Battery-ultracapacitor hybrid energy storage system architecture for mild-hybrid power systems

1. A mild-hybrid energy storage system architecture, comprising:

a battery;

an ultracapacitor connected in parallel with the battery;

a passive battery pre-charge circuit connected between a terminal of the battery and a DC bus;

a battery main contactor connected in parallel with the battery pre-charge circuit between the terminal of the battery and the DC bus;

a passive ultracapacitor pre-charge circuit connected between a terminal of the ultracapacitor and the DC bus;

an ultracapacitor main contactor connected in parallel with the ultracapacitor pre-charge circuit between the terminal of the ultracapacitor and the DC bus; and

a control module configured to independently control operation of the battery pre-charge circuit, the battery main contactor, the ultracapacitor pre-charge circuit and the ultracapacitor main contactor.

2. The mild-hybrid energy storage system architecture of claim 1 , wherein the battery includes at least one Lithium-ion cell.

3. The mild-hybrid energy storage system architecture of claim 1 , wherein the passive battery pre-charge circuit includes a resistor connected between the terminal of the battery and an input of a pre-charge switch, an output of the pre-charge switch being connected to the DC bus.

4. The mild-hybrid energy storage system architecture of claim 1 , wherein the passive ultracapacitor pre-charge circuit includes a resistor connected between the terminal of the ultracapacitor and an input of a pre-charge switch, an output of the pre-charge switch being connected to the DC bus.

5. The mild-hybrid energy storage system architecture of claim 1 , wherein the terminal of the battery is a positive terminal and the terminal of the ultracapacitor is a positive terminal.

6. The mild-hybrid energy storage system architecture of claim 1 , further comprising a first voltage sensor configured to provide ultracapacitor voltage measurements to the control module.

7. The mild-hybrid energy storage system architecture of claim 6 , further comprising a second voltage sensor configured to provide DC bus voltage measurements to the control module.

8. The mild-hybrid energy storage system architecture of claim 7 , further comprising a third voltage sensor configured to provide battery voltage measurements to the control module.

9. The mild-hybrid energy storage system architecture of claim 1 , wherein the control module is further configured to:

respond to a voltage of the ultracapacitor being approximately zero by closing a battery pre-charge switch of the passive battery pre-charge circuit to pre-charge the DC bus;

after pre-charging the DC bus, activate an engine starter to start an engine; and

after activating the engine starter, closing an ultracapacitor pre-charge switch of the passive ultracapacitor pre-charge circuit to charge the ultracapacitor.

10. The mild-hybrid energy storage system architecture of claim 9 , wherein the control module is further configured to:

shut down the engine; and

after shutting down the engine, open the ultracapacitor pre-charge switch and the ultracapacitor main contactor to isolate the ultracapacitor.

11. The mild-hybrid energy storage system architecture of claim 9 , wherein the control module is further configured to respond to the voltage being above a predetermined threshold voltage by closing the ultracapacitor pre-charge switch and closing the ultracapacitor main contactor before closing the battery pre-charge switch.

12. The mild-hybrid energy storage system architecture of claim 9 , wherein the control module is further configured to respond to the voltage being above a predetermined threshold voltage by closing the ultracapacitor pre-charge switch, closing the ultracapacitor main contactor and closing the battery pre-charge switch approximately simultaneously.

13. A method for controlling an engine in a mild-hybrid system, comprising:

sensing a voltage of an ultracapacitor;

responding to the voltage being approximately zero by closing a battery pre-charge switch of a passive battery pre-charge circuit connected between a terminal of a battery and a DC bus coupled to an engine starter to pre-charge the DC bus;

after pre-charging the DC bus, activating the engine starter to start the engine; and

after activating the engine starter, closing an ultracapacitor pre-charge switch of a passive ultracapacitor pre-charge circuit connected between a terminal of the ultracapacitor and the DC bus to charge the ultracapacitor.

14. The method of claim 13 , further comprising:

shutting down the engine; and

after shutting down the engine, opening the ultracapacitor pre-charge switch and a main contactor connected in parallel with the passive ultracapacitor pre-charge circuit to isolate the ultracapacitor.

15. The method of claim 13 , further comprising responding to the voltage being above a predetermined threshold voltage by closing the ultracapacitor pre-charge switch and closing a main contactor connected in parallel with the passive ultracapacitor pre-charge circuit before closing the battery pre-charge switch.

16. The method of claim 13 , further comprising responding to the voltage being above a predetermined threshold voltage by closing the ultracapacitor pre-charge switch, closing a main contactor connected in parallel with the passive ultracapacitor pre-charge circuit and closing the battery pre-charge switch approximately simultaneously.