System and method for balancing voltage of individual battery cells within a battery pack

1. A method for actively balancing charge of a battery pack, comprising:

discharging a plurality of battery cells through a first current path during a battery discharge cycle;

charging a first battery cell via a second current path by transferring energy from a primary coil to a secondary coil of a first transformer during said battery discharge cycle; and

discharging a second battery cell via said second current path during said battery discharge cycle by transferring energy from a secondary coil to a primary coil of a second transformer, said second battery cell discharging at least during a portion of time when said first battery cell is charging.

2. The method of claim 1 , wherein said second battery cell is discharged for a longer period of time than said first battery cell.

3. The method of claim 1 , further comprising charging a third battery cell during said battery discharge cycle, said third battery cell charged at a different timing than said first battery cell.

4. The method of claim 3 , wherein charging of said first battery cell and said third battery cell overlaps.

5. The method of claim 3 , wherein charging of said first battery cell and said third battery cell does not overlap.

6. A method for actively balancing charge of a battery pack, comprising:

discharging a plurality of battery cells via a first current path during a second battery discharge cycle;

providing charge to a first battery cell via a first flyback transformer during said second battery discharge cycle, said flyback transformer in a second current path;

providing charge to a second battery cell via a second flyback transformer during said second battery discharge cycle, said second flyback transformer in said second current path, said charge provided to said second battery cell by providing charge to said second flyback transformer at a different timing than charge provided to said first flyback transformer; and

controlling a net current flow in the secondary current path to zero while providing charge to said first and second battery cells.

7. The method of claim 6 , wherein said charge provided to said second flyback transformer is provided after said charge is provided to said first flyback transformer.

8. The method of claim 6 , wherein said charge provided to said first flyback transformer is provided on a periodic basis during said second battery discharge cycle.

9. The method of claim 8 , wherein a period of said periodic basis is related to a characteristic of said first flyback transformer.

10. The method of claim 9 , wherein said characteristic is a coil saturation time.

11. The method of claim 6 , wherein said first flyback transformer and said second flyback transformer are in different groups of transformers that are supplied charge during said second battery discharge cycle.

12. The method of claim 6 , wherein said second battery discharge cycle is after a first battery discharge cycle, and wherein said charge provided to said first flyback transformer is related to an amount of charge remaining in said first battery cell after said first battery discharge cycle.

13. A system for actively balancing battery cells of a battery pack, comprising:

a plurality of battery cells;

a first current path for charging and discharging said plurality of battery cells external said battery pack;

a second current path for charging battery cells during a battery pack discharge cycle; and

a controller, said controller including instructions for supplying a first charge to a first battery cell via a first flyback transformer and draining charge from a second battery cell via switching a secondary side of a second flyback transformer to a low side of the second battery cell, said first and second flyback transformers in said second current path.

14. The system of claim 13 , wherein said controller includes further instructions for supplying said first charge and draining charge at different timings.

15. The system of claim 13 , wherein said first flyback transformer supplies charge to the first battery cell and wherein said second flyback transformer drains charge from the second battery cell.

16. The system of claim 15 , further comprising a third flyback transformer.

17. The system of claim 16 , wherein said controller includes further instructions for drawing charge from a third battery cell via said third flyback transformer.

18. The system of claim 13 , wherein said battery cells are lithium-ion battery cells.

19. The system of claim 13 , wherein said controller includes instructions for supplying charge to a first group of flyback transformers, said first group of flyback transformers including said first flyback transformer, and wherein said controller includes instructions for draining charge from a second group of flyback transformers, said second group of flyback transformers including said second flyback transformer.

20. The system of claim 13 , further comprising a first switch and a second switch, said first switch electrically coupled to a secondary coil of said first flyback transformer and a low side of the first battery cell, and said second switch electrically coupled to a secondary coil of said second flyback transformer and a low side of the second battery cell.