Method and apparatus for determining a capacity of a battery

• US 9,651,628 B2
• Filed: 07/13/2015
• Issued: 05/16/2017
• Est. Priority Date: 09/18/2012
• Status: Active Grant

• Current Assignee: Apple Inc.
• Original Assignee: Apple Inc.
• Inventors: Greening, Thomas C., Koller, Jeffrey G., Mattisson, Nils E.
• Primary Examiner(s): DIAO, M BAYE
• Application Number:
  US14/798,147
• Publication Number:
  US 20160011273A1
• Time in Patent Office: 673 Days
• Field of Search: 320107, 320162, 320132
• CPC Class Codes: G01R 19/0069 measuring voltage or current standards
• CPC Class Codes: G01R 19/165 Indicating that current or voltage is either above or below a predetermined value or within or outside a predetermined range of values
• CPC Class Codes: G01R 21/06 by measuring current and voltage G01R21/08 - G01R21/133 take precedence
• 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 31/382 Arrangements for monitoring battery or accumulator variables, e.g. SoC
• CPC Class Codes: G01R 31/3828 using current integration
• CPC Class Codes: G01R 31/3835 involving only voltage measurements
• CPC Class Codes: G01R 31/3842 combining voltage and current measurements
• CPC Class Codes: G01R 31/392 Determining battery ageing or deterioration, e.g. state of health
• CPC Class Codes: H02J 7/00 Circuit arrangements for charging or depolarising batteries or for supplying loads from batteries
• CPC Class Codes: Y02E 60/10 Energy storage using batteries

Abstract

Some embodiments of the present invention provide a system that accurately and reliably updates a full charge capacity of a battery. During operation, the system charges the battery from an initial state to a rest point prior to reaching a fully charged state. The system then interrupts the charging process to allow the battery to relax to a resting voltage. Next, the system measures the resting voltage. The system then resumes the charging process toward the fully charged state. The system subsequently estimates the capacity of the battery based on the measured resting voltage and one or more other parameters.

Claims

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

determining, using a controller, a current state of charge of the battery;

charging, using a charger, the battery to a first state of charge when the current state of charge is lower than a threshold state of charge;

measuring, using a voltmeter, a first resting voltage corresponding to the first state of charge;

charging, using the charger, the battery from the first state toward a second state of charge;

measuring, using the voltmeter, a second resting voltage corresponding to the second state of charge; and

estimating, using the controller, the capacity of the battery based on at least the first and second resting voltages.

2. The method of claim 1, wherein the capacity is a full charge capacity.

3. The method of claim 1, wherein charging the battery to the first state of charge involves charging the battery with a constant current during a time period immediately preceding the first state of charge.

4. The method of claim 3, wherein measuring the first resting voltage involves immediately dropping the charging current to zero.

5. The method of claim 3, wherein measuring the first resting voltage involves reducing the charging current to a low level above zero.

6. The method of claim 1, wherein estimating the capacity of the battery based on at least the first resting voltage and the second resting voltage involves:

computing, using the controller, the first state of charge of the battery corresponding to the first resting voltage;

computing, using the controller, the second state of charge of the battery corresponding to the second resting voltage;

determining a coulomb count between the first state of charge and the second state of charge; and

determining the capacity of the battery based on the first state of charge, the second state of charge, and the coulomb count.

7. The method of claim 6, wherein determining the coulomb count between the first state of charge and the second state of charge involves:

initiating a coulomb counting from the first state of charge after resuming the charging process;

concluding the coulomb counting when the second state of charge is reached; and

determining the coulomb count based on the coulomb counting between the first state of charge and the second state of charge.

8. The method of claim 6, wherein each of the first resting voltage and the second resting voltage is an open circuit voltage (OCV).

9. The method of claim 1, wherein prior to charging the battery, the method further comprises selecting the first state of charge based at least on one or more of the following parameters:

a time required for the battery to relax at the first state of charge;

an error associated with a coulomb counting between the state of charge and the second state of charge;

a likelihood of a user discharging the battery below the first state of charge; and

an open-circuit-voltage measurement accuracy required to compute a state of charge associated with the first state of charge.

10. The method of claim 9, wherein selecting the first state of charge involves ensuring that the time required for the battery to relax at the first state of charge is significantly shorter than a time to charge the battery from the current state to the second state of charge without interruption.

11. The method of claim 9, wherein the state of charge is in the vicinity of or above a 60% state of charge of the battery.

12. The method of claim 9, wherein the first state of charge is in the vicinity of or below an 80% state of charge of the battery.

