Method for determining state of charge of lead-acid batteries of various specific gravities

• US 20060170426A1
• Filed: 01/31/2005
• Published: 08/03/2006
• Est. Priority Date: 01/31/2005
• Status: Active Grant

• Current Assignee: Honeywell International Inc.
• Original Assignee: Honeywell International Inc.
• Inventors: Hoeing, Steven, Palanisamy, Thirumalai G., Singh, Harmohan N., Atehortua, Hector M.
• Granted Patent: US 7,429,436 B2
• Time in Patent Office: Days
• US Class Current: 324/429
• CPC Class Codes: G01R 31/367 Software therefor, e.g. for battery testing using modelling or look-up tables
• CPC Class Codes: G01R 31/379 for lead-acid batteries
• CPC Class Codes: H01M 10/06 Lead-acid accumulators semi-lead accumulators H01M10/20
• CPC Class Codes: H01M 10/42 Methods or arrangements for servicing or maintenance of secondary cells or secondary half-cells H01M10/60 takes precedence
• CPC Class Codes: Y02E 60/10 Energy storage using batteries

Abstract

In accordance with various embodiments, there is a method for determining the state of charge of a battery. Various embodiments include the steps of determining the specific gravity of the battery and measuring an open circuit voltage of the battery at rest. The open circuit voltage at rest can be used to determine the battery state of charge from a correlation function dependent on the battery specific gravity.

Claims

1. A method of determining a state of charge (SOC) of a lead-acid battery comprising:

determining a specific gravity of a lead-acid battery;

determining an open circuit voltage of the lead-acid battery at a rested state; and

determining the SOC of the lead-acid battery from the open circuit voltage using a correlation function, wherein the correlation function depends on the specific gravity of the lead-acid battery.

2. The method of claim 1, wherein the step of determining a specific gravity of the lead-acid battery comprises:

applying a current ramp to the lead-acid battery and measuring a battery voltage and detecting gas points;

calculating a maximum voltage per cell by dividing a maximum measured battery voltage by a number of cells of the lead-acid battery; and

determining the specific gravity of the lead-acid battery when one or more gas points are detected by comparing the maximum voltage per cell to an SG_Lim,

wherein the SG_Lim is an upper limit of a maximum measured battery voltage for a low specific gravity battery divided by a number of cells of the low specific gravity battery, and

wherein the maximum voltage per cell being less than the SG_Lim indicates a lower specific gravity lead-acid battery.

3. The method of claim 2, wherein the maximum voltage per cell being greater than the SG_Lim indicates a higher specific gravity lead-acid battery.

4. The method of claim 2, wherein applying the current ramp comprises:

applying a linearly increasing current until the battery voltage reaches a predetermined limit; and

applying a linearly decreasing current after reaching the predetermined limit.

5. The method of claim 1, wherein the step of determining a specific gravity of the lead-acid battery comprises removing a portion of an electrolyte from the lead-acid battery and measuring the specific gravity of the electrolyte.

6. The method of claim 1, wherein the specific gravity is about 1.28 and wherein the correlation function is:

SOC=((OCV per cell/0.20)−

9.55)×

100.

7. The method of claim 1, wherein the specific gravity is about 1.30 and wherein the correlation function is:

SOC_H=((OCV per cell/0.12)−

16.75)×

100.

8. The method of claim 1, further comprising determining the correlation function by applying a charge/discharge cycle to the lead-acid battery comprising:

discharging the lead-acid battery to a predetermined cutoff voltage;

charging the lead-acid battery to a plurality of predetermined state of charge intervals;

measuring the rested state open circuit voltage of the lead-acid battery during charging at each of the plurality of predetermined state of charge intervals;

discharging the lead-acid battery to the plurality of predetermined state of charge intervals; and

measuring the rested state open circuit voltage during discharging of the lead-acid battery at each of the plurality of predetermined state of charge intervals.

9. The method of claim 8, wherein the plurality of predetermined state of charge intervals is determined by dividing a battery capacity by an integer.

10. The method of claim 1, wherein the rested state is a period of inactivity of at least 30 minutes or more.

11. An apparatus for determining a state of charge (SOC) of a lead-acid battery comprising:

a power supply;

a control module comprising an analog-to-digital converter, a digital-to-analog converter, and an electronics control system, wherein the control module controls the power supply to apply a current to a battery;

a control relay electrically connected to the power supply;

a sensor that detects at least one of a battery voltage response to a current ramp and an open circuit voltage at a rested state, the sensor electrically connected to the control relay; and

a processor configured to determine a specific gravity of the battery based on a maximum battery voltage detected in the battery voltage response and configured to determine the SOC of the lead-acid battery based on the rested state open circuit voltage and the specific gravity of the lead-acid battery.

12. The apparatus of claim 11, wherein the processor determines the specific gravity of the lead-acid battery by comparing the maximum battery voltage detected in the battery voltage response divided by a number of cells of the battery, to an SG_Lim, where the SG_Lim is an upper limit of the maximum measured battery voltage for a low specific gravity battery divided by the number of cells of the low specific gravity battery.

13. The apparatus of claim 11, wherein the control module controls application of the current ramp to the lead-acid battery.

14. The apparatus of claim 11, wherein the current ramp comprises:

a linearly increasing current to a predetermined limit; and

a linearly decreasing current after reaching the predetermined limit.

15. The apparatus of claim 11, wherein the control module controls application of a charge/discharge cycle to the lead-acid battery.

16. The apparatus of claim 11, wherein the charge/discharge cycle comprises:

discharging the lead-acid battery to a predetermined cutoff voltage;

charging the lead-acid battery to a plurality of predetermined state of charge intervals;

measuring the rested state open circuit voltage of the lead-acid battery during charging at each of the plurality of predetermined state of charge intervals;

discharging the lead-acid battery to the plurality of predetermined state of charge intervals; and

measuring the rested state open circuit voltage during discharging of the lead-acid battery at each of the plurality of predetermined state of charge intervals.

17. A computer readable medium for determining a state of charge of a battery, the computer readable medium comprising:

program code for applying a current to a battery at a rested state;

program code for measuring an open circuit voltage in response to the current; and

program code for determining the state of charge from the open circuit voltage and a correlation function, wherein the correlation function depends on the specific gravity of the lead-acid battery.

18. The computer readable medium of claim 17, wherein the specific gravity is about 1.28 and wherein the correlation function is:

SOC=((OCV per cell/0.20)−

9.55)×

100.

19. The computer readable medium of claim 17, wherein the specific gravity is about 1.30 and wherein the correlation function is:

SOC_H=((OCV per cell/0.12)−

16.75)×

100.

20. The computer readable medium of claim 17, further comprising:

program code for applying a current ramp to the battery;

program code for measuring a battery voltage response;

program code for detecting gas points;

program code for calculating a maximum voltage per cell of the battery; and

program code for comparing a maximum battery voltage detected in the battery voltage response divided by the number of cells of the battery, to an SG_Lim, where the SG_Lim is an upper limit of a maximum measured battery voltage of a low specific gravity battery divided by the number of cells of the low specific gravity battery.

21. The computer readable medium of claim 20, wherein the program code for applying the current ramp to the battery comprises program code for applying:

a linearly increasing current to a predetermined limit; and

a linearly decreasing current after reaching the predetermined limit.

22. The computer readable medium of claim 17, further comprising program code for indicating the specific gravity of the battery.

23. The computer readable medium of claim 17, further comprising program code for indicating the state of charge of the battery.