Method for opportunistically balancing charge between battery cells
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.
1. 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 the 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 in a second voltage 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, where a slope of battery voltage versus battery charge of the first voltage region is higher than a slope of battery voltage versus battery charge of the second voltage region, and where a battery voltage of the first voltage region is lower than a battery voltage of the second voltage region; 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.
2. The method of claim 1 , where the timing of balancing is related to a voltage of the first or second battery cell.
3. The method of claim 1 , where the balancing of charge of the first or second battery cell includes decreasing battery charge.
4. The method of claim 1 , where balancing of the first battery cell is performed via passive balancing circuitry.
5. The method of claim 1 , where adjusting timing of the balancing of charge makes balancing of charge occur at a lower battery voltage during battery cell charging.
6. 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 the 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 in a second voltage 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, where in the first voltage region of battery cell 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 cell output voltage, battery voltage changes in an amount less than the first voltage amount as battery charge changes in the first charge amount.
7. The method of claim 6 , where the timing of balancing is related to a voltage of the first or second battery cell.
8. The method of claim 6 , where the balancing of charge of the first or second battery cell includes decreasing battery charge.
9. The method of claim 6 , where balancing of the first battery cell is performed via passive balancing circuitry.
10. The method of claim 6 , where adjusting timing of the balancing of charge makes balancing of charge occur at a lower battery voltage during battery cell charging.