13. A non-transitory computer-readable storage medium storing instructions that when executed by a controller cause the controller to perform a method for accurately estimating a capacity of a battery, the method comprising:

determining, at the controller, a current state of charge of the battery;

charging, using a charger, the battery to a first state of charge when the current state of charge is lower than a threshold state of charge;

measuring a first resting voltage corresponding to the first state of charge;

charging, using the charger, the battery from the first state of charge toward a fully charged state corresponding to a second resting voltage; and

estimating, at the controller, the capacity of the battery based on at least the first and second resting voltages.

14. The non-transitory computer-readable storage medium of claim 13, wherein the capacity is a full charge capacity.

15. The computer-readable storage medium of claim 13, wherein charging the battery to the first state of charge involves charging the battery with a constant current during a time period immediately preceding the first state of charge.

16. The non-transitory computer-readable storage medium of claim 15, wherein interrupting the charging process involves immediately dropping the charging current to zero.

17. The non-transitory computer-readable storage medium of claim 13, wherein after reaching the second state of charge, the method further comprises:

allowing the battery to relax to a second resting voltage; and

measuring the second resting voltage corresponding to the second state of charge.

18. The non-transitory computer-readable storage medium of claim 17,

wherein estimating the capacity of the battery based at least on the first resting voltage involves:

computing a first state of charge of the battery corresponding to the first resting voltage;

computing a second state of charge of the battery corresponding to the second resting voltage;

determining a coulomb count between the first state of charge and the second state of charge; and

determining the capacity of the battery based on the first state of charge, the second state of charge, and the coulomb count.

19. The non-transitory computer-readable storage medium of claim 18, wherein determining the coulomb count between the first state of charge and the second state of charge involves:

initiating a coulomb counting from the first state of charge after resuming the charging process;

concluding the coulomb counting when the second state of charge is reached; and

determining the coulomb count based on the coulomb counting between the first state of charge and the second state of charge.

20. The non-transitory computer-readable storage medium of claim 13, wherein prior to charging the battery, the method further comprises selecting the first state of charge based at least on one or more of the following parameters:

a time required for the battery to relax at the first state of charge;

an error associated with a coulomb counting between the first state of charge and the second state of charge;

a likelihood of a user discharging the battery below the first state of charge; and

an open-circuit-voltage measurement accuracy required to compute a state of charge associated with the first state of charge.

21. The non-transitory computer-readable storage medium of claim 20, wherein selecting the first state of charge involves ensuring that the time required for the battery to relax at the first state of charge is significantly shorter than a time to charge the battery from the initial state to the second state of charge without interruption.

22. A battery with a capacity estimation mechanism, comprising:

a cell;

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

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

a controller configured to receive inputs from the voltage sensor and the current sensor, and to generate a capacity estimate;

wherein the controller is configured to;

determine a current state of charge of the battery;

charge the battery to a first state of charge when the current state of charge is lower than a threshold state of charge;

measure a first resting voltage corresponding to the first state of charge;

charge the battery from the first state toward a second state of charge;

measure a second resting voltage corresponding to a second state of charge; and

estimate the capacity of the battery based on at least the first and second resting voltages.

23. The battery of claim 22, wherein the controller is configured to charge the battery to the first state of charge by charging the battery with a constant current during a time period immediately preceding the first state of charge.

24. The battery of claim 23, wherein the controller is configured to interrupt the charging process by immediately dropping the charging current to zero.

25. The battery of claim 22, wherein after reaching the second state of charge, the controller is configured to:

allow the battery to relax to a second resting voltage; and

measure the second resting voltage corresponding to the second state of charge.

26. The battery of claim 25, wherein the controller is configured to determine the capacity of the battery by:

computing a first state of charge of the battery corresponding to the first resting voltage;

computing a second state of charge of the battery corresponding to the second resting voltage;

determining a coulomb count between the first state of charge and the second state of charge; and

determining the capacity of the battery based on the first state of charge, the second state of charge, and the coulomb count.

27. The battery of claim 26, wherein the controller is configured to determine the coulomb count between the first state of charge and the second state of charge by:

initiating a coulomb counting from the state of charge after resuming the charging process;

concluding the coulomb counting when the second state of charge is reached; and

determining the coulomb count based on the coulomb counting between the first state of charge and the second state of charge.

28. A method for evaluating a state of charge of a battery during a charging process, the method comprising:

determining, by a controller, whether a current state of charge of a battery is greater than or equal to a first state of charge corresponding to a predetermined first rest point;

in event the current state of charge is lower than the first state of charge;

charging, using a charger, the battery to the first predetermined rest point;

allowing the battery to relax from the first rest point to a resting voltage;

measuring, using a voltmeter, the resting voltage;

charging, using the charger, the battery toward a second rest point corresponding to a second state of charge; and

determining a state of charge of the battery at the first rest point based at least on the resting voltage.