Introduction

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 Maxim Integrated. 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

   
   
   Figure 1. OCV characterization data and the nine approximated breakpoints.
   
   
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.
   
   
   Figure 2. The Parameters tab of the DS2786K evaluation software.

Conclusion

APP 4189: Mar 10, 2008 APPLICATION NOTE 4189, AN4189, AN 4189, APP4189, Appnote4189, Appnote 4189