Battery capacity estimating method and apparatus

• US 20100138178A1
• Filed: 04/08/2009
• Published: 06/03/2010
• Est. Priority Date: 04/08/2009
• Status: Active Grant

• Current Assignee: Tesla Inc.
• Original Assignee: Tesla Motors Incorporated (Tesla Inc.)
• Inventors: Carison, Craig Bruce, Kohn, Scott Ira, Paryani, Anil, Boggs, Brian, Baglino, Andrew David
• Granted Patent: US 8,004,243 B2
• Time in Patent Office: Days
• US Class Current: 702/63
• CPC Class Codes: G01R 31/3842 combining voltage and current measurements

Abstract

A method for accurately estimating battery capacity based on a weighting function is provided. The disclosed system monitors battery current and uses the monitored battery current to calculate the state of charge (SOCbyAh) of the battery. The system also measures the open circuit voltage (OCV) of the battery when the system is at rest, rest being determined by achieving a current of less than a preset current value for a period of time greater than a preset time period. The state of charge of the battery is calculated from the OCV (SOCbyOCV). The weighting function is based on ΔSOCbyAh and ΔSOCbyOCV, where ΔSOCbyAh is equal to SOCbyAh_{First time} minus SOCbyAh_{Second time}, and where ΔSOCbyOCV is equal to SOCbyOCV_{First time} minus SOCbyOCV_{Second time}. The weighting function also takes into account the errors associated with determining SOCbyAh and SOCbyOCV.

Claims

1. A method for estimating battery capacity of a battery, the method comprising the steps of:

reading CAC_Current, wherein CAC_Current is the current computed amp-hour capacity of the battery;

determining SOCbyAh_{First time} at a first time, wherein SOCbyAh_{First time} is the state of charge of the battery calculated by amp-hours at said first time;

determining SOCbyOCV_{First time} at said first time, wherein SOCbyOCV_{First time} is the state of charge of the battery calculated from the battery open circuit voltage (OCV) at said first time;

determining SOCbyAh_{Second time} at a second time, wherein SOCbyAh_{Second time} is the state of charge of the battery calculated by amp-hours at said second time;

determining SOCbyOCV_{Second time} at said second time, wherein SOCbyOCV_{Second time} is the state of charge of the battery calculated from the battery OCV at said second time;

calculating Δ

SOCbyAh, wherein

Δ

SOCbyAh=SOCbyAh_{First time}−

SOCbyAh_{Second time};

calculating Δ

SOCbyOCV, wherein

Δ

SOCbyOCV=SOCbyOCV_{First time}−

SOCbyOCV_{Second time};

calculating a weighting function, W, defined as

$W=100-\left(k<br><br><br><br>1*\frac{100*\mathrm{SOCbyAhError}}{\mathrm{\Delta <br><br>}<br><br><br><br>\mathrm{SOCbyAh}}+k<br><br><br><br>2*\frac{100*\mathrm{SOCbyOCVError}}{\mathrm{\Delta <br><br>}<br><br><br><br>\mathrm{SOCbyOCV}}+k<br><br><br><br>3*\mathrm{SOCbyOCVErrorPrev}\right)$

wherein SOCbyAhError is the error associated with determining SOCbyAh, SOCbyOCVError is the error associated with determining SOCbyOCV, SOCbyOCVErrorPrev is the previously determined error associated with SOCbyOCV, and k1, k2 and k3 are constants; and

calculating CAC_Modified, wherein said CAC_Modified=(W)(CAC_Current).

2. The method of claim 1, wherein said step of determining SOCbyAh_{First time} is calculated in accordance with the formula:

SOCbYAh_{First time}=1−

(Ah_used_{First time}/CAC_Current);

wherein said step of determining SOCbYAh_{Second time} is calculated in accordance with the formula;

SOCbyAh_{Second time}=1−

(Ah_used_{Second time}/CAC_Current).

3. The method of claim 2, further comprising the steps of:

monitoring battery current;

integrating battery current over a first time period to determine Ah_used_{First time}, wherein said first time period is defined as the time between the last battery charge and said first time; and

integrating battery current over a second time period to determine Ah_used_{Second time}, wherein said second time period is defined as the time between the last battery charge and said second time.

4. The method of claim 1, further comprising the steps of monitoring battery current and comparing said battery current to a preset current value, wherein said battery current is less than said preset current value prior to performing said step of determining SOCbyOCV_{First time}, and wherein said battery current is less than said preset current value prior to performing said step of determining SOCbyOCV_{Second time}.

