Circuit and method for monitoring battery state of charge

• US 20040128086A1
• Filed: 05/02/2003
• Published: 07/01/2004
• Est. Priority Date: 12/29/2002
• Status: Active Grant

• Current Assignee: Texas Instruments, Inc.
• Original Assignee: Texas Instruments, Inc.
• Inventors: Freeman, David L., Barsoukov, Evgenij, Poole, Dan R.
• Granted Patent: US 6,789,026 B2
• Time in Patent Office: Days
• US Class Current: 702/63
• CPC Class Codes: G01R 31/367 Software therefor, e.g. for battery testing using modelling or look-up tables
• CPC Class Codes: G01R 31/3842 combining voltage and current measurements

Abstract

A processor (13) operates to determine amount of charge presently stored in a battery by determining that the battery is in a zero-current relaxed condition. Circuitry (16,23) including a first ADC (16) measures an open circuit voltage (OCV) of the battery prior to a period of time during which flow of current through the battery is not negligible. A program executed by the processor correlates the measured open circuit voltage (OCV0) with a corresponding value of the variable and selects the corresponding value as a value of the variable.

Claims

1. A method of determining a value of a variable representing the amount of charge presently stored in a battery, comprising:

(a) determining that the battery is in a zero-current relaxed condition by establishing that no more than a negligible amount of external current is flowing through the battery and that a sufficient amount of time has passed since a last significant current flow through the battery to ensure that the battery voltage is no longer significantly changing;

(b) measuring an open circuit voltage (OCV) of the battery prior to a period of time during which flow of current through the battery is not negligible;

(c) using a predetermined dependents of the variable on the open circuit voltage of the battery to obtain a value of the variable which corresponds to the measured open circuit voltage.

2. The method of claim 1 including measuring current flowing through the battery at a moment in an active period during which current flowing through the battery is not negligible, and calculating the value of the variable at that moment using the measured current and an earlier value of the variable determined immediately prior to re-connecting a load or a charger.

3. The method of claim 2 including calculating the value of the variable by integrating the current during the active period and modifying the earlier value of the variable by a value of charge passed through the battery during the active period divided by a value of total no-load capacity of the battery.

4. The method of claim 1 wherein the variable is the state of charge (SOC) of the battery.

5. The method of claim 1 wherein the variable is the depth of discharge (DOD) of the battery.

6. The method of claim 1 wherein step (a) includes determining that a predetermined amount of time has elapsed since the last significant flow of current through the battery.

7. The method of claim 6 wherein the predetermined amount of time is at least 10 minutes.

8. The method of claim 1 wherein sufficiently low average current flows through the battery during the OCV measurement so as to cause a voltage drop or voltage rise of less than 1% of the total no-load voltage of the battery due to internal impedance of the battery.

9. The method of claim 1 including performing a correction of the measured voltage of the battery to compensate for a voltage drop due to the internal impedance of the battery while a nearly negligible amount of current is flowing through the battery in order to obtain the open circuit voltage.

10. The method of claim 1 including detecting the rate of change of the open circuit voltage of the battery to determine when to perform step (b).

11. The method of claim 5 including storing a database in a memory, data of the database representing a relationship between the OCV of the battery and the DOD of the battery, wherein step (c) includes utilizing the database to correlate the measured open circuit voltage with a corresponding value of the DOD.

12. The method of claim 11 including monitoring the temperature of the battery by means of a temperature sensor in thermal contact with the battery, the database including data representing corresponding values of the OCV of the battery and DOD of the battery as K values of OCV of the battery at known corresponding values of DOD of the battery at each of N temperatures T1, T2 . . . TN, the database also including K intercepts A[k] and K slopes B[k] defining K equations V[k](T)=A[k]+B[k]*T, wherein k is an index and T is a variable representing the temperature of the battery, and the values A[k] and B[k] have been obtained by linear regression of the OCV and DOD values, the method including operating a processor to execute a first algorithm utilizing a table search algorithm and a root-finding algorithm in conjunction with the database to determine values of DOD corresponding to values of OCV measured from the battery.

13. The method of claim 12 including operating the processor to execute a second algorithm to determine a value of total run-time that would be required for a load, when operatively connected to the battery, to reduce the open circuit voltage of the battery to a predetermined lower limit.

14. The method of claim 13 wherein the second algorithm also determines a remaining run-time of the load operatively connected to the battery by integrating a load current supplied by the battery to the load to determine a net transfer of charge from the battery and a duration of the integrating and determines the remaining run-time by subtracting the duration of the integrating from the total run-time.

