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

1. A circuit for passively managing power between a fuel cell stack and a battery in a hybrid system, the circuit comprising:

a) a buck-boost converter circuit;

b) a direct charge circuit; and

c) a network interconnecting the buck-boost converter, the direct charge, the network being configured such that:

i) in response to a first 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

ii) in response to a second voltage level in the network being higher than the maximum battery charge voltage, the battery is charged via the buck-boost converter circuit.

2. The circuit, according to claim 1 , further includes:

a) a mode selector; and

b) a current limit circuit; and wherein the network interconnects the buck-boost converter, the direct charge, 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.

3. The circuit, according to claim 2 , 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.

4. The circuit, according to claim 1 , 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.

5. The circuit, according to claim 1 , in which the network includes a bus which interconnects the fuel cell stack with the battery.

6. The circuit, according to claim 5 , 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.

7. The circuit, according to claim 5 , in which first and second voltage levels are measured in the bus.

8. The circuit, according to claim 1 , in which the battery has low internal resistance.

9. The circuit, according to claim 1 , in which the bus has a low voltage limit.

10. The circuit, according to claim 1 , further includes an onboard balancing circuit to monitor individual battery voltages.

11. The circuit according to claim 1 , in which the number of fuel cell stack unit cell active area and fuel cell number are characterized by a specific overall voltage and current characteristic which matches the battery's overall voltage and current characteristic and cell number, thereby permitting passive power management between the battery and fuel cell stack depending on the resulting combined voltage and current characteristic of the fuel cell stack and battery.

12. A device for passively managing power, the device comprising:

a) a fuel cell stack;

b) a battery; and

c) the circuit, according to claim 1 .

13. A method of passively managing power between a fuel cell stack and a battery in a hybrid system, the method comprising:

i) in response to a first voltage level in a network being lower than or equal to a maximum battery charge voltage, charging a battery via a direct charge circuit; and

ii) in response to a second voltage level in the network being higher than the maximum battery charge voltage, charging the battery via a buck-boost converter circuit.

14. The method, according to claim 13 , 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 the 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 the buck-boost converter circuit so that the battery is charged.

15. The method, according to claim 14 , further includes limiting current delivered to the battery by connecting either the buck-boost converter circuit or the direct charge circuit to the battery.

16. The method, according to claim 13 , in which the battery is passively selectively charged.