Method and apparatus for determining the state of charge of a lithium-ion battery

• US 20030112011A1
• Filed: 01/23/2003
• Published: 06/19/2003
• Est. Priority Date: 11/22/2000
• Status: Active Grant

• Current Assignee: Harmohan Singh, James V. Guiheen, Thirumalai G. Palanisamy
• Original Assignee: Harmohan Singh, James V. Guiheen, Thirumalai G. Palanisamy
• Inventors: Singh, Harmohan, Palanisamy, Thirumalai G., Guiheen, James V.
• Granted Patent: US 6,774,636 B2
• Time in Patent Office: Days
• US Class Current: 324/429
• CPC Class Codes: G01R 31/3648 comprising digital calculation means, e.g. for performing an algorithm
• 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

Computer-assisted methods for determining the state of charge of a specific lithium ion battery, without the need for charging and discharging the battery, by utilizing look-up tables or algorithms which store the relationships of state of charge to open-circuit voltage or to ramp-peak current, or to both for that type of specific lithium-ion battery to determine the state of charge for that specific lithium-ion battery.

Claims

1. A method for creating a look-up table of open-circuit voltage versus state of charge for a particular type of lithium-ion battery and determining the capacity thereof, said method comprising the steps of:

a) charging the battery at an initial constant current until a voltage across said battery increases to a predetermined maximum voltage;

b) continuing to charge the battery at said predetermined maximum voltage until an input current to said battery decreases to a predetermined minimum current;

c) measuring an open-circuit voltage for said battery, said open-circuit voltage being defined as the open-circuit voltage at 100% state of charge for said particular type of lithium-ion battery;

d) recording said open-circuit voltage at 100% state of charge;

e) discharging said battery at a predetermined discharge rate to a predetermined lower cutoff voltage and recording a discharge time period;

f) multiplying said discharge time period times said predetermined discharge rate and storing the product, said multiplication product being defined as the battery capacity for said particular type of lithium-ion battery;

g) measuring an open-circuit voltage for said battery, said open-circuit voltage being defined as the open-circuit voltage at 0% state of charge for said particular type of lithium-ion battery;

h) recording said open-circuit voltage at 0% state of charge;

i) charging said battery with a predetermined percentage of the battery capacity so that a present battery state of charge exceeds a previous battery state of charge;

j) measuring an open-circuit voltage for said battery, said open-circuit voltage being defined as the open-circuit voltage at the present state of charge for said particular type of lithium-ion battery; and

k) recording said open-circuit voltage at the present state of charge;

l) repeating said charging said battery with a predetermined percentage of the battery capacity step (i), said measuring an open-circuit voltage for said battery step (j), and said recording said open-circuit voltage at the present state of charge step (k) until the present state of charge of the battery equals the battery capacity for said particular type of lithium-ion battery; and

m) creating the look-up table of open-circuit voltage versus state of charge for state of charge values from 0% to 100%.

2. The method in accordance with claim 1 further comprising the step of:

a) allowing the battery to stabilize for a period of time, where said time period is the time required for the rate of change of the open-circuit voltage to decrease below a predefined threshold.

3. The method in accordance with claim 2 wherein said predefined threshold for rate of change of the open-circuit voltage is 0.0001 volts/minute.

4. The method in accordance with claim 1 wherein the predetermined percentage of the battery capacity is 10%.

5. The method in accordance with claim 1 wherein all the steps of claim 1 are repeated at a set of predetermined temperatures.

6. The method in accordance with claim 5 wherein the set of predetermined temperatures consists of −

30°

C., 0°

C., +20°

C., and +50°

C.

7. A method for determining the state of charge of a lithium-ion battery of a particular type using a look-up table of open-circuit voltage versus state of charge for that particular type of lithium-ion battery, said method comprising the steps of:

a) inputting a battery type into a computer;

b) measuring an open-circuit voltage for said battery, said open-circuit voltage being input into said computer;

c) executing an open-circuit voltage correlation algorithm based on inputs comprising,

i) the type of said battery inputted into said computer and

ii) the measured open-circuit voltage for said battery; and

d) determining the state of charge of the lithium-ion battery based on said open-circuit voltage correlation algorithm.

