Battery charge measurement and discharge reserve time prediction technique and apparatus

A method of testing one or more cells and parameterising the results in order to obtain a characteristic curve/function from which cell discharge reserve time can be predicted from cell voltage. The test involves obtaining a plurality of data points representing the voltage of a cell as a function of charge remaining, and parameterising the data points to obtain a function representing cell voltage and charge remaining. The function allows charge remaining to be calculated from cell voltage. The invention also provides for a device for measuring capacity and predicting discharge reserve time of a cell, the device including a voltage and current measuring means adapted to measure the voltage and load current of a cell. The device also includes a timing means so that a number of voltage and current data points can be obtained with respect of time. The data points are parameterised and the device produces a function relating charge remaining to cell voltage whereby the charge remaining can be determined by measuring the cell voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described by way of example and with reference to the Figures in which:

FIG.

1: illustrates schematically a testing arrangement for strings of cells;

FIG.

2: illustrates a curve representing the charge remaining domain discharge characteristic;

FIGS. 3

a and

b: illustrate time and charge remaining domain discharge characteristics of six different cells of the same type;

FIGS. 4

a and

b: illustrate time and charge remaining domain discharge characteristics of different cells of the same type at different discharge rates;

FIGS. 5

a and

b: illustrate the effects of discharge rate switching during the discharge;

FIGS. 6

a and

b: illustrate time and charge remaining domain discharge characteristics of six different cells of the same type at different temperatures;

FIGS. 7

a and

b: illustrate time and charge remaining domain discharge characteristics of eight different cells of the same type with different discharge rate and temperature combinations;

FIGS. 8

a and

b: illustrate time and charge remaining domain discharge characteristics of four different cells of the same type with different charge levels;

FIG.

9: illustrates actual and measured charge remaining and measurement error for a new forty-eight volt string of cells discharged at a rate of 68 amps;

FIG.

10: illustrates the discharge characteristics of two cells, one complying and one not complying with the nominal characteristic ; and

FIG.

11: illustrates the ratio of actual and measured charge remaining for the compliant and noncompliant cells shown in FIG.

10.

1. A method of calculating the discharge reserve time of one or more test cells including the steps of:

2. A method as claimed in claim 1 wherein said test cells are different from said nominal cells.

3. A method as claimed in claim 1 wherein the discharge reserve time is calculated by dividing the charge remaining by a constant power discharge rate.

4. A method as claimed in claim 1 wherein the discharge reserve time is calculated by dividing the charge remaining by a constant current discharge rate.

5. A method as claimed in claim 1 wherein a fully charged cell or cells is/are subjected to a partial discharge, the charge released during the partial discharge being added to the charge remaining to obtain a measurement of capacity.

6. A method as claimed in claim 1 wherein a fully charged cell or cells is/are subjected to a partial discharge, the charge released during the partial discharge being added to the charge remaining to obtain a meausrement of capacity, and wherein the partial discharge is one that is long enough to avoid the Coup de Fouet region, but is much shorter than a complete discharge of the cell or cells.

7. A method as claimed in claim 1 wherein the data points are obtained by measuring the cell(s) voltage and current over specific time intervals.

8. A method as claimed in claim 1 wherein the parameterisation is effected by means of collecting data points equidistant in the voltage domain, a least squares fit, interpolation, extrapolation, or an analytical approach adapted to target the best fit to the characteristics.

9. A method as claimed in claim 1 wherein a minimum number of data points sufficient for a set level of measurement accuracy are obtained and parameterised.

10. A method as claimed in claim 1 wherein the data points are selected over intervals selected so as to minimise the errors inherent in the parameterisation process.

11. A method as claimed claim 1 wherein the steps are repeated due to changes in cell characteristic from ageing, environmental and usage conditions.

12. A method as claimed in claim 1 wherein the steps are repeated due to changes in cell characteristics from aging, environmental and usage conditions, and wherein the decision to repeat the steps is determined by comparison of the change in charge remaining measured for a cell(s) derived from a previous characterisation and the change in actual charge remaining of the cell(s).

13. A battery charge remaining and capacity measurement and discharge reserve time prediction device including:

14. A device as claimed in claim 13 which includes a microprocessor adapted to manipulate the voltage, time and current to provide data points representing the charge remaining as a function of voltage, wherein the charge remaining is expressed in amp/hours.

