Systems and methods for determining battery state of charge

1. A system for determining the state of charge (SOC) of a lithium battery comprising:

a controller measuring an initial open circuit voltage (OCV) at a first time;

the controller determining the SOC of the battery from the measurement of the OCV at the first time;

a power source charging the battery at a predetermined DC current;

the controller determining a subsequent SOC of the battery at a second time based on charging current and the amount of time the battery is being charged;

the controller utilizing the subsequent SOC of the battery to determine a subsequent OCV of the battery;

the controller utilizing the subsequent OCV to determine a corresponding impedance of the battery;

a memory coupled to the controller for storing correspondence between an impedance of the battery and corresponding SOC of the battery; and

the controller, during a discharge time, utilizing the correspondence of the SOC of the battery and the impedance of the battery, which was determined during battery charging, to determine a discharge SOC by determining the present impedance of the battery through measurement or prediction.

2. The system of claim 1 , wherein the impedance of the battery during the charge time period for a given time instance is determined based at least on a measured voltage of the battery at the given time instance, the state of charge of the battery at the given time instance and a current provided by the power source at the given time instance.

3. The system of claim 2 , wherein the controller is configured to determine impedances of the battery for a plurality of different time instances during the charge time period to determine an average scaling factor for the battery that is employed by the controller to predict an impedance of the battery.

4. The system of claim 1 , wherein the impedance of the battery varies as a function of the state of charge of the battery.

5. The system of claim 4 , wherein the controller employs the state of charge of the battery and a change in the state of charge of the battery to determine a subsequent state of charge of the battery for a given time instance.

6. The system of claim 5 , wherein the controller employs the subsequent state of charge for the given time instance to determine an open circuit voltage for the subsequent state of charge.

7. The system of claim 6 , wherein the controller accesses a lookup table to determine the relationship between the given state of charge of the battery and the open circuit voltage for the subsequent state of charge of the battery.

8. The system of claim 1 , wherein the controller is configured to determine a remaining battery run time of the battery based on the state of charge of the battery.

9. The system of claim 1 wherein the controller is configured to determine a plurality of impedances of the battery during a battery charge period, wherein each impedance of the plurality of impedances of the battery corresponds with a given state of charge of the battery;

the controller determines an open circuit voltage (OCV) corresponding to the subsequent state of charge (SOC) of a relaxed battery;

the controller calculates an impedance for the battery corresponding to the subsequent SOC of the battery; and

the controller determines a scaling factor for the battery.

10. The system of claim 9 , wherein a memory is configured to store the plurality of impedances and the corresponding states of charge of the battery.

11. The system of claim 10 , wherein the controller is further configured to determine a state of charge of the battery during a discharge time period based on the stored plurality of impedances and the corresponding states of charge of the battery.

12. The system of claim 9 , wherein a given impedance of the plurality of impedances of the battery is based on the state of charge for the battery corresponding to the given impedance, a measured voltage of the battery and the charging current of the battery.

13. The system of claim 12 , wherein the controller is further configured to report a remaining battery run time for a given time instance via a battery status indicator based on a state of charge of the battery at the given time instance.

14. A method for determining the state of charge (SOC) of a lithium battery comprising:

measuring an initial open circuit voltage (OCV) at a first time;

determining the SOC of the battery from the measurement of the OCV at the first time;

charging the battery with a predetermined DC current;

determining a subsequent SOC of the battery at a second time based on charging current and the amount of time the battery is being charged;

utilizing the subsequent SOC of the battery to determine a subsequent OCV of the battery;

utilizing the subsequent OCV to determine a corresponding impedance of the battery;

storing correspondence between an impedance of the battery and corresponding SOC of the battery in a memory; and

during a discharge time, utilizing the correspondence of the SOC of the battery and the impedance of the battery, which was determined during battery charging, to determine a discharge SOC by determining the present impedance of the battery through measurement or prediction.

15. The method of claim 14 , further comprising determining a scaling factor for the battery based on a calculated impedances of the battery.

16. The method of claim 15 , further comprising predicting an impedance associated with a given SOC of the battery based on the scaling factor.

17. The method of claim 14 , further comprising:

connecting a device for charging the battery to a power outlet;

detecting a disconnection from a power outlet; and

determining an SOC for the battery near the time of the disconnection.

18. The system of claim 1 wherein the battery is charged for a predetermined period of time.

19. The system of claim 18 wherein the measurements are made on a relaxed battery.

20. The method of claim 14 further comprising charging the battery for a predetermined period of time.

21. The method of claim 20 further comprising measuring values on a relaxed battery.