State of charge tracking system for battery systems based on relaxation voltage

1. A battery control module for a battery system, comprising:

a voltage measuring module that measures battery voltage;

a current measuring module that measures battery current; and

a state of charge (SOC) module that communicates with said current and voltage measuring modules and that estimates SOC based on relaxation voltage, wherein said SOC module enables said estimation of said SOC based on one of a qualified charge swing following a discharge swing and a qualified discharge swing following a charge swing.

2. The battery control module of claim 1 wherein said SOC module enables said estimation of said SOC when said qualified charge swing follows said discharge swing and relaxation.

3. The battery control module of claim 1 wherein said SOC module enables said estimation of said SOC when said qualified discharge swing follows said charge swing and relaxation.

4. The battery control module of claim 2 wherein said SOC module accumulates charge swing during charging and identifies said qualified charge swing when said accumulated charge swing is within a charge swing window.

5. The battery control module of claim 3 wherein said SOC module accumulates discharge swing during discharging and identifies said qualified discharge swing when said accumulated discharge swing is within a discharge swing window.

6. The battery control module of claim 1 wherein said SOC module monitors rest periods during which said battery is neither charging nor discharging.

7. The battery control module of claim 6 wherein said SOC module enables said estimation of said SOC when said rest period is greater than a threshold.

8. The battery control module of claim 2 wherein said SOC module enables SOC estimation during charging when a first period between said qualified charge swing and said prior discharge swing and relaxation is less than a predetermined allowed time.

9. The battery control module of claim 3 wherein said SOC module enables SOC estimation during charging when a second period between said qualified discharge swing and said prior charge swing and relaxation is less than a predetermined allowed time.

10. A method for operating a battery control module for a battery system, comprising:

measuring battery voltage;

measuring battery current;

estimating state of charge (SOC) based on relaxation voltage, said battery voltage and said battery; and

enabling said SOC estimation when one of a qualified charge swing follows a discharge swing and a qualified discharge swing follows a charge swing.

11. The method of claim 10 further comprising enabling said SOC estimation when said qualified charge swing follows said discharge swing and relaxation.

12. The method of claim 10 further comprising enabling said SOC estimation when said qualified discharge swing follows said charge swing and relaxation.

13. The method of claim 11 further comprising:

accumulating charge swing during charging; and

identifying said qualified charge swing when said accumulated charge swing is within a charge swing window.

14. The method of claim 12 further comprising:

accumulating discharge swing during discharging; and

identifying said qualified discharge swing when said accumulated discharge swing is within a discharge swing window.

15. The method of claim 10 further comprising monitoring rest periods during which said battery is neither charging nor discharging.

16. The method of claim 15 further comprising enabling said SOC estimation when said rest period is greater than a threshold.

17. The method of claim 11 further comprising enabling SOC estimation during charging when a first period between said qualified charge swing and said discharge swing and relaxation is less than a predetermined allowed time.

18. The method of claim 12 further comprising enabling SOC estimation during charging when a second period between said qualified discharge swing and said charge swing and relaxation is less than a predetermined allowed time.