Method and apparatus for battery fuel gauging

• US 6,388,447 B1
• Filed: 11/07/2000
• Issued: 05/14/2002
• Est. Priority Date: 11/07/2000
• Status: Expired due to Fees

• Current Assignee: Moltech Power Systems Incorporated (City of Shanghai)
• Original Assignee: Moltech Power Systems Incorporated (City of Shanghai)
• Inventors: Hudson, Richard A., Hall, A. Daniel
• Primary Examiner(s): Wong, Peter S.
• Assistant Examiner(s): TIBBITS, PIA FLORENCE
• Application Number:
  US09/707,476
• Time in Patent Office: 553 Days
• Field of Search: 324/426, 324/430, 320/132, 320/130, 702/63
• US Class Current: 324/426
• CPC Class Codes: H02J 7/0048 Detection of remaining charge capacity or state of charge [SOC]
• CPC Class Codes: H02J 7/00718 in response to charge current gradient
• CPC Class Codes: H02J 7/007184 in response to battery voltage gradient

Abstract

The subject invention pertains to a method and apparatus for recalibrating and/or estimating a battery'"'"'s state of charge (SOC). The subject invention is advantageous in situations where the battery infrequently, if ever, reaches a full charge and/or a full discharge. In a specific embodiment, the subject method and apparatus can utilize a battery'"'"'s voltage and/or current, and/or changes in a battery'"'"'s voltage and/or current with time, to recalibrate and/or estimate the battery'"'"'s SOC. The method and apparatus of the subject invention can monitor various parameters relating to the condition of the battery in order to determine where on the open circuit voltage (OCV) versus SOC curve, the dOCV/dSOC versus SOC curve, and/or d²OCV/dSOC² versus SOC curve the curve the battery is and, in a specific embodiment, when the battery reaches inflection point of the OCV versus SOC curve. Alternatively, the second derivative of the curve can be monitored to determine when the battery is outside a certain region, such as the 30% to 70% SOC region. If desired, the variation of the OCV versus SOC curve with respect to temperature and/or battery age can be taken into account during the determination of SOC in order to enhance the accuracy of such SOC determination.

Claims

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

generating a curve which expresses the relationship between a parameter and a state of charge (SOC) of a battery over a region of interest;

measuring a voltage and a current of the battery during a period of time to create a set of voltage and current data;

processing the set of voltage and current data to create a set of processed data;

regressing the set of processed data with respect to an equation which is representative of the battery to obtain a value of the parameter; and

comparing the value of the parameter to the curve in order to obtain an estimate of the state of charge of the battery.

2. The method according to claim 1,

wherein processing the set of voltage and current data comprises differentiating voltage and current data from the set of voltage and current data.

3. The method according to claim 1,

wherein processing the set of voltage and current data comprises integrating voltage and current data from the set of voltage and current data.

4. The method according to claim 1,

wherein processing the set of voltage and current data comprises integrating current data from the set of voltage and current data.

5. The method according to claim 1, wherein said curve is generated prior to normal operation of the battery.

6. The method according to claim 1, wherein said curve is generated during normal operation of the battery.

7. The method according to claim 1, wherein the battery is electrically isolated during the period of time.

8. The method according to claim 7, wherein the battery is cycled through a predetermined regime during the period of time.

9. The method according to claim 1, wherein said curve is stored in one or more look-up tables.

10. The method according to claim 1, wherein said curve is represented as a mathematical function.

11. The method according to claim 10, wherein the mathematical function is a straight line.

12. The method according to claim 1, wherein said region of interest is between about 20% state of charge and about 80% state of charge, wherein said method is utilized for estimating a state of charge of a battery which has a state of charge between about 20% state of charge and about 80% state of charge.

13. The method according to claim 1, wherein said region of interest is between about 30% state of charge (SOC) and about 70% SOC, wherein said method is utilized for estimating a SOC of a battery which has a state of charge between about 30% SOC and about 70% SOC.

14. The method according to claim 1, wherein said parameter is selected from the group consisting of:

open circuit voltage (OCV), dOCV/dSOC, d²OCV/dSOC², battery internal “

polarization”

resistance (R_P), dR_P/dSOC battery shunt capacitance around polarization resistance (C), battery internal “

ohmic”

resistance (R_O), and dR_O/dSOC.

