Determining the state of-charge of batteries via selective sampling of extrapolated open circuit voltage

• US 8,872,518 B2
• Filed: 06/25/2010
• Issued: 10/28/2014
• Est. Priority Date: 06/25/2010
• Status: Active Grant

• Current Assignee: Atieva, Inc.
• Original Assignee: Atieva, Inc.
• Inventors: Chen, Li-Wei, Liu, Albert Shih-Young
• Primary Examiner(s): Berhanu, Samuel
• Assistant Examiner(s): Robbins, Jerry D
• Application Number:
  US12/823,268
• Publication Number:
  US 20110316547A1
• Time in Patent Office: 1,586 Days
• Field of Search: 324/427
• US Class Current: 324/427
• 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 31/3842 combining voltage and current measurements

Abstract

A method for estimating the state-of-charge of a battery. The method includes collecting a plurality of voltage measurements during operation of the system containing the battery and determining a time-constant of relaxation and an open-circuit voltage corresponding to the battery based, at least in part, on the voltage measurements. The method further includes estimating the state-of-charge of the battery based, at least in part, on the open-circuit voltage.

Claims

1. A method of determining a state-of-charge of a battery within a system powered at least in part by the battery comprising:

collecting a plurality of voltage measurements from the battery during operation of the system;

deriving a time-constant of relaxation and an open-circuit voltage corresponding to the battery from the voltage measurements and a mathematical model of the battery that relates the voltage measurements to the open-circuit voltage and the time-constant of relaxation; and

,

estimating the state-of-charge of the battery based, at least in part, on the open-circuit voltage;

wherein the mathematical model comprises a closed mathematical expression that includes the open-circuit voltage and the time-constant of relaxation; and

,

wherein the closed mathematical expression is of the form;

V(t)=OCV−

α

e^t/τ

, wherein V(t) is one of the voltage measurements at a given point in time, OCV is the open-circuit voltage, α

is an overpotential, and τ

is the time-constant of relaxation.

2. The method of claim 1, wherein determining the time-constant of relaxation and the open-circuit voltage comprises regressing the time-constant of relaxation and the open-circuit voltage using the closed expression as a regression function.

3. The method of claim 2, further comprising determining an overpotential value by regressing the overpotential together with the time-constant of relaxation and open-circuit voltage.

4. The method of claim 2, further comprising determining a regression interval based at least in part on the time-constant of relaxation.

5. The method of claim 4, wherein the regression interval is used, at least in part, to estimate the uncertainty of the open-circuit voltage.

6. The method of claim 4, wherein collecting a plurality of voltage measurements comprises collecting the plurality of voltage measurements over a first time interval.

7. The method of claim 6, wherein the first time interval encompasses and has a duration greater than the regression interval.

8. The method of claim 1, wherein a plurality of amperage measurements are also collected during operation of the system.

9. The method of claim 1, wherein a mathematical relationship between the open-circuit voltage and the state-of-charge is used to estimate the uncertainty of the prediction of the state-of-charge from the open-circuit voltage.

10. The method of claim 9, wherein the mathematical relationship comprises a set of measurements relating the open-circuit voltage and the state-of-charge.

11. A method of operation within a system powered at least in part by a battery, the method comprising:

collecting a plurality of voltage measurements of the battery over a first time interval and during operation of the system, wherein the plurality of voltage measurements are to be used to estimate a state-of-charge of the battery;

adjusting a time-constant value based on the plurality of voltage measurements collected over the first time interval, the time-constant value indicating a rate at which the battery voltage transitions from a voltage under load to an open-circuit voltage; and

adjusting the duration of the first time interval based at least in part on the time-constant value;

wherein adjusting the time-constant value comprises regressing the time-constant value using the function V(t)=OCV−

α

e^t/τ

, wherein V(t) is one of the voltage measurements at a given point in time within a regression interval, OCV is the open-circuit voltage, α

is an overpotential, and τ

is the time-constant value.

12. The method of claim 11, wherein the first time interval encompasses and has a duration equal or greater than the regression interval.

13. A system powered at least in part by a battery, the system comprising:

voltage measurement circuitry coupled to the battery to generate a plurality of measurements of the battery voltage during operation of the system;

battery management circuitry coupled to the voltage measurement circuitry, the battery management circuitry configured to determine a time-constant of relaxation and an open-circuit voltage for the battery from (i) the plurality of measurements of the battery voltage and (ii) a mathematical model of the battery that relates the battery voltage to the open-circuit voltage and the time-constant of relaxation; and

estimating a state-of-charge of the battery based, at least in part, on the open-circuit voltage;

wherein the mathematical model comprises a closed mathematical expression that includes the open-circuit voltage and the time-constant of relaxation; and

,

wherein the closed mathematical expression is of the form;

V(t)=OCV−

α

e^t/τ

, wherein V(t) is one of the voltage measurements at a given point in time, OCV is the open-circuit voltage, α

is an overpotential, and τ

is the time-constant of relaxation.

14. The system of claim 13, wherein the battery management circuitry comprises circuitry to determine the time-constant of relaxation and the open-circuit voltage comprising regressing the time-constant of relaxation and the open-circuit voltage using the closed expression as a regression function.

15. The system of claim 14, wherein the battery management circuitry further comprises circuitry to determine a regression interval based at least in part on the time-constant of relaxation.

16. The system of claim 15, wherein the regression interval is used, at least in part, to estimate an uncertainty of the open-circuit voltage.

17. The system of claim 13, wherein the open-circuit voltage is used to estimate an uncertainty of the estimation of the state-of-charge.

18. The system of claim 17, wherein the system contains a user interface that presents visual information about the state-of-charge and the uncertainty of the estimation of the state-of-charge, to an operator of the system.

19. The system of claim 13, wherein the battery is used in an automotive vehicle powered, at least in part, by the battery.

20. The system of claim 13, wherein the battery is comprised of a plurality of battery cells.