     
         6 . The device, according to  claim 5 , in which the network is configured such that when the first voltage level is higher than or equal to the maximum voltage charge of the battery, the mode selector passively connects the fuel cell stack and a buck-boost converter circuit to the battery so as to charge the battery. 
     
     
         7 . The device, according to  claim 5 , in which a voltage comparator controls a switch located in the mode selector to passively switch charging of the battery between the buck-boost converter circuit and the direct charge circuit. 
     
     
         8 . The device, according to  claim 5 , in which the current limit circuit is connected to the buck-boost converter circuit and the direct charge circuit to limit current delivered to the battery. 
     
     
         9 . The device, according to  claim 1 , in which the network includes a first diode and a second diode, the first diode being connected to the fuel cell stack, the second diode being connected between the battery and the bus. 
     
     
         10 . The device, according to  claim 1 , further includes an onboard balancing circuit to monitor individual battery voltages. 
     
     
         11 . A method for passively managing the power between a fuel cell stack and a battery in a hybrid system, the method comprising:
 in a network having a bus, matching a fuel cell stack, having a first voltage and current characteristic, with a battery, having a second voltage and current characteristic, so as to permit passive power sharing between the fuel cell stack and the battery depending on the amount of current being drawn from the hybrid system, so as to share power based on the resulting voltage in the bus.   
     
     
         12 . The method, according to  claim 11 , comprising: charging the battery charged via the direct charge circuit when the bus voltage level is lower than the maximum recommended battery charge voltage thereby coupling the bus directly to the battery. 
     
     
         13 . The method, according to  claim 11 , comprising: charging the battery via the buck-boost converter charging circuit when the bus voltage level is higher than the maximum recommended battery charge voltage. 
     
     
         14 . The method, according to  claim 11 , in which the fuel cell stack has plurality of unit cells each being sized so as to provide a desired current and voltage characteristic of the cell. 
     
     
         15 . The method, according to  claim 14 , comprises selecting the number of fuel cells to provide the overall voltage characteristic of the fuel stack. 
     
     
         16 . The method, according to  claim 16 , further comprises matching the number of fuel cells with the voltage and current characteristic of the battery so that as long as the fuel cell stack voltage is higher than that of the battery, power will be supplied from the fuel cell stack. 
     
     
         17 . The method, according to  claim 11 , further includes: i) in response to the first voltage level in the network being lower than or equal to the maximum battery charge voltage, selectively connecting the battery to a direct charge circuit so that the battery is charged; and ii) in response to the second voltage level in the network being higher than the maximum battery charge voltage, selectively connecting the battery to a buck-boost converter circuit so that the battery is charged. 
     
     
         18 . The method, according to  claim 17 , further includes limiting current delivered to the battery by connecting either the buck-boost converter circuit or the direct charge circuit to the battery.