ESTIMATING STATE OF CHARGE (SOC) AND UNCERTAINTY FROM RELAXING VOLTAGE MEASUREMENTS IN A BATTERY

• US 20140079969A1
• Filed: 09/19/2012
• Published: 03/20/2014
• Est. Priority Date: 09/19/2012
• Status: Active Grant

• Current Assignee: Apple Inc.
• Original Assignee: Apple Inc.
• Inventors: Greening, Thomas C., Koller, Jeffrey G., Mattisson, Nils E., Ungar, P. Jeffrey
• Granted Patent: US 9,128,162 B2
• Time in Patent Office: Days
• US Class Current: 429/91
• CPC Class Codes: G01R 31/3648 comprising digital calculation means, e.g. for performing an algorithm
• CPC Class Codes: G01R 31/367 Software therefor, e.g. for battery testing using modelling or look-up tables
• CPC Class Codes: G01R 31/3835 involving only voltage measurements
• CPC Class Codes: G01R 31/387 Determining ampere-hour charge capacity or SoC
• CPC Class Codes: G01R 31/392 Determining battery ageing or deterioration, e.g. state of health
• CPC Class Codes: H01M 10/0525 Rocking-chair batteries, i.e. batteries with lithium insertion or intercalation in both electrodes; Lithium-ion batteries
• CPC Class Codes: H01M 10/4285 Testing apparatus
• CPC Class Codes: H01M 10/48 Accumulators combined with arrangements for measuring, testing or indicating the condition of cells, e.g. the level or density of the electrolyte constructional details of current conducting connections for detecting conditions inside cells or batteries, e.g. details of voltage sensing terminals, H01M50/569
• CPC Class Codes: Y02E 60/10 Energy storage using batteries

Abstract

Some embodiments of the present invention provide a system that estimates a state of charge for a battery. During operation, while the battery is relaxing toward a fully rested state, the system determines if a modified state of charge of the battery is substantially consistent with a predetermined decay pattern. If so, the system estimates the state of charge of the battery as a value of the modified state of charge determined at the time when the consistency is observed. Otherwise, the system continues monitoring the modified state of charge of the battery. In one embodiment, the predetermined decay pattern is a single exponential decay. After estimating the state of charge of the battery, the system may determine an uncertainty of the estimated state of charge. More specifically, the system can estimate the uncertainty as substantially equal to a fraction of the difference between the value of the modified state of charge and the value of the voltage-based state of charge at the time when the consistency is observed.

Claims

1. A method for estimating a state of charge for a battery, comprising:

while the battery is relaxing toward a fully rested state, determining if a modified state of charge of the battery is substantially consistent with a predetermined decay pattern; and

if so, estimating the state of charge of the battery as a value of the modified state of charge determined at the time when the consistency is observed;

otherwise, continuing monitoring the modified state of charge of the battery.

2. The method of claim 1, wherein the predetermined decay pattern is a single exponential decay.

3. The method of claim 1, wherein determining if the modified state of charge of the battery is substantially consistent with the predetermined decay pattern involves:

computing a rate of change of the modified state of charge; and

determining whether the rate of change of the modified state of charge has become less than a threshold rate of change.

4. The method of claim 3, wherein computing the rate of change of the modified state of charge involves computing both a first value of the modified state of charge at a first time and a second value of the modified state of charge at a second time, wherein the first time and the second time are associated with a predetermined time interval.

5. The method of claim 1, wherein the method further comprises deriving the modified state of charge from a voltage-based state of charge of the battery, and wherein determining if the modified state of charge of the battery is substantially consistent with the predetermined decay pattern is equivalent to determining if the voltage-based state of charge substantially matches the predetermined decay pattern.

6. The method of claim 5, wherein deriving the modified state of charge from a voltage-based state of charge of the battery involves:

determining one or more sample values of the voltage-based state of charge of the battery;

computing a first time derivative of the voltage-based state of charge;

computing a time constant associated with the predetermined decay pattern; and

determining the value of the modified state of charge based on the one or more sample values of the voltage-based state of charge, the first time derivative of the voltage-based state of charge, and the time constant.

7. The method of claim 6, wherein computing the time constant associated with the predetermined decay pattern further involves:

computing a second time derivative of the voltage-based state of charge; and

computing the time constant as the ratio of the first time derivative and the second time derivative of the voltage-based state of charge.

8. The method of claim 6, wherein determining the value of the voltage-based state of charge involves:

measuring an open circuit voltage of the battery; and

determining the value of the voltage-based state of charge based at least on the open circuit voltage.

9. The method of claim 6, wherein after estimating the state of charge of the battery, the method further comprises determining an uncertainty of the estimated state of charge.

10. The method of claim 9, wherein the uncertainty of the estimated state of charge is substantially equal to a fraction of the difference between the value of the modified state of charge and the value of the voltage-based state of charge at the time when the consistency is observed.

11. The method of claim 10, wherein the fraction is ⅓

.

12. The method of claim 3, wherein the threshold rate of change is ˜

0.4%/hour.

13. The method of claim 1, wherein the method further comprises:

determining a coulomb-counting-based state of charge of the battery; and

computing an updated state of charge of the battery based on both the modified state of charge and the coulomb-counting-based state of charge at the time when the consistency is observed.

