Enhanced voltage-based fuel gauges and methods

• US 8,203,305 B1
• Filed: 07/02/2008
• Issued: 06/19/2012
• Est. Priority Date: 07/02/2008
• Status: Active Grant

• Current Assignee: Maxim Integrated Products, Inc. (Analog Devices, Inc.)
• Original Assignee: Maxim Integrated Products, Inc. (Analog Devices, Inc.)
• Inventors: Wortham, Jason Allen, Ghaseminejad, Parviz
• Primary Examiner(s): Fantu, Yalkew
• Application Number:
  US12/166,584
• Time in Patent Office: 1,448 Days
• Field of Search: 320/110
• US Class Current: 320/110
• CPC Class Codes: G01R 31/367 Software therefor, e.g. for battery testing using modelling or look-up tables
• CPC Class Codes: H01M 10/44 Methods for charging or discharging circuits for charging H02J7/00
• CPC Class Codes: H01M 10/48 Accumulators combined with arrangements for measuring, testing or indicating the condition of cells, e.g. the level or density of the electrolyte constructional details of current conducting connections for detecting conditions inside cells or batteries, e.g. details of voltage sensing terminals, H01M50/569
• CPC Class Codes: H01M 50/569 Constructional details of current conducting connections for detecting conditions inside cells or batteries, e.g. details of voltage sensing terminals battery terminal connectors with integrated measuring arrangements G01R31/364
• CPC Class Codes: Y02E 60/10 Energy storage using batteries

Abstract

Enhanced voltage-based fuel gauges and methods that increase the accuracy of voltage-based fuel gauges and allow the use of voltage-based fuel gauges to detect current, and particularly excessive current from a battery without the use of a sense resistor. When used with a coulomb counter, the outputs of a voltage-based fuel gauge and a coulomb counter may be combined in a manner that allows the combination to provide better performance that either alone may provide. Various embodiments and methods of operation are disclosed.

Claims

1. A method of monitoring current delivered by a battery comprising:

providing a model of a battery circuit that includes the battery dynamic electrical characteristics;

monitoring the battery circuit terminal voltage;

estimating the open circuit voltage of the battery circuit using the battery circuit model and the battery circuit terminal voltage, including during the application of a load on the battery circuit; and

,

using the battery circuit model, estimating the current being delivered by the battery circuit responsive to the estimated open circuit voltage of the battery circuit and the battery circuit terminal voltage.

2. The method of claim 1 wherein the battery circuit consists of the battery.

3. The method of claim 1 wherein excessive current in the battery circuit is detected.

4. The method of claim 1 wherein the current being delivered by the battery circuit is estimated by the difference between the estimated open circuit voltage of the battery circuit and the battery circuit voltage.

5. The method of claim 1 wherein the battery circuit model is a single pole RC model.

6. The method of claim 5 wherein the time constant of the RC model is determined by applying a constant charging voltage to the battery circuit and determining the time constant of the decay of the charging current with time.

7. The method of claim 6 wherein the value of R in the RC model is determined from the time constant using the value of C for the present state of charge of the battery circuit.

8. The method of claim 1 wherein the battery circuit model is a multiple pole RC model.

9. The method of claim 8 wherein the battery circuit has multiple time constants, and wherein the longest time constant of the RC model is determined by applying a constant charging voltage to the battery circuit and determining the longest time constant of the decay of the charging current with time.

10. The method of claim 9 wherein the value of R is determined from the longest time constant using the value of C for that time constant and for the state of charge of the battery circuit.

11. The method of claim 1 wherein:

the battery circuit model is a single pole RC model characterized by the equation OCV_n+1=OCV_n·

e<sup>−

tcalc/τ</sup>

+V_CELL·

(1−

e<sup>−

t/τ</sup>

), where OCV is the open circuit voltage of the battery circuit, t_calc is the time between calculations to update a value of open circuit voltage OCV_n of the battery circuit to a new value OCV_n+1, τ

is the RC time constant of the RC model and V_cell is the terminal voltage of the battery circuit; and

,

the time between calculations

12. The method of claim 11 wherein the time constant of the RC model is determined by applying a constant charging voltage to the battery circuit and determining the time constant of the decay of the charging current with time using the value of C for the present state of charge of the battery circuit to determine R of the RC model.

13. The method of claim 12 wherein the time between calculations t_calc is set by two cascaded timers.

14. The method of claim 1 wherein:

the battery circuit model is defined or approximated by equations having the open circuit voltage OCV_n+1 on each node in the battery circuit model a function of the prior open circuit voltage OCV_n on that node and the prior open circuit voltage on at least one other node of the battery circuit model, or the terminal voltage of the battery circuit, or both at least one other node of the battery circuit model and the terminal voltage of the battery circuit, the equations also having a plurality of exponential terms, each in the form e<sup>−

t/τ

n</sup>, where t is time and τ

_n is one of multiple time constants 1 through n; and

,

the time between calculations for each node, is

15. The method of claim 14 wherein the largest time constant of the battery circuit model is determined by applying a constant charging voltage to the battery circuit and determining the time constant of the decay of the charging current with time using the value of C for the present state of charge of the battery circuit.