5. The method of claim 1, further comprising the steps of monitoring battery current and comparing said battery current to a preset current value, wherein said battery current is less than said preset current value for a period of time greater than a preset time period prior to performing said step of determining SOCbyOCV_{First time}, and wherein said battery current is less than said preset current value for a period of time greater than said preset time period prior to performing said step of determining SOCbyOCV_{Second time}.

6. The method of claim 1, wherein said SOCbyOCV_{First time} determining step further comprises the steps of measuring the battery OCV at said first time and retrieving said SOCbyOCV_{First time} from a look-up table, wherein said look-up table provides SOC as a function of battery OCV, and wherein said SOCbyOCV_{Second time} determining step further comprises the steps of measuring the battery OCV at said second time and retrieving said SOCbyOCV_{Second time} from said look-up table.

7. The method of claim 1, further comprising the step of calculating SOCbyAhError in accordance with the formula:

SOCbyAhError=∫

(g×

iBat+k)dt, wherein g is gain error, iBat is battery current, and k is current offset error.

8. The method of claim 1, further comprising the step of retrieving SOCbyOCVError from a look-up table, wherein said look-up table provides SOCbyOCVError as a function of SOC.

9. The method of claim 1, further comprising the step of setting CAC_Current to CAC_Modified, wherein said CAC_Current setting step is performed after said CAC_Modified calculating step.

10. The method of claim 9, further comprising the step of repeatedly calculating CAC_Modified for subsequent times.

11. The method of claim 1, wherein said determining and calculating steps are performed by an electrical circuit.

12. A method for estimating battery capacity of a battery, the method comprising the steps of:

monitoring battery current;

calculating Ah_used from said battery current, wherein Ah_used corresponds to the amp-hours used;

calculating SOCbyAh from said Ah_used, wherein SOCbyAh is the state of charge of the battery calculated using said Ah_used;

comparing said battery current to a preset current value;

measuring the open circuit voltage (OCV) of the battery when said battery current is less than said preset current value for a period of time greater than a preset time period;

determining SOCbyOCV from said measured OCV, wherein SOCbyOCV is the state of charge of the battery calculated from the OCV;

calculating Δ

SOCbyAh, wherein

Δ

SOCbyAh=SOCbyAh_{First time}−

SOCbyAh_{Second time}, wherein said first time and said second time correspond to two separate times when said battery current was less than said preset current value for a period of time greater than said preset time period;

calculating Δ

SOCbyOCV, wherein

Δ

SOCbyOCV=SOCbyOCV_{First time}−

SOCbyOCV_{Second time};

calculating a weighting function, W, defined as

$W=100-\left(k<br><br><br><br>1*\frac{100*\mathrm{SOCbyAhError}}{\mathrm{\Delta <br><br>}<br><br><br><br>\mathrm{SOCbyAh}}+k<br><br><br><br>2*\frac{100*\mathrm{SOCbyOCVError}}{\mathrm{\Delta <br><br>}<br><br><br><br>\mathrm{SOCbyOCV}}+k<br><br><br><br>3*\mathrm{SOCbyOCVErrorPrev}\right)$

wherein SOCbyAhError is the error associated with determining SOCbyAh, SOCbyOCVError is the error associated with determining SOCbyOCV, SOCbyOCVErrorPrev is the previously determined error associated with SOCbyOCV, and k1, k2 and k3 are constants; and

calculating a current computed amp-hour capacity, CAC_Current, from a previously calculated computed amp-hour capacity, CAC_Prior, wherein

CAC_Current=(W)(CAC_Prior).

13. The method of claim 12, wherein said step of calculating SOCbyAh is calculated in accordance with the formula:

SOCbyAh=1−

(Ah_used/CAC_prior).

14. The method of claim 12, wherein said SOCbyOCV determining step further comprises the step of retrieving said SOCbyOCV from a look-up table, wherein said look-up table provides SOC as a function of battery OCV.

15. The method of claim 12, further comprising the step of calculating SOCbyAhError in accordance with the formula:

SOCbyAhError=∫

(g×

iBat+k)dt, wherein g is gain error, iBat is battery current, and k is current offset error.

16. The method of claim 12, further comprising the step of retrieving SOCbyOCVError from a look-up table, wherein said look-up table provides SOCbyOCVError as a function of SOC.

17. The method of claim 12, further comprising the step of retrieving SOCbyOCVError from a look-up table, wherein said look-up table provides SOCbyOCVError as a function of temperature.

18. The method of claim 12, further comprising the step of repeatedly calculating CAC_Current for subsequent times.

19. The method of claim 12, wherein said monitoring, calculating, comparing and determining steps are performed by an electrical circuit.