METHOD AND SYSTEM FOR DETERMINING STATE OF CHARGE OF AN ENERGY DELIVERY DEVICE

• US 20090326842A1
• Filed: 06/03/2009
• Published: 12/31/2009
• Est. Priority Date: 06/05/2008
• Status: Active Grant

• Current Assignee: A123 Systems Incorporated
• Original Assignee: A123 Systems Incorporated
• Inventors: THOMAS-ALYEA, Karen E.
• Granted Patent: US 8,855,956 B2
• Time in Patent Office: Days
• US Class Current: 702/64
• CPC Class Codes: G01R 31/3648 comprising digital calculation means, e.g. for performing an algorithm
• CPC Class Codes: G01R 31/382 Arrangements for monitoring battery or accumulator variables, e.g. SoC
• CPC Class Codes: G01R 31/3835 involving only voltage measurements
• CPC Class Codes: G01R 31/392 Determining battery ageing or deterioration, e.g. state of health

Abstract

A method of determining state of charge of an energy delivery device includes sampling voltage values of the energy delivery device during relaxation of the device. The method further includes regressing an open circuit voltage value and the total overpotential being relaxed. The regression includes a predetermined time constant of relaxation associated with the energy delivery device. One embodiment uses the equation V(t)=OCV−α exp(−t/tau), where V(t) represents the sampled voltage values, t represents times at which each of the voltage values are sampled, OCV represents the open circuit voltage value of the energy delivery device, α represents the overpotential value, and tau represents the time constant of relaxation. The method uses a predetermined profile that relates open circuit voltage of the energy delivery device to state of charge of the device, to determine a particular state of charge corresponding to the regressed open circuit voltage value.

Claims

1. A method of determining state of charge of an energy delivery device, comprising:

sampling voltage values presented by the energy delivery device during a relaxation period of the energy delivery device;

regressing, with respect to the sampled voltage values, (i) an open circuit voltage value of the energy delivery device and (ii) an overpotential value that describes a total overpotential being relaxed, wherein the regression includes a predetermined time constant of relaxation associated with the energy delivery device; and

,

using a predetermined profile that relates open circuit voltage of the energy delivery device to state of charge of the energy delivery device, to determine a particular state of charge corresponding to the regressed open circuit voltage value.

2. The method of claim 1, wherein the regression uses the equation

V(t)=OCV−

α

exp(−

t/tau),

where V(t) represents the sampled voltage values, t represents times at which each of the voltage values are sampled, OCV represents the open circuit voltage value of the energy delivery device, α

represents the overpotential value, and tau represents the predetermined time constant of relaxation associated with the energy delivery device.

3. The method of claim 1, wherein the predetermined time constant of relaxation associated with the energy delivery device is generated by

(i) applying a current to the energy delivery device,

(ii) discontinuing the current after a period of time,

(iii) recording voltages presented by the energy delivery device after the current is discontinued,

(iv) plotting a logarithm of a plurality of differences V_t—

_last−

V_t, wherein V<sub>t—

last</sub> is a final voltage recorded after the current is discontinued and V_t, is a voltage recorded at time t;

(v) determining a slope of a linear fit to the plurality of differences V<sub>t—

last</sub>−

V_t;

wherein the predetermined time constant of relaxation is the inverse of the slope.

4. The method of claim 3, wherein the linear fit is determined using the differences V_t—

_last−

V_t only for t greater than N seconds and less than M seconds after discontinuing the current, wherein M is a value selected according to a first time constant associated with a large diffusion component of the energy delivery device, and N is a value selected according to a second time constant associated with the energy delivery device that is substantially smaller than the first time constant.

5. The method of claim 3, wherein the predetermined time constant of relaxation is generated as a function of temperature of the energy delivery device.

6. The method of claim 3, wherein the predetermined time constant is determined as a function of state of health of the energy delivery device.

7. The method of claim 1, wherein the predetermined time constant of relaxation is chosen with respect to a measured temperature of the energy delivery device.

