Method for monitoring remaining battery capacity

• US 5,808,445 A
• Filed: 12/06/1996
• Issued: 09/15/1998
• Est. Priority Date: 12/06/1995
• Status: Expired due to Fees

• Current Assignee: University of Virginia
• Original Assignee: University of Virginia Patent Foundation (University of Virginia)
• Inventors: Lepsch, Mark A., Aylor, James H.
• Primary Examiner(s): TSO, EDWARD H
• Application Number:
  US08/760,956
• Time in Patent Office: 648 Days
• Field of Search: 320/5, 320/29, 320/30, 320/34, 320/43, 320/44, 320/48, 320/132, 320/DIG. 21, 324/427, 324/432, 324/433, 340/636, 429/90
• US Class Current: 320/132
• CPC Class Codes: G01R 31/379 for lead-acid batteries
• CPC Class Codes: G01R 31/3828 using current integration
• 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: H02J 7/00041 in response to measured battery parameters, e.g. voltage, current or temperature profile
• CPC Class Codes: H02J 7/00047 with provisions for charging different types of batteries
• CPC Class Codes: H02J 7/00714 in response to battery charging or discharging current
• CPC Class Codes: H02J 7/007182 in response to battery voltage
• CPC Class Codes: Y02E 60/10 Energy storage using batteries
• CPC Class Codes: Y10S 320/21 State of charge of battery

Abstract

A complete energy management system for lead-acid batteries. The system is designed for deep discharge batteries in traction vehicle type applications such as electric wheelchairs, electric vehicles, golf carts, or industrial equipment, where more efficient use of lead-acid batteries during both the charging and discharging cycle is needed. This system utilizes current measurements, open circuit voltage measurements during the recovery from charging or discharging, and an adaptation calculation to account for variations in physical battery characteristics, battery treatment, battery environment, and battery aging.

Claims

1. The method of producing a predetermined amount of overcharging performed while replenishing a full charge to lead-acid batteries, comprising the steps of:

a. obtaining a coulometric summation during battery operation, with open circuit voltage measurements taken during the first few minutes of said battery'"'"'s recovery from use, and

b. obtaining open circuit voltage measurements during an initial period of recovery of the battery, and

c. adaptively learning said battery'"'"'s size and conditions, for tracking by storing historic operational data in computer memory and calculating from stored data, an adaptation factor, thereby rendering said tracking essentially independent of variations in battery treatment during said tracking, including rest period durations and frequency, cycling frequency, discharge patterns, and battery environment including temperature variations and degree of agitation of said battery,

d. visually displaying at least one of the state of charge, remaining ampere hours, time remaining to discharge of full useful current at the current discharge rate, miles remaining to discharge of full useful current, based on the discharge rate,

e. providing a warning signal which is at least one of visual or audio signal, when the state of charge is less than a predetermined minimum level, as determined by steps (a), (b) and (c), whereby said overcharging is sufficient to produce sufficient gassing to destratify the battery, but below the level which deteriorates the battery beyond a predetermined acceptable limit.

2. The method of claim 1, wherein said discharge rate of step (d) is the current discharge rate.

3. The method of claim 1, wherein said discharge rate of step (d) includes an average of the prior discharge rate.

4. The method of claim 1, wherein said coulometric summation comprises essentially continual measurement of the battery current flow and the duration of discharge and the multiplying of said current and duration of discharge, whereby the removed current capacity is obtained.

5. The method of claim 4, wherein the determination of the removed current capacity is adjusted inversely, relative to the measured discharge rate.

6. The method of claim 5, further comprising obtaining consecutive open circuit voltage recovery calculations, said consecutive reading having intervals in which at least about 5% of the battery capacity has be removed, determining an error factor in accordance with the formula, ##EQU10## wherein state of charge is the state of charge EF is the error factor,

SOC is the state of charge,

ocv is the open circuit voltage,

SOC_isum is the difference between removed capacity and the remaining capacity,

SOC_ocv is state of charge defined with respect to the percentage of sulfuric acid in the electrolytic solution when battery is at is under no load,

SOC_ocvprev is previous state of charge, open circuit determination.

7. The method of claim 6, wherein said adaptation factor is a weighted average of the adaptation factor for contemporary operation and the adaptation factor for previous operations, where the previous adaptation factor is weighted more heavily than the new adaptation factor, in accordance with the formula,

8. The method of claim 7, wherein following a charging cycle, at least about 10% of the capacity must be removed before the open circuit voltage recovery process is monitored and the open circuit voltage recovery determination is not made when the state of charge is less than about 10%.

