Multi-cell battery pack charge balancing circuit

• US 6,114,835 A
• Filed: 07/26/1999
• Issued: 09/05/2000
• Est. Priority Date: 07/26/1999
• Status: Expired due to Term

• Current Assignee: Unitrode Corp. (Texas Instruments, Inc.)
• Original Assignee: Unitrode Corp. (Texas Instruments, Inc.)
• Inventors: Price, Burt L.
• Primary Examiner(s): Wong, Peter S.
• Assistant Examiner(s): TIBBITS, PIA FLORENCE
• Application Number:
  US09/360,746
• Time in Patent Office: 407 Days
• Field of Search: 320/118, 320/119
• US Class Current: 320/118
• CPC Class Codes: H02J 7/0018 using separate charge circuits

Abstract

A charge balancing circuit incorporates a voltage threshold which determines when to initiate a charge balance mode in order to equalize the level of charge in at least two cells of a multi-cell battery pack. Charge balancing is initiated when the voltage level of a first cell reaches this second threshold. Charge balancing then continues by modifying the charges of the first balance cell and a second reference lesser charged cell until the voltage levels of the first and second cells are equal. A subsequent charge cycle will result in the cell with the greatest charge being balanced with another of lesser charge. In this manner, all of the cells of a multi-cell battery pack are charge balanced over the course of plural charge cycles.

Claims

1. A multi-cell battery pack charge balancing circuit comprising:

a cell voltage comparison subcircuit for connection to a plurality of rechargeable cells, said cell voltage comparison subcircuit having a first voltage threshold and a second voltage threshold and producing cell voltage output signals indicative of a voltage associated with each of said cells exceeding said first voltage threshold and said second voltage threshold;

a charge balance trigger connected to said cell voltage comparison subcircuit and operative to receive said cell voltage output signals and to generate a balance signal in response thereto, wherein said balance signal is indicative of the voltage level of at least one of said plurality of cells exceeding said first voltage threshold and at least another of said plurality of cells not exceeding said second voltage threshold;

cell balance selection logic connected to said cell voltage comparison subcircuit and to said charge balance trigger, and operative to identify a balance cell and a reference cell from among said plurality of rechargeable cells in response to said balance signal, wherein

a voltage level of said balance cell exceeds said first voltage threshold, and

a voltage level of said reference cell does not exceed said second voltage threshold; and

a charge switching subcircuit connected to each of said cells and adapted to electrically connect said balance and said reference cells such that the voltage level of each becomes equal.

2. The charge balancing circuit of claim 1 wherein said cell voltage comparison subcircuit produces said cell voltage output signals in response to comparing the differential obtained by coupling individual ones of said plurality of rechargeable cells relative to a ground.

3. The charge balancing circuit of claim 1 wherein said charge switching subcircuit is further connected to said cell balance selection logic and operative to switch each of said cells to a charging source and to a discharge source, wherein said charge switching subcircuit selectively couples said balance and said reference cells such that their voltage levels are made equal.

4. The charge balancing circuit of claim 3 wherein said charge switching subcircuit couples said balance cell to said discharge source.

5. The charge balancing circuit of claim 4 wherein said charge switching subcircuit uncouples said balance cell from said charging source.

6. The charge balance circuit of claim 1 wherein said cell balance selection logic is adapted to select the cell having the greatest voltage as the balance cell.

7. The charge balance circuit of claim 1 wherein said cell balance selection logic is adapted to select the cell to first attain said first voltage threshold as the balance cell.

8. The charge balance circuit of claim 1 wherein said cell balance selection logic is adapted to select the cell with the least voltage as the reference cell.

9. The charge balance circuit of claim 1 wherein said cell balance selection logic is adapted to select a predetermined cell as the reference cell.

10. The charge balance circuit of claim 1 wherein said charge balance trigger continues to assert said balance signal to said cell balance selection logic until said balance cell and said reference cell have equal voltages.

11. The charge balance circuit of claim 1 wherein said charge balance trigger is asserted:

after a period when said voltage level of all cells is lower than said second threshold, and

said voltage level of at least one cell is above said first threshold.

12. The charge balance circuit of claim 1 wherein said cell voltage comparison subcircuit has only a single comparator connected to said cells to produce said cell voltage output signals.

13. The charge balance circuit of claim 1 wherein said second voltage threshold is about 4V.

14. The charge balance circuit of claim 13 wherein said first voltage threshold is about 50 mV above said second voltage threshold.

15. A method of balancing charge levels in a battery pack having a plurality of individual cells comprising the steps of:

sampling the voltage levels of each of said individual cells;

comparing the sampled voltage to a first voltage threshold for each cell;

comparing the sampled voltage to a second voltage threshold for each cell;

enabling a balance state if the voltage level of at least one cell is above said first voltage threshold and the voltage level of at least one cell is below said second voltage threshold;

selecting, if said balance state is enabled, a balancing pair of cells comprising a first cell having a voltage level above said first voltage level and a second cell having a voltage level below the voltage level of said first cell;

modifying the voltage level of at least one cell of said balancing pair until the voltage levels of the balancing pair are substantially equal.

16. The method of balancing charge levels according to claim 15 wherein said step of modifying further comprises the step of decoupling said first cell from a charging source.

17. The method of balancing charge levels according to claim 16 wherein said step of modifying further comprises the step of coupling said first cell to a discharge source.

18. The method of balancing charge levels according to claim 15 wherein said step of selecting of said first cell comprises selecting the cell with the greatest charge.

19. The method of balancing charge levels according to claim 15 wherein said step of selecting of said first cell comprises selecting the first cell to reach said first voltage threshold.

20. The method of balancing charge levels according to claim 15 wherein said step of selecting of said second cell comprises selecting a cell below said second voltage level.

21. The method of balancing charge levels according to claim 15 wherein said step of selecting of said second cell comprises selecting a cell with the lowest charge.

22. The method of balancing charge levels according to claim 15 wherein said step of selecting of said second cell comprises selecting a predetermined cell.

23. The method of balancing charge levels according to claim 15 wherein entering said balance state does not occur if all cells have not fallen below said second threshold since the previous balance state.

24. The method of balancing charge levels according to claim 15 wherein said step of sampling includes switching each cell to be sampled to a single comparator for each of said first voltage threshold and said second voltage threshold.

25. The method of balancing charge levels according to claim 15 wherein said step of sampling further comprises comparing the voltage levels of individual ones of said plurality of cells relative to a ground.