8. The method of claim 1, wherein a sampling interval associated with the sampling is chosen as a function of the predetermined time constant of relaxation of the energy delivery device.

9. The method of claim 1, wherein the regression uses only the sampled voltage values that were sampled more than N seconds after beginning the relaxation period.

10. The method of claim 1, further including combining the determined particular state of charge with one or more state of charge estimates determined by alternative estimating techniques.

11. The method of claim 10, wherein the alternative estimating techniques includes at least one of (i) coulomb counting, (ii) evaluation of an equivalent circuit model corresponding to the energy delivery device, (iii) evaluation of a physical cell model corresponding to the energy delivery device, (iv) measurements of one or more of voltage, temperature, a derivative of voltage and a derivative of temperature.

12. The method of claim 10, wherein the combining includes calculating a weighted sum of the determined particular state of charge and the one or more state of charge estimates determined by alternative techniques.

13. An apparatus for determining state of charge of an energy delivery device, comprising:

a voltage measurement device for sampling voltage values presented by the energy delivery device during a relaxation period of the energy delivery device;

a processor for

a) regressing, with respect to the sampled voltage values, (i) an open circuit voltage value of the energy delivery device and (ii) an overpotential value that describes a total overpotential being relaxed, wherein the regression includes a predetermined time constant of relaxation associated with the energy delivery device; and

,

b) using a predetermined profile that relates open circuit voltage of the energy delivery device to state of charge of the energy delivery device, to determine a particular state of charge corresponding to the regressed open circuit voltage value.

14. The apparatus of claim 13, wherein the regression uses the equation

V(t)=OCV−

α

exp(−

t/tau),

where V(t) represents the sampled voltage values, t represents times at which each of the voltage values are sampled, OCV represents the open circuit voltage value of the energy delivery device, α

represents the overpotential value, and tau represents the predetermined time constant of relaxation associated with the energy delivery device.

15. The apparatus of claim 13, wherein the predetermined time constant of relaxation is chosen with respect to a measured temperature of the energy delivery device.

16. The apparatus of claim 13, wherein a sampling interval associated with the sampling is chosen as a function of the predetermined time constant of relaxation of the energy delivery device.

17. The apparatus of claim 13, wherein the regression uses only the sampled voltage values that were sampled more than N seconds after beginning the relaxation period.

18. The apparatus of claim 13, wherein the determined particular state of charge is combined with one or more state of charge estimates determined by alternative estimating techniques.

19. The apparatus of claim 18, wherein the alternative estimating techniques includes at least one of (i) coulomb counting, (ii) evaluation of an equivalent circuit model corresponding to the energy delivery device, (iii) evaluation of a physical cell model corresponding to the energy delivery device, (iv) measurements of one or more of voltage, temperature, a derivative of voltage and a derivative of temperature.

20. The apparatus of claim 18, wherein the combination of the determined particular state of charge and the one or more state of charge estimates determined by alternative estimating includes a weighted sum of the determined particular state of charge and the one or more state of charge estimates determined by alternative techniques.

21. A computer program product, including a computer usable medium having a computer readable program code embodied therein, the computer readable program code adapted to be executed to implement a method of determining state of charge of an energy delivery device, the medium holding one or more instructions for:

sampling voltage values presented by the energy delivery device during a relaxation period of the energy delivery device;

regressing, with respect to the sampled voltage values, (i) an open circuit voltage value of the energy delivery device and (ii) an overpotential value that describes a total overpotential being relaxed, wherein the regression includes a predetermined time constant of relaxation associated with the energy delivery device; and

,

using a predetermined profile that relates open circuit voltage of the energy delivery device to state of charge of the energy delivery device, to determine a particular state of charge corresponding to the regressed open circuit voltage value.

22. The computer program product of claim 21, wherein the regression uses the equation

V(t)=OCV−

α

exp(−

t/tau),

where V(t) represents the sampled voltage values, t represents times at which each of the voltage values are sampled, OCV represents the open circuit voltage value of the energy delivery device, α

represents the overpotential value, and tau represents the predetermined time constant of relaxation associated with the energy delivery device.