System and method for a rechargeable battery

1. A method for a rechargeable battery comprising:

applying a charge voltage to the rechargeable battery;

monitoring a battery voltage and a battery current of the rechargeable battery; and

identifying a top of charge condition when for a defined time period the battery voltage is within a voltage tolerance of the charge voltage and the battery current is within a current tolerance of a threshold current;

establishing a state of charge corresponding to a known available energy capacity of the rechargeable battery when the top of charge condition is identified, the known available energy capacity being determined based at least in part on the charge voltage and a float voltage of the rechargeable battery.

2. The method of claim 1 further comprising:

establishing a state of charge of the rechargeable battery when the top of charge condition is identified.

3. The method of claim 2 , wherein the state of charge corresponds to a known available energy capacity of the rechargeable battery.

4. The method of claim 2 , wherein the charge voltage is a float voltage of the rechargeable battery, and wherein the state of charge is established as 100%.

5. The method of claim 2 , wherein the charge voltage is not a float voltage of the rechargeable battery, and wherein the state of charge is established based on the charge voltage and the float voltage.

6. The method of claim 1 further comprising:

ceasing to charge the rechargeable battery when the top of charge condition is identified.

7. The method of claim 1 , wherein the defined time period is no more than 120seconds.

8. The method of claim 1 , wherein the voltage tolerance is no more than 10 millivolts per cell of the rechargeable battery.

9. The method of claim 1 , wherein the current tolerance is no more than 500 milliamps.

10. The method of claim 1 , wherein the threshold current is zero amps.

11. A method for a rechargeable battery comprising:

applying a charge voltage to the rechargeable battery at a voltage that is greater than a float voltage of the rechargeable battery;

monitoring a battery voltage and a battery current of the rechargeable battery;

reducing the charge voltage to the float voltage;

calculating a battery resistance from the battery voltage and the battery current; and

identifying a top of charge condition when for a defined time period the battery voltage is within a tolerance of the float voltage and the calculated battery resistance is greater than a threshold resistance.

12. The method of claim 11 , wherein calculating the battery resistance comprises calculating a per cell resistance for a plurality of energy storage cells of the rechargeable battery.

13. The method of claim 11 , wherein the threshold resistance is 80 milliohms.

14. A method for a rechargeable battery comprising:

monitoring a battery voltage and battery current of the rechargeable battery;

applying a charge voltage to the rechargeable battery at a voltage level that is greater than a float voltage of the rechargeable battery;

reducing the charge voltage to the float voltage; and

identifying a top of charge condition when for a defined time period the battery voltage is within a voltage tolerance of the float voltage and the battery current is within a tolerance of zero amps.

15. The method of claim 14 , further comprising:

establishing a state of charge of the rechargeable battery when the top of charge condition is identified.

16. The method of claim 15 , wherein the state of charge is established as 100% when the top of charge condition is identified.

17. The method of claim 14 , wherein the voltage tolerance is no more than 10 millivolts.

18. The method of claim 14 , wherein the voltage level is at least 1% greater than the float voltage.

19. The method of claim 14 , wherein reducing the charge voltage to the float voltage further comprises:

reducing the charge voltage to the float voltage upon achieving a designated state of charge.

20. The method of claim 14 , wherein reducing the charge voltage to the float voltage further comprises:

reducing the charge voltage to the float voltage after a designated charging time.

21. The method of claim 14 , wherein reducing the charge voltage to the float voltage further comprises:

reducing the charge voltage to the float voltage when the battery current falls below a designated threshold current.

22. The method of claim 14 , wherein reducing the charge voltage to the float voltage further comprises:

reducing the charge voltage to the float voltage when the battery voltage exceeds a designated threshold voltage.

23. The method of claim 14 , wherein the rechargeable battery comprises a plurality of sodium-metal-halide cells.

24. A rechargeable battery system comprising:

means for applying a charge voltage to the rechargeable battery;

means for monitoring a battery voltage and a battery current of the rechargeable battery; and

means for identifying a top of charge condition when for a defined time period the battery voltage is within a voltage tolerance of the charge voltage and the battery current is within a current tolerance of a threshold current;

wherein the rechargeable battery comprises a plurality of sodium-metal-halide cells.

25. A rechargeable battery system comprising:

a rechargeable battery;

a source configured to apply a charge voltage to the rechargeable battery, the charge voltage being greater than a float voltage of the rechargeable battery;

a voltage sensor configured to monitor a battery voltage of the rechargeable battery;

a current sensor configured to monitor a battery current of the rechargeable battery; and

a controller in communication with the voltage sensor and the current sensor, the controller configured to reduce the charge voltage to a float voltage and to identify a top of charge condition of the rechargeable battery when for a defined time period the battery voltage is within a voltage tolerance of the float voltage and the battery current is within a current tolerance of a threshold current.