15. The method according to claim 14, wherein said parameter is OCV.

16. The method according to claim 14, wherein said parameter is dOCV/dSOC.

17. The method according to claim 14, wherein said parameter is d²OCV/dSOC².

18. The method according to claim 14, wherein said parameter is R_P.

19. The method according to claim 14, wherein said parameter is R_O.

20. The method according to claim 14, wherein said parameter is dR_P/dSOC.

21. The method according to claim 14, wherein said parameter is dR_O/dSOC.

22. The method according to claim 1, wherein said battery is discharging during a first portion of said period of time.

23. The method according to claim 22, wherein said battery is charging during a second portion of said period of time, wherein said second portion is before or after said first portion.

24. The method according to claim 23, wherein said battery is at rest during a third portion of said period of time, wherein said third portion is before or after said first portion, and is before or after said second portion.

25. The method according to claim 1, wherein said equation is a generalized equation corresponding to the battery.

26. The method according to claim 25, wherein coefficients of the generalized equation are related to coefficients of an equation representing an equivalent circuit corresponding to the battery.

27. The method according to claim 10, wherein said parameter is d²OCV/dSOC² and said curve is represented as a straight line.

28. The method according to claim 27, wherein when d²OCV/dSOC² has a value of zero the state of charge of the battery is recalibrated to a known state of charge.

29. The method according to claim 28, wherein a value of zero occurs when d²OCV/dSOC² transitions from positive to negative or from negative to positive.

30. The method according to claim 1, wherein said battery is cycled through a predetermined regime during said period of time.

32. The method according to claim 1, wherein said battery is a NiMH battery.

33. The method according to claim 1, wherein said battery comprises one or more cells selected from the group consisting of a:

D cell, a C cell, and a sub C cell.

34. The method according to claim 1, wherein the voltage measurements and the current measurements are monitored in a continuous manner, and wherein said period of time is selected based on at least one predetermined criterium relating to the voltage measurements and the current measurements.

35. The method according to claim 34, wherein said period of time is less than or equal to 60 seconds.

36. The method according to claim 35, wherein one of said at least one predetermined criterium is at least 15 seconds of discharging during said period of time.

37. The method according to claim 36, wherein another of said at least one predetermined criterium is at least 15 seconds of charging during said period of time.

38. The method according to claim 1, wherein the variation of the curve with respect to one or more of the following is taken into account during comparing the value of the parameter to the curve:

battery temperature, battery age, internal pressure, and charging/discharging rate.

39. The method according to claim 26, wherein the equation representing the equivalent circuit is a function of the voltage and the current of the battery and derivatives and integrals of the voltage and the current of the battery.

40. The method according to claim 25, wherein the equation representing the equivalent circuit has the following form:

$0=a+b<br><br>\int <br><br>I_L<br><br><br><br>t+{\mathrm{cI}}_L+d<br><br>\frac{<br><br>I_L}{<br><br>t}+e<br><br>\frac{<br><br>V}{<br><br>t}+\mathrm{fV}+g<br><br>\int <br><br>V<br><br><br><br>t+h<br><br>\frac{{<br><br>}^2<br><br>I_L}{<br><br>t^2}+\mathrm{\dots <br><br>}<br><br>\text{\hspace{1em}<br><br>}.$

41. The method according to claim 25, wherein the equation representing the equivalent circuit includes square or higher power terms.

42. The method according to claim 26, wherein the equivalent circuit is as shown in FIG. 5B.

43. The method according to claim 26, wherein the equivalent circuit is as shown in FIG. 6.

31. The method according to claim 31, wherein said predetermined regime is the HPPC dynamic load profile.

44. A method for recalibrating a state of charge of a battery, comprising:

determining from the relationship between a parameter and a state of charge of a battery a recalibration state of charge of the battery at which the relationship between the parameter and the state of charge of the battery has an inflection point;

measuring a voltage and a current of the battery during a period of time to create a set of voltage and current data;

processing the set of voltage and current data to create a set of processed data;

regressing the set of processed data with respect to an equation which is representative of the battery to obtain a value of a second derivative of the parameter; and

recalibrating the state of charge of the battery to the recalibration state of charge of the battery when the second derivative of the parameter has a value of zero.

45. The method according to claim 44, wherein a value of zero occurs when the second derivative of the parameter transitions from positive to negative or from negative to positive.

46. The method according to claim 44, wherein said parameter is an open circuit voltage (OCV) of the battery.