Vehicle propulsion system

1. A vehicle propulsion system comprising:

an alternating current (AC) traction drive;

a first energy storage system electrically coupled to said fraction drive through a direct current (DC) link;

a second energy storage system electrically coupled to said traction drive such that the voltage output from said second energy storage system is decoupled from said DC link using a bi-directional boost converter; and

a uni-directional current device that is poled to conduct current from a low voltage side of a boost converter to a high voltage side of said boost converter,

wherein said first energy storage system comprises a first ultracapacitor that is coupled on said boost converter high voltage side,

wherein said second energy storage system comprises a second ultracapacitor and a high-specific energy battery that are each coupled on said boost converter low voltage side.

2. A vehicle propulsion system in accordance with claim 1 wherein said first ultracapacitor is comprised of multiple cells coupled in a series configuration with cell capacitance that is greater than 500 Farads.

3. A vehicle propulsion system in accordance with claim 1 wherein said second ultracapacitor has a voltage rating that is between approximately two and approximately three times greater than a nominal voltage rating of said high specific energy battery.

4. A vehicle propulsion system in accordance with claim 1 further comprising a second uni-directional current device that is poled to conduct current from said high specific energy battery to said second ultracapacitor coupled on said boost converter low voltage side.

5. A vehicle propulsion system in accordance with claim 1 wherein said first ultracapacitor has a voltage rating that is between approximately two and approximately three times greater than the maximum voltage rating of said second ultracapacitor.

6. A vehicle propulsion system in accordance with claim 1 further comprising a pre-charge circuit configured to at least partially pre-charge said first ultracapacitor.

7. A vehicle propulsion system in accordance with claim 6 further comprising a diode poled between said pre-charge circuit and said first ultracapacitor.

8. A vehicle propulsion system in accordance with claim 6 wherein said pre-charge circuit is configured to partially charge said first ultracapacitor utilizing said high-specific energy battery.

9. A vehicle comprising:

a propulsion system comprising

an alternating current (AC) traction drive;

a first energy storage system electrically coupled to said traction drive through a direct current (DC) link;

a second energy storage system electrically coupled to said traction drive such that the voltage output from the second energy storage system is decoupled from said DC link using a bi-directional boost converter; and

a uni-directional current device that is poled to conduct current from a low voltage side of a boost converter to a high voltage side of said boost converter,

wherein said first energy storage system comprises a first ultracapacitor that is coupled on said boost converter high voltage side,

wherein said second energy storage system comprises a second ultracapacitor and a high-specific energy battery that are each coupled on said boost converter low voltage side.

10. A vehicle in accordance with claim 9 wherein said first ultracapacitor comprises multiple cells that are coupled in a series configuration, each said cell having a capacitance that is greater than 500 Farads.

11. A vehicle in accordance with claim 9 further comprising a second uni-directional current device that is poled to conduct current from said high specific energy battery to said second ultracapacitor coupled on said boost converter low voltage side.

12. A vehicle in accordance with claim 9 wherein said second ultracapacitor has a voltage rating that is between approximately two and approximately three times greater than the nominal voltage rating of said high specific energy battery.

13. A vehicle in accordance with claim 9 wherein said first ultracapacitor has a voltage rating that is between approximately two and approximately three times greater than the maximum voltage rating of said second ultracapacitor.

14. A vehicle in accordance with claim 9 further comprising a pre-charge circuit configured to at least partially pre-charge said first ultracapacitor.

15. A vehicle in accordance with claim 9 wherein said vehicle is a parallel hybrid vehicle.

16. A vehicle in accordance with claim 9 wherein said vehicle is an electric vehicle.