15. The method of claim 13 wherein the second algorithm determines the total run-time by iterating a variable t in the equation

V(DOD0+t*I/Qmax)=OCV(DOD0+t*I/Qmax)−

I*R_battery(DOD0+t*I/Qmax)

to a value that causes OCV(DOD0+t*I/Qmax)−

I*R_battery(DOD0+t*I/Qmax) to be equal to the predetermined lower limit and using that value as the total run-time, where Qmax is a total zero-current capacity of the battery, I is the average current supplied by the battery to the load during the active phase while the load is operatively connected to the battery, DOD0 is a depth of discharge corresponding to the measured open circuit voltage, V(DOD0+t*I/Qmax) is the voltage between terminals of the battery as a function of DOD0+t*I/Qmax, and R_battery(DOD0+t*I/Qmax) is the internal resistance of the battery as a function of DOD0+t*I/Qmax.

16. The method of claim 13 wherein the second algorithm determines the total run-time by iterating a variable t in the equation

V(DOD0+t*I/Qmax,T(t))=OCV(DOD0+t*I/Qmax,T(t))−

I*R_battery(DOD0+t*I/Qmax,T(t))

to a value that causes OCV(DOD0+t*I/Qmax,T(t))−

I*R_battery(DOD0+t*I/Qmax,T(t)) to be equal to the predetermined lower limit and using that value as the total run-time, where Qmax is a total zero-current capacity of the battery, I is the average current supplied by the battery to the load during active phase, DOD0 is a depth of discharge corresponding to the measured open circuit voltage, V(DOD0+t*I/Qmax,T(t)) is the voltage between terminals of the battery as a function of DOD0+t*I/Qmax and predicted temperature dependence on time T(t), and R_battery(DOD0+t*U/Qmax, T(t)) is the internal resistance of the battery as a function of DOD0+t*I/Qmax and predicted temperature dependence on time T(t).

17. A system for determining a value of variable representing the amount of charge presently stored in a battery, comprising:

(a) a processor coupled to a digital bus, and a memory for storing data representing the relationship between the OCV (open circuit voltage) and DOD (depth of discharge) of the battery, a first ADC, and a second ADC all coupled to the digital bus;

(b) a first program stored in the memory for execution by the processor for determining that the battery is in a zero-current relaxed condition by establishing that no more than a negligible amount of external current is flowing through the battery and that a sufficient amount of time has passed since a last significant current flow through the battery to ensure that the battery voltage is no longer significantly changing;

(c) circuitry including the first ADC for measuring an open circuit voltage (OCV) of the battery prior to a period of time during which flow of current through the battery is not negligible; and

(d) a second program stored in the memory for execution by the processor for correlating the measured open circuit voltage with a corresponding value of the variable and selecting the corresponding value as a value of the variable.

18. The system of claim 17 wherein the first program causes the processor to measure current flowing through the battery at a moment in an active period during which current flowing through the battery is not negligible, and the second program causes the processor to calculate the value of the variable at that moment using the measured current and an earlier value of the variable determined immediately prior to re-connecting a load or a charger.

19. The system of claim 18 wherein the second program causes the processor to calculate the value of the variable by integrating the current during the active period and modifying the earlier value of the variable by a value of charge passed through the battery during the active period divided by a value of total no-load capacity of the battery.

20. The system of claim 17 wherein the variable is the state of charge (SOC) of the battery.

21. The system of claim 17 wherein the first program causes the processor to determine that the battery is in a zero-current relaxed condition by determining that a predetermined amount of time has elapsed since the last significant flow of current through the battery.

22. The system of claim 19 wherein the second program causes the processor to perform a correction of the measured voltage of the battery to compensate for a voltage drop due to the internal impedance of the battery while a nearly negligible amount of current is flowing through the battery in order to obtain the open circuit voltage.

23. The system of claim 17 wherein the first program causes the processor to determine that the battery is in a zero-current relaxed condition by detecting the rate of change of the open circuit voltage of the battery.

24. The system of claim 17 wherein the second program accesses the stored data to correlate the measured open circuit voltage with a corresponding value of the DOD.

25. The system of claim 17 including a temperature sensor in thermal contact with the battery and a third ADC coupled to the digital bus.

26. The system of claim 25 wherein the processor monitors the temperature of the battery by means of a temperature sensor in thermal contact with the battery, and the stored data includes data representing corresponding values of the OCV of the battery and DOD of the battery as K values of OCV of the battery at known corresponding values of DOD of the battery at each of N temperatures T1, T2 . . . TN, the database also including K intercepts A[k] and K slopes B[k] defining K equations V[k](T)=A[k]+B[k]*T, wherein k is an index and T is a variable representing the temperature of the battery, and the values A[k] and B[k] have been obtained by linear regression of the OCV and DOD values, the processor operating to execute a first algorithm utilizing a table search algorithm and a root-finding algorithm in conjunction with the database to determine values of DOD corresponding to values of OCV measured from the battery.