14. The method of claim 1, wherein after determining that the modified state of charge of the battery is not consistent with the predetermined decay pattern, the method further comprises estimating the state of charge of the battery using the coulomb-counting-based state of charge.

15. A computer-readable storage medium storing instructions that when executed by a controller for a battery cause the controller to perform a method for estimating a state of charge of the battery, the method comprising:

while the battery is relaxing toward a fully rested state, determining if a modified state of charge of the battery is substantially consistent with a predetermined decay pattern; and

if so, estimating the state of charge of the battery as a value of the modified state of charge determined at the time when the consistency is observed;

otherwise, continuing monitoring the modified state of charge of the battery.

16. The computer-readable storage medium of claim 15, wherein the predetermined decay pattern is a single exponential decay.

17. The computer-readable storage medium of claim 15, wherein determining if the modified state of charge of the battery is substantially consistent with the predetermined decay pattern involves:

computing a rate of change of the modified state of charge; and

determining whether the rate of change of the modified state of charge has become less than a threshold rate of change.

18. The computer-readable storage medium of claim 17, wherein computing the rate of change of the modified state of charge involves computing both a first value of the modified state of charge at a first time and a second value of the modified state of charge at a second time, wherein the first time and the second time are associated with a predetermined time interval.

19. The computer-readable storage medium of claim 15, wherein the method further comprises deriving the modified state of charge from a voltage-based state of charge of the battery, and wherein determining if the modified state of charge of the battery is substantially consistent with the predetermined decay pattern is equivalent to determining if the voltage-based state of charge substantially matches the predetermined decay pattern.

20. The computer-readable storage medium of claim 19, wherein deriving the modified state of charge from a voltage-based state of charge of the battery involves:

determining one or more sample values of the voltage-based state of charge of the battery;

computing a first time derivative of the voltage-based state of charge;

computing a time constant associated with the predetermined decay pattern; and

determining the value of the modified state of charge based on the one or more sample values of the voltage-based state of charge, the first time derivative of the voltage-based state of charge, and the time constant.

21. The computer-readable storage medium of claim 20, wherein computing the time constant associated with the predetermined decay pattern further involves:

computing a second time derivative of the voltage-based state of charge; and

computing the time constant as the ratio of the first time derivative and the second time derivative of the voltage-based state of charge.

22. The computer-readable storage medium of claim 20, wherein determining the value of the voltage-based state of charge involves:

measuring an open circuit voltage of the battery; and

determining the value of the voltage-based state of charge based at least on the open circuit voltage.

23. The computer-readable storage medium of claim 20, wherein after estimating the state of charge of the battery, the method further comprises determining an uncertainty of the estimated state of charge.

24. The computer-readable storage medium of claim 23, wherein the uncertainty of the estimated state of charge is substantially equal to a fraction of the difference between the value of the modified state of charge and the value of the voltage-based state of charge at the time when the consistency is observed.

25. A battery with a state-of-charge estimation mechanism, comprising:

a cell;

a voltage sensor configured to measure a voltage for the battery; and

a controller configured to receive inputs from the voltage sensor and to generate a state-of-charge estimate;

wherein while the battery is relaxing toward a fully rested state, the controller is configured to;

determine if a modified state of charge of the battery is substantially consistent with a predetermined decay pattern; and

if so, estimate the state of charge of the battery as a value of the modified state of charge determined at the time when the consistency is observed;

otherwise, continue monitoring the modified state of charge of the battery.

26. The battery of claim 25, wherein the predetermined decay pattern is a single exponential decay.

27. The battery of claim 25, wherein the controller determines if the modified state of charge of the battery is substantially consistent with the predetermined decay pattern by:

computing a rate of change of the modified state of charge; and

determining whether the rate of change of the modified state of charge has become less than a threshold rate of change.

28. The battery of claim 27, wherein the controller is configured to compute the rate of change of the modified state of charge by computing both a first value of the modified state of charge at a first time and a second value of the modified state of charge at a second time, wherein the first time and the second time are associated with a predetermined time interval.

29. The battery of claim 25, wherein the controller is configured to derive the modified state of charge from a voltage-based state of charge of the battery.

30. The battery of claim 29, wherein the controller is configured to derive the modified state of charge from a voltage-based state of charge of the battery by:

determining one or more sample values of the voltage-based state of charge of the battery;

computing a first time derivative of the voltage-based state of charge;

computing a time constant associated with the predetermined decay pattern; and

determining the value of the modified state of charge based on the one or more sample values of the voltage-based state of charge, the first time derivative of the voltage-based state of charge, and the time constant.

31. The battery of claim 30, wherein the controller is configured to compute the time constant associated with the predetermined decay pattern by:

computing a second time derivative of the voltage-based state of charge; and

computing the time constant as the ratio of the first time derivative and the second time derivative of the voltage-based state of charge.

32. The battery of claim 30, wherein the controller is configured to determine the value of the voltage-based state of charge by:

measuring an open circuit voltage of the battery; and

determining the value of the voltage-based state of charge based at least on the open circuit voltage.

33. The battery of claim 30, wherein after estimating the state of charge of the battery, the controller is further configured to compute an uncertainty of the estimated state of charge as a fraction of the difference between the value of the modified state of charge and the value of the voltage-based state of charge at the time when the consistency is observed.