Characterizing a Lithium-Ion (Li+) Cell for Use with an Open-Circuit-Voltage (OCV) Based Fuel Gauge

Introduction

The DS2786 stand-alone, open-circuit-voltage (OCV) based fuel gauge estimates the available capacity of rechargeable lithium-ion (Li+) batteries based on the cell voltage in an open-circuit state following a relaxation period. The open-circuit voltage is used to determine relative cell capacity based on a lookup table stored in the IC. The DS2786 leaves the factory with a best-fit OCV profile loaded into the EEPROM of the device. The accuracy of the DS2786 can be improved by customizing the OCV profile of the Li+ cell with the charge and discharge parameters that are used in the application.

This application note outlines a method for characterizing a Li+ cell for use with an OCV-based fuel gauge, collecting and interpreting the data, and then loading the data into the DS2786K evaluation software.

Procedure for Characterizing a Li+ Cell

1. Determine the full and empty point
   
   The best way to characterize a Li+ cell is to create an environment as close as possible to the actual application in which it will be used. This includes the protection circuitry, a discharge profile consisting of the typical active and standby currents of the application, and a charging profile. This requires a method to simulate the charging and discharging of the cell and a method to monitor and record the current and voltage.
   
   The full point (100% capacity) is defined as the capacity where the Li+ cell is considered fully charged by the charging circuitry. The empty point (0% capacity) can be defined to be the active empty point, the standby empty point, or the absolute energy remaining in the cell.
2. Characterize the cell
   
   The procedure to characterize a Li+ cell for use with an OCV-based fuel gauge can be performed at room temperature. It is best to perform the following steps on at least three cells to obtain an average OCV profile.
   
   
   1. Charge the cell to the full point.
   2. Discharge the cell to the 0% point to learn the capacity of the cell, which should be recorded in mAh.
   3. Charge the cell to the full point.
   4. Allow the cell to relax for 60 minutes.
   5. Record the open-circuit voltage for the 100% point.
   6. Discharge the cell 5% (based on the capacity of the cell from Step 2) at a rate of approximately 0.2C.
   7. Allow the cell to relax for 60 minutes.
   8. Record the open-circuit voltage for the 95% point.
   9. Repeat Steps 6–8 19 times until the capacity reaches 0%.
3. Analyze the data
   
   Table 1 contains sample data of a typical cell that was characterized by Analog Devices. The twenty Capacity/OCV pairs must be summarized into nine pairs that are stored in the EEPROM of the DS2786. The 100% and 0% pairs must be stored in the device. The other seven points should be selected to approximate the full set of data. Table 2 shows the nine data pairs that were chosen to approximate the data in Table 1. Figure 1 compares the data in Table 1 and Table 2.
   

   Table 1. OCV Characterization Data for a Typical Li+ Cell

   Capacity (%)	OCV (V)
   100	4.177454
   95	4.129486
   90	4.085934
   85	4.045427
   80	4.008118
   75	3.974769
   70	3.945074
   65	3.917968
   60	3.884009
   55	3.841219
   50	3.820965
   45	3.805737
   40	3.79325
   35	3.783504
   30	3.775129
   25	3.762185
   20	3.741018
   15	3.7098
   10	3.686654
   5	3.674776
   0	3.305545

   Table 2. OCV Characterization Data Stored in the DS2786

   Breakpoint	Capacity (%)	OCV (V)
   8	100.0	4.177454
   7	82.5	4.026773
   6	60.0	3.884009
   5	55.0	3.841219
   4	40.0	3.79325
   3	25.0	3.762185
   2	10.0	3.686654
   1	5.0	3.674776
   0	0.0	3.305545

   

4. Store the data
   
   The last step is to program the data into the EEPROM of the DS2786. The DS2786K evaluation software can be used to easily convert the data into the appropriate format that will be stored in EEPROM. Simply load the values from Table 2 into the Parameters tab of the DS2786K as shown in Figure 2. Additionally, the capacity of the cell (learned in Step 2) should be entered into the Initial Cell Capacity text box. Then click the Write & Copy button. Make sure a programming voltage is applied to the V_PROG pin of the DS2786 in order for the EEPROM to be properly programmed.
   
   

   

Conclusion

Analog's OCV-based fuel-gauging algorithm provides an accurate means of tracking the capacity of a Li+ cell as the cell is charged and discharged by the application. Once the characterization data is collected and stored using the evaluation software provided by Analog, the fuel gauge will accurately estimate the remaining capacity of the cell.