8. The method in accordance with claim 7 wherein said open-circuit voltage correlation algorithm comprises a table look-up followed by linear interpolation.

9. The method in accordance with claim 7 wherein said inputs to said open-circuit voltage correlation algorithm further comprise measured battery temperature.

10. A method for creating a look-up table of ramp-peak current versus state of charge for a particular type of lithium-ion battery, said method comprising the steps of:

a) bringing the battery to a known present state of charge;

b) applying a monotonically increasing current to said battery from a power supply, where there is a dependence of current increase with time;

c) recording said dependence of current increase with time;

d) increasing said monotonically increasing current to said battery until the current reaches a value being defined as the ramp-peak current at the present state of charge for said particular type of lithium-ion battery where either,

i) a measured voltage for said battery reaches a predetermined maximum voltage, or

ii) said monotonically increasing current equals a maximum current rating of the power supply;

e) recording said ramp-peak current at the present state of charge;

f) repeating said bringing the battery to a known present state of charge step (a), said applying a monotonically increasing current step (b), said recording said dependence of current increase with time step (c), said increasing said monotonically increasing current step (d), and said recording said ramp-peak current at the present state of charge step (e) at a set of predetermined present states of charge for said battery; and

g) creating the look-up table of ramp-peak current versus state of charge for the particular type of lithium-ion battery using the set of predetermined present states of charge for said battery.

11. The method in accordance with claim 10 wherein said monotonically increasing current is an electrical current increasing from 0 amps to 20 amps in 60 seconds.

12. The method in accordance with claim 10 wherein the set of predetermined present states of charge for said battery consists of 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90% and 100%.

13. The method in accordance with claim 10 further comprising the steps of:

a) checking the look-up table of ramp-peak current versus state of charge for duplicate ramp-peak current values;

b) repeating the steps of claim 10 for the states of charge with duplicate ramp-peak current values, except wherein the monotonically increasing current to said battery from a power supply, has a different dependence of current increase with time.

14. The method in accordance with claim 10 wherein all the steps of claim 10 are repeated at a set of predetermined temperatures.

15. The method in accordance with claim 14 wherein the set of predetermined temperatures consists of −

30°

C., 0°

C., +20°

C., and +50°

C.

16. A method for determining the state of charge of a lithium-ion battery of a particular type using a look-up table of ramp-peak current versus state of charge for that particular type of lithium-ion battery, said method comprising the steps of:

a) inputting a battery type into a computer;

b) measuring a ramp-peak current for said battery, said ramp-peak current being input into said computer;

c) executing a ramp-peak current correlation algorithm based on inputs comprising,

i) the type of said battery inputted into said computer and

ii) the measured ramp-peak current for said battery; and

d) determining the state of charge of the lithium-ion battery based on said ramp-peak current correlation algorithm.

17. The method in accordance with claim 16 wherein said ramp-peak current correlation algorithm comprises a table look-up followed by linear interpolation.

18. The method in accordance with claim 16 wherein said inputs to said ramp-peak current correlation algorithm further comprise measured battery temperature.

19. A method for determining the state of charge of a lithium-ion battery of a particular type using both a look-up table of open-circuit voltage versus state of charge and a look-up table of ramp-peak current versus state of charge for that particular type of lithium-ion battery, said method comprising the steps of:

a) inputting a battery type into a computer;

b) measuring an open-circuit voltage for said battery, said open-circuit voltage being input into said computer;

b) measuring a ramp-peak current for said battery, said ramp-peak current being input into said computer;

c) executing a correlation algorithm based on inputs comprising,

i) the type of said battery inputted into said computer,

ii) the measured open-circuit voltage for said battery, and

iii) the measured ramp-peak current for said battery; and

d) determining the state of charge of the lithium-ion battery based on said correlation algorithm.

20. The method in accordance with claim 19 wherein said inputs to said correlation algorithm further comprise measured battery temperature.