16. The method of claim 1 further comprised of executing the method simultaneously for models of battery circuits having batteries of differing chemistries to determine which chemistry battery the method is being practiced on.

17. A method of improving performance of a battery fuel gauge comprising:

providing a voltage based fuel gauge by;

modeling a battery circuit by a battery circuit model that includes the battery dynamic electrical characteristics;

monitoring the battery circuit terminal voltage;

estimating the open circuit voltage of the battery circuit using the battery circuit model and the battery circuit terminal voltage, including during the application of a load on the battery circuit;

estimating the state of charge of the battery based on the estimated open circuit voltage of the battery circuit;

providing a coulomb counter to monitor current into and out of the battery circuit, and;

determining the remaining capacity of the battery using a combination of the state of charge determined by the voltage based fuel gauge and an output of the coulomb counter.

18. The method of claim 17 wherein the battery circuit consists of a battery.

19. The method of claim 17 wherein the battery circuit comprises the battery and a sense resistor for the coulomb counter.

20. The method of claim 17 further comprising determining the capacity of the battery by determining the ratio of a net change in amp hours as determined by the coulomb counter and a net change in the state of charge as determined by the voltage based fuel gauge.

21. The method of claim 20 further comprising repetitively mixing the remaining capacity of the battery, as determined by using a combination of the state of charge determined by the voltage based fuel gauge and an output of the coulomb counter, with a remaining capacity of the battery as determined from the state of charge and battery capacity, whereby the output of the coulomb counter will dominate the change in the remaining capacity of the battery at high battery currents and the output of the voltage based fuel gauge will dominate the change in remaining capacity of the battery at low and no battery currents.

22. The method of claim 21 further comprised of repetitively determining the battery capacity and filtering the successive results.

23. The method of claim 17 further comprising comparing the coulomb-counter dQ to the change in the voltage fuel-gauge state of charge to determine any nonlinearity in the voltage fuel-gauge, where dQ is an integral of current over time through the coulomb-counter.

24. A method of improving performance of a battery fuel gauge comprising:

providing a voltage based fuel gauge that monitors a terminal voltage of a battery to determine the battery'"'"'s state of charge;

providing a coulomb counter to provide a measure of current into and out of the battery to current;

determining a first remaining charge in the battery using the product of a full capacity of the battery times the state of charge as determined by the voltage based fuel gauge;

determining a second remaining charge in the battery using a combination of the first remaining charge in the battery and the current into and out of the battery; and

repeatedly correcting the second remaining charge by continually subtracting a value responsive to the second remaining charge from the first remaining charge to continually obtain an updated second remaining charge.

25. The method of claim 24 wherein:

the coulomb counter also provides a measure of the accumulated current times time into and out of the battery, and further comprises determining a full capacity of the battery using the ratio of a difference in accumulated current into and out of the battery times time divided by the change in state of charge indicated by the voltage based fuel gauge.

26. The method of claim 25 wherein the currents into and out of the battery are converted to counts, and the measure of current times time into and out of the battery is maintained as a total count responsive to the count of the current into the battery and the count of the current out of the battery.

27. The method of claim 26 wherein the current into and out of the battery is provided by a count responsive to the current into or out of the battery.

28. The method of claim 25 wherein the method is practiced in one or more custom integrated circuits or in a microprocessor or microcontroller under software control.

29. The method of claim 24 wherein the voltage based fuel gauge uses a battery model comprising at least one resistor capacitor (RC) time constant.

30. The method of claim 29 wherein a range of state of charge of the battery is divided into multiple segments, and wherein the battery model uses a fixed resistor and a capacitor that is constant over a segment, but which varies from at least one segment to an adjacent segment.

31. A computer readable storage medium having stored thereon a sequence of instructions which, when executed by a computer, cause the computer to perform the following:

determine the battery'"'"'s state of charge from the terminal voltage of a battery using a voltage based fuel gauge;

determine a first remaining charge in the battery using the product of a full capacity of the battery times the state of charge as determined by the voltage based fuel gauge;

determine a second remaining charge in the battery using a combination of the first remaining charge in the battery and the current into and out of the battery from a coulomb counter; and

continually correct the second remaining charge by continuously subtracting a value responsive to the second remaining charge from the first remaining charge to continuously obtain an updated second remaining charge.

32. The computer readable storage medium of claim 31 wherein the sequence of instructions, when executed by a computer, further cause the computer to:

provide a measure of the accumulated current times time into and out of the battery, and

determine a full capacity of the battery using the ratio of a difference in accumulated current into and out of the battery times time divided by the state of charge indicated by the voltage based fuel gauge when the voltage based fuel gauge indicates a predetermined state of charge.

33. The computer readable storage medium of claim 31 wherein the voltage based fuel gauge uses a battery model comprising at least one resistor capacitor (RC) time constant.

34. The computer readable storage medium of claim 33 wherein a range of state of charge of the battery is divided into multiple segments, and wherein the battery model uses a fixed resistor and a capacitor that is constant over a segment, but which varies from at least one segment to an adjacent segment.