15. A device as claimed in claim 13 adapted to calculate the discharge rate of a cell or cells, the device using the charge remaining and discharge rate to determine the discharge reserve time, wherein the discharge rate is calculated for either constant power or constant current discharge, and the discharge reserve time is expressed in hours and fractions of an hour.

16. A device as claimed in claim 13 which further includes a discharge means, the discharge means being adapted to at least partially discharge the cell or cells and measure the charge released from said cell or cells during the discharge, the cell or cells capacity being derived from the charge released during the discharge and the charge remaining.

17. A device as claimed in claim 13 which includes an output means adapted to graphically, numerically or otherwise indicate, in real time, the charge remaining, capacity measurement and/or discharge reserve time of the cell or cells being measured.

18. A device as claimed in claim 13 adapted to, at the initiation of a user, measure the data points and effect the parameterisation automatically.

19. A device as claimed in claim 13 further adapted to include a means for sensing variations in the environmental conditions in which the cell or cells are used and further adapted so that, in response to predetermined criteria, the device remeasures the data points and establishes an updated parameterisation.

20. A device as claimed in claim 13 wherein the device outputs the charge remaining, capacity measurement and/or discharge reserve time of the cell or cells constantly.

21. A device as claimed in claim 13 wherein the charge remaining, capacity measurement and/or discharge reserve time may be output in response to a user activation or request.

22. A method of characterising one or more test cells including the steps of:

23. A method as claimed in claim 22 wherein a fully charged cell or cells is/are subjected to a partial discharge, the charge released during the partial discharge being added to the charge remaining to obtain a measurement of capacity.

24. A method as claimed in claim 22 wherein a fully charged cell or cells is/are subjected to a partial discharge, the charge released during the partial discharge being added to the charge remaining to obtain a measurement of capacity wherein the partial discharge is one that is long enough to avoid the Coup de Fouet region, but is much shorter than a complete discharge of the cell or cells.

25. A method as claimed in claim 22 wherein the data points are obtained by measuring the cell(s) voltage and current over specific time intervals.

26. A method as claimed in claim 22 wherein the parameterisation is effected by means of collecting data points equidistant in the voltage domain, a least squares fit, interpolation, extrapolation, or an analytical approach adapted to target the best fit to the characteristics.

27. A method as claimed in claim 22 wherein a minimum number of data points sufficient for a set level of measurement accuracy are obtained and parameterized.

28. A method as claimed in claim 22 wherein the data points are selected over intervals selected so as to minimise the errors inherent in the parameterisation process.

29. A method as claimed in claim 22 wherein the steps are repeated due to changes in cell characteristic from ageing, environmental and usage conditions.

30. A method as claimed in claim 22 wherein the steps are repeated due to changes in cell characteristic from ageing, environmental and usage conditions, and wherein the decision to repeat the steps is determined by comparison of the change in charge remaining measured for a cell(s) derived from a previous characterisation and the change in actual charge remaining of the cell(s).

31. A battery charge remaining and capacity measurement and discharge reserve time prediction device including:

32. A device as claimed in claim 31 which includes a microprocessor adapted to manipulate the voltage, time and current to provide data points representing the charge remaining as a function of voltage, wherein the charge remaining is expressed in amp/hours.

33. A device as claimed in claim 31 which further includes a discharge means, the discharge means being adapted to at least partially discharge the cell or cells and measure the charge released from said cell or cells during the discharge, the cell or cells capacity being derived from the charge released during the discharge and the charge remaining.

34. A device as claimed in claim 31 which includes an output means adapted to graphically, numerically or otherwise indicate, in real time, the charge remaining, capacity measurement and/or discharge reserve time of the cell or cells being measured.

35. A device as claimed in claim 31, adapted to, at the initiation of a user, measure the data points and effect the parameterisation automatically.

36. A device as claimed in claim 31 further adapted to include a means for sensing variations in the environmental conditions in which the cell or cells are used and further adapted so that, in response to predetermined criteria, the device remeasures the data points and establishes an updated parameterisation.

37. A device as claimed in claim 31 wherein the device outputs the charge remaining, capacity measurement and/or discharge reserve time of the cell or cells constantly.

38. A device as claimed in claim 31 wherein the charge remaining, capacity measurement and/or discharge reserve time may be output in response to a user activation or request.