9. The method of claim 8, wherein following a charging cycle, at least about 20% of the capacity must be removed before the open circuit voltage recovery process is monitored and the open circuit voltage recovery determination is not made when the state of charge is less than about 20%.

10. The method of claim 7, wherein 2(x) is about equal to (y).

11. The method of claim 1, wherein during the recovery process from a discharging cycle, the open circuit voltage is monitored for about the first seven minutes.

12. The method of claim 9, wherein during the recovery process from a discharging cycle, the open circuit voltage is monitored for about the first seven minutes.

13. The method of claim 1, wherein the open circuit voltage is monitored during rest periods provided that:

1) greater than 20% of the capacity has been removed since previous discharging activity,

2) greater than 5% of the capacity has been removed since since the last time the open circuit voltage recovery was monitored, and

3) the state of charge is greater than 20%.

14. The method of claim 1, wherein when the state of charge, as monitored by coulometric summation, reaches 80%, charging is terminated, and the open current voltage recovery process is monitored at predetermined rest time interval, and a plurality of consecutive open circuit voltage measurements are taken and averaged, the open circuit voltage recovery slope is calculated, and the state of charge is determined based on said averaged measurements.

15. The method of tracking with high accuracy, a lead-acid battery'"'"'s state of charge during discharging, comprising the steps of:

a. obtaining a coulometric summation during battery operation, with open circuit voltage measurements taken during the first few minutes of said battery'"'"'s recovery from use, and

b. obtaining open circuit voltage measurements during an initial period of recovery of the battery, and

c. adaptively learning said battery'"'"'s size and conditions by storing historic operational data in computer memory and calculating from stored data, an adaptation factor,

whereby said tracking is essentially independent of variations in battery treatment during said tracking, including rest period durations and frequency, cycling frequency, discharge patterns, and battery environment including temperature variations and degree of agitation of said battery.

16. A process of battery energy management utilizing a plurality of methods for monitoring the state of charge of a battery, comprising the steps of:

a. obtaining a coulometric summation during battery operation by continually measuring said battery'"'"'s current flow and the duration of discharge;

b. accessing, for a predetermined time period, the open circuit voltage following discharge,

c. storing said coulometric summation and said open circuit voltage reading in a data processor,

d. calculating the current state of charge of said battery by applying a preprogrammed formula to said accessed data;

e. providing compensation for differences which are created by variations due to battery replacement, battery aging, steady state temperature differentials and the decrease in usable capacity at increasing discharge rates;

thereby providing an accurate state of charge of said battery.

17. The process of claim 16, wherein said battery is a lead-acid battery.

18. The process of claim 17, wherein said open circuit voltage reading is obtained during the charging of said battery.

19. The process of claim 18, wherein the accuracy of said open circuit voltage reading is further increased by:

a. determining at least 5% of the battery capacity has been removed between consecutive readings;

b. determining an adaptation factor by taking an average of the current adaptation factor and the previous adaptation factor, weighing said previously adaptation factor twice as heavily as said current adaptation factor;

c. removing at least 20% of the capacity of said battery;

d. reading the open circuit voltage only after removal of said 20% of said capacity;

whereby said accuracy is increased by eliminating the effects of overvoltages, small errors in open circuit voltage readings and reading said open circuit voltage when said battery has discharge substantially above said 20%.

20. The process of claim 19, wherein said adjusting for differences comprises the step of calculating an adaptation factor to minimize the error created by variations due to battery replacement, battery aging, steady state temperature differentials, and the decrease in usable capacity at increasing discharge rates.

21. The process of claim 16 wherein said preprogrammed formula uses a first algorithm if said battery is charging and a second algorithm is said battery is discharging.

22. The process of claim 20 wherein said adaptation factor is calculated during quiescent voltage measuring cycles.

23. The process of monitoring the remaining capacity of a battery, said monitoring incorporating coulometric summation during operation of said battery, said summation requiring the steps of:

a. continually monitoring the current of the battery;

b. detecting a zero current;

c. waiting a predetermined time from said zero current;

d. taking a first voltage measurement after said predetermined time;

e. waiting an additional predetermined time;

f. taking a second voltage measurement after said additional predetermined time;

g. predicting the final recovered voltage using a predetermined formula

h. relating said predicted final recovered voltage to the remaining capacity

whereby compensation is provided for differences which are created by variations due to battery replacement, battery aging, steady state temperature differentials, and the decrease in usable capacity at increasing discharge rates.