27. The system of claim 26 including operating the processor to execute a second algorithm to determine a value of total run-time that would be required for a load, when operatively connected to the battery, to reduce the open circuit voltage of the battery to a predetermined lower limit.

28. The system of claim 27 wherein the second algorithm also determines a remaining run-time of the load operatively connected to the battery by integrating a load current supplied by the battery to the load to determine a net transfer of charge from the battery and a duration of the integrating and determines the remaining run-time by subtracting the duration of the integrating from the total run-time.

29. The system of claim 27 wherein the second algorithm determines the total run-time by iterating a variable t in the equation

V(DOD0+t*I/Qmax)=OCV(DOD0+t*I/Qmax)−

I*R_battery(DOD0+t*I/Qmax)

to a value that causes OCV(DOD0+t*I/Qmax)−

I*R_battery(DOD0+t*I/Qmax) to be equal to the predetermined lower limit and using that value as the total run-time, where Qmax is a total zero-current capacity of the battery, I is the average current supplied by the battery to the load during the active phase while the load is operatively connected to the battery, DOD0 is a depth of discharge corresponding to the measured open circuit voltage, V(DOD0+t*I/Qmax) is the voltage between terminals of the battery as a function of DOD0+t*I/Qmax, and R_battery(DOD0+t*I/Qmax) is the internal resistance of the battery as a function of DOD0+t*I/Qmax.

30. The system of claim 27 wherein the second algorithm determines the total run-time by iterating a variable t in the equation

V(DOD0+t*I/Qmax,T(t))=OCV(DOD0+t*I/Qmax,T(t))−

I*R_battery(DOD0+t*I/Qmax,T(t))

to a value that causes OCV(DOD0+t*I/Qmax,T(t))−

I*R battery(DOD0+t*I/Qmax,T(t)) to be equal to the predetermined lower limit and using that value as the total run-time, where Qmax is a total zero-current capacity of the battery, I is the average current supplied by the battery to the load during active phase, DOD0 is a depth of discharge corresponding to the measured open circuit voltage, V(DOD0+t*I/Qmax,T(t)) is the voltage between terminals of the battery as a function of DOD0+t*I/Qmax and predicted temperature dependence on time T(t), and R_battery(DOD0+t*I/Qmax, T(t)) is the internal resistance of the battery as a function of DOD0+t*I/Qmax and predicted temperature dependence on time T(t).

31. A system for determining a value of variable representing the amount of charge presently stored in a battery, comprising:

(a) means for determining that the battery is in a zero-current relaxed condition by establishing that no more than a negligible amount of external current is flowing through the battery and that a sufficient amount of time has passed since a last significant current flow through the battery to ensure that the battery voltage is no longer significantly changing;

(b) means for measuring an open circuit voltage (OCV) of the battery prior to a period of time during which flow of current through the battery is not negligible;

(c) means for correlating the measured open circuit voltage with a corresponding value of the variable; and

(d) means for selecting the corresponding value as the value of variable.

32. The system of claim 31 including means for measuring current flowing through the battery at a moment in an active period during which current flowing through the battery is not negligible, and means for calculating the value of the variable at that moment using the measured current and an earlier value of the variable determined immediately prior to re-connecting a load or a charger.

33. The system of claim 32 including means for calculating the value of the variable by integrating the current during the active period and modifying the earlier value of the variable by a value of charge passed through the battery during the active period divided by a value of total no-load capacity of the battery.

34. The system of claim 31 wherein the variable is the state of charge (SOC) of the battery.

35. The system of claim 31 wherein sufficiently low average current flows during the OCV measurement so as to cause a voltage drop or voltage rise of less than 1% of the total no-load voltage of the battery due to internal impedance of the battery.

36. The system of claim 31 including means for performing a correction of the measured voltage of the battery to compensate for a voltage drop due to the internal impedance of the battery in order to obtain the open circuit voltage while a nearly negligible amount of current is flowing through the battery.

37. The system of claim 35 including means for storing a database in a memory, data of the database representing a relationship between the OCV of the battery and the DOD of the battery, and means for utilizing the database to correlate the measured open circuit voltage with a corresponding value of the DOD.

38. The system of claim 37 including means for monitoring the temperature of the battery, the database including data representing corresponding values of the OCV of the battery and DOD of the battery as K values of OCV of the battery at known corresponding values of DOD of the battery at each of N temperatures T1, T2 . . . TN, the database also including K intercepts A[k] and K slopes B[k] defining K equations V[k](T)=A[k]+B[k]*T, wherein k is an index and T is a variable representing the temperature of the battery, and the values A[k] and B[k] have been obtained by linear regression of the OCV and DOD values, and a processor configured to execute a first algorithm utilizing a table search algorithm and a root-finding algorithm in conjunction with the database to determine values of DOD corresponding to values of OCV measured from the battery.