Method for Opportunistically Balancing Charge Between Battery Cells

• US 20140049222A1
• Filed: 03/07/2012
• Published: 02/20/2014
• Est. Priority Date: 03/07/2011
• Status: Active Grant

• Current Assignee: A123 Systems Incorporated
• Original Assignee: A123 Systems Incorporated
• Inventors: Moorhead, Brian C., Rutkowski, Brian D.
• Granted Patent: US 9,450,426 B2
• Time in Patent Office: Days
• US Class Current: 320/134
• CPC Class Codes: H01M 10/441 for several batteries or cells simultaneously or sequentially
• CPC Class Codes: H02J 7/00 Circuit arrangements for charging or depolarising batteries or for supplying loads from batteries
• CPC Class Codes: H02J 7/0013 acting upon several batteries simultaneously or sequentially H02J7/1423 takes precedence
• CPC Class Codes: H02J 7/0014 Circuits for equalisation of charge between batteries
• CPC Class Codes: H02J 7/0016 using shunting, discharge or bypass circuits
• CPC Class Codes: Y02E 60/10 Energy storage using batteries

Abstract

Systems and methods for balancing battery cells of a battery pack are disclosed. In one example, a charge imbalance is determined while battery cells operate in a high charge resolution voltage range. The charge imbalance determined during operation in the high charge resolution voltage range may be removed when the battery cells are operated in a low charge resolution voltage range. The system and method may be particularly useful for balancing battery cells that operate in the low charge resolution voltage range for a large portion of their operating time.

Claims

1. A method for balancing charge between battery cells, comprising:

operating a first battery cell with a first voltage and a second battery cell with a second voltage, the first and second battery cells both operating in a first charge region; and

charge balancing between the first battery cell and the second battery cell in a second charge region different from the first charge region, the charge balancing responsive to a voltage difference between the first voltage and the second voltage as measured in the first charge region.

2. The method of claim 1, where the first charge region is a lower charge region than the second charge region, and where charge balancing is iteratively performed in response to a lowest capacity battery cell of a battery cell stack.

3. The method of claim 1, where the charge balancing includes adding or draining charge from the first battery cell or the second battery cell.

4. The method of claim 1, where the second charge region is related to a battery cell voltage of a battery cell where a measurement error in the battery cell voltage results in a change in state of charge estimate of the first battery cell greater than a threshold amount.

5. The method of claim 1, where the first charge region has a higher voltage versus capacity slope than the second charge region.

6. The method of claim 1, where an amount of charge added to or drained from the first or second battery cell is based on an amount of charge inferred from the voltage difference.

7. A method for balancing charge between battery cells, comprising:

operating a first battery cell and a second battery cell in a first region of battery charge, the first and second battery cells operating with a charge imbalance indicated by a voltage difference between voltage of the first batter cell and voltage of the second battery cell;

charging both the first and second battery cells to transition both the first and second battery cells to a second region of battery charge; and

operating the first and second battery cells in the second region to reduce the charge imbalance, including adding or subtracting an amount of charge to the first or second battery cell in the second region of battery charge, the amount of charge based on the voltage difference in the first region of battery charge.

8. The method of claim 7, where subtracting charge of the first or second battery cell balances charge between the first and second battery cells, and where the first region of battery charge is a lower battery charge than the second region of battery charge.

9. The method of claim 7, where the first region of battery charge is a higher charge than the second region of battery charge.

10. The method of claim 7, where a slope of battery voltage versus battery cell charge of the first region is higher than the slope of battery voltage versus battery cell charge of the second region.

11. The method of claim 7, where charge is added to the first or second battery cell via a transformer.

12. The method of claim 7, where charge is subtracted from the first or second battery cell via a resistor.

13. The method of claim 7, further comprising basing a timing of adding or subtracting charge from the first or second battery cell on an amount of charge stored in the first or second battery cell estimated from an amount of current the first or second battery cell sinks or sources and an open circuit voltage of the first or second battery cell in the second region of battery charge.

14. The method of claim 7, further comprising integrating an amount of charge added or subtracted from the first battery cell after estimating the amount of charge stored in the first battery cell.

15. A method for balancing charge between battery cells, comprising:

estimating an amount of charge stored in a first battery cell in response to an open circuit voltage of the first battery cell, the open circuit voltage in a first voltage region of battery cell output voltage;

estimating an amount of charge stored in a second battery cell in response to an open circuit voltage of the second battery cell, the open circuit voltage in a first voltage region of battery cell output voltage;

reducing a total charge capacity of the first or second battery cell from a rated charge storage capacity in response to an open circuit voltage of the first or second battery cell and an integrated charge capacity of the first or second battery cell;

balancing charge of the first or second battery cell battery cell in a second region of battery cell output voltage in an amount of charge related to a difference between the amount of charge stored in the first battery cell in the first voltage region of battery cell output voltage and the amount of charge stored in the second battery cell in the first voltage region of battery cell output voltage; and

adjusting a timing of the balancing of charge of the first or second battery cell in response to reducing the rated charge storage capacity of the first or second battery cell.

16. The method of claim 15, where the timing of balancing is related to a voltage of the first or second battery cell.

17. The method of claim 15, where the balancing of charge of the first or second battery cell includes decreasing battery charge.

18. The method of claim 15, where in the first voltage region of battery output voltage, battery voltage changes in at least a first voltage amount as battery charge changes in a first charge amount, and where in the second voltage region of battery output voltage, battery voltage changes in an amount less than the first voltage amount as battery charge changes in the first charge amount.

19. The method of claim 15, where balancing of the first battery cell is performed via passive balancing circuitry.

20. The method of claim 15, where adjusting timing of the balancing of charge makes balancing of charge occur at a lower battery voltage during battery cell charging.