Characterizing Lithium Batteries for Fuel Gauge Applications

introduction

To accurately estimate the remaining charge in a Li+ battery , it is necessary to understand how the battery characteristics change with temperature and load current. This application note describes a method for characterizing a Li+ battery and discusses how to acquire and process the data and load it into the evaluation software for the Dallas battery management device for use in a fuel gauge application. The device monitors the current flowing into and out of the Li+ battery through the accumulated current register (ACR) and compares the ACR data with the calculated full and empty battery charges to determine the remaining capacity.

Steps to obtain Li+ battery characteristics

1. Determine the charge and discharge curves
The best way to obtain the characteristics of Li+ batteries is to create an environment that is as similar to the actual application as possible. This includes protection circuits, discharge curves (including typical values ​​of active current and standby current in actual applications), charging curves, and the ambient temperature of the application. Therefore, it is required to simulate the battery charging and discharging process and adjust the operating temperature accordingly. Generally, various battery characteristic parameters should be obtained in the range of 0°C to 40°C with a step size of 10°C. At the same time, the temperature point interval required by the evaluation software is also 10°C.

Active current refers to the typical output current of a Li+ battery during use. Standby current refers to the typical output current of a Li+ battery in an idle state.

Active Empty and Standby Empty in the Fuel Gauge section of the EV kit software correspond to the points at which the Li+ cell is discharged to an empty voltage (defined by the user) at the active and standby currents, respectively. The empty points are shown in Figure 1 and described in Step 5. The user can define different active empty and standby empty points. The full point is defined by the charging circuit as the point at which the Li+ cell is fully charged. For detailed instructions on using the Dallas battery management device with an integrated fuel gauge, see Application Note 131: Lithium-Ion Cell Fuel Gauging with Dallas Semiconductor.

Figure 1. Relationship between voltage and current during gradual discharge.

2. Calibrate the device offset register
After properly connecting the Dallas battery management device to the Li+ battery, the device offset should be calibrated according to the device data sheet. The device offset can be easily calibrated using the evaluation software for the selected device. Verify that there is no load connected to the circuit and click the Calibrate Offset button in the Meters tab. If you are not using the evaluation software, you can follow the steps in Application Note 224: Calibrating the Offset Register of the DS2761 to calibrate the offset.

3. Start Logging Data
Logging data is easy with the evaluation software. Simply go to the Data Log tab, set the Sample Interval to 15 seconds and click Log Data. A 15 second interval is recommended because it allows all the required data points to be logged without generating a very large file. All real-time data will be logged to the specified file until the Stop Logging Data button is clicked.

4. Activate the battery at room temperature
The battery must first be activated (break-in). Typically, the capacity of a Li+ battery will fluctuate by a few percent at the beginning of its life. Therefore, it is recommended to allow the battery to undergo 20 full charge and discharge cycles before testing its characteristics. Data logging is not required during this process, but it helps the user monitor other battery imbalance parameters for final data analysis.

5. Calibrate from the highest temperature
It is usually recommended to test the characteristics of the battery from the highest temperature, because the Li+ battery capacity is the largest at this time, which is suitable as a reference point for other data. Set the battery to operate at the highest temperature and fully discharge the battery to the standby empty point. Then, fully charge the battery according to the charging curve required by the actual application, which corresponds to the full charge point at this temperature. After that, fully discharge the battery with an effective current to the user-defined effective empty voltage to determine the effective empty point. Finally, change the load to the standby current and continue discharging until the voltage drops to the standby empty voltage to determine the standby empty point.

If the user wants to speed up the process, they can gradually reduce the current from the active current to the standby current. As shown in Figure 1, the active current is set to 200mA and the standby current is set to 5mA. The empty voltage is defined as 3.3V in both cases. The battery is discharged to 3.3V with a current of 200mA to reduce the voltage to the active empty point, and then, after a few seconds, the battery is discharged with a current of 100mA to reach the empty voltage point again. The discharge current is then gradually reduced from 50mA, 20mA, 10mA to 5mA, until the battery voltage finally stabilizes at the empty voltage, which is the standby empty point. In this way, the battery can quickly reach the same empty point without going through a long 5mA discharge process.

6. Repeat at each temperature
Once the standby empty point is reached at a certain temperature, immediately switch to the next temperature and start charging the battery until the battery is fully charged. When charging is complete, the full point at that temperature is reached. Then discharge the battery to the active empty point and the standby empty point. Repeat the above operation at all required temperatures to complete the measurement of battery characteristics.

Filter key data points from characteristic parameters

The evaluation software records the real-time data in a tab-delimited text file for easy import into a spreadsheet. You can then filter the data to find the desired data by sorting or drawing a chart.

7. Find all the required data points
The user can sort the data in the log file and mark all the full points, active empty points, and standby empty points. An easy way to do this is to go through the data, find the Current column, watch the current readings change, and insert "x"s in the unused columns in the spreadsheet. For example, when the battery changes from charging to discharging, it is recorded as full; when the battery stops discharging at an active current, it is recorded as active empty; when the battery changes from discharging to charging, it is recorded as standby empty. Then use the spreadsheet's AutoFilter function to easily view the important marked points.

Table 1 shows an example of using the DS2761 to collect data when obtaining Li+ battery characteristics, and after filtering, marking each important data point. In this example, the battery is charged at a constant current of 900mA until the voltage reaches 4.2V. Then the battery voltage is kept stable at 4.2V until the current gradually decreases to 70mA, which is the full charge point. The battery is discharged at a current of 350mA until the voltage drops to 3.0V, which corresponds to the effective empty point. The battery is discharged at a current of 3mA until the voltage drops to 2.7V, which corresponds to the standby empty point. The battery characteristic parameters are obtained at 40°C, 30°C, 20°C, 10°C, and 0°C.

If data was recorded during the battery activation process in step 4, the empty points can be compared to determine if they have increased or decreased, and therefore if there is an offset in the current value. Because this activation process is completed at a constant temperature, if there is no offset, all empty points will be exactly the same. If there is an offset, the data should be corrected accordingly based on the offset introduced by the ACR column to accurately measure the Li+ battery characteristics.

Table 1. Li+ battery characteristic parameters

8. Determine the data related to battery capacity
By recording the data in the ACR column of the file, the full and empty points of the Li+ battery at different temperatures can be determined. The full and empty points are only relative values, and a fixed reference point needs to be selected. The standby empty point at the highest temperature is selected as a reference because this point is often the minimum value of ACR in the process of obtaining battery characteristic parameters (the highlighted part in Table 1). Therefore, all readings are greater than the value at this point, which is easy to store data.

Table 2 lists all the important ACR values ​​from Table 1. The standby empty point at 40°C is chosen as the reference point and 71.04mAhrs is subtracted from all other ACR values ​​to get the full and empty points at other points. Table 3 lists all the full and empty points with the 40°C standby empty point as the reference point, which can be easily stored in the device using the evaluation software.

Table 2. Full and Empty points from Table 1

Table 3. Full and Empty Points Using the Standby Empty Point at 40°C as Reference Point

9. Determine the turning point
The turning point is an important data point for estimating the remaining time of charging. With the help of the turning point, a 2-point polyline is used to approximate the ACR curve during charging, as shown in Figure 2. The turning point is selected by the user, and the error between the 2-point polyline determined by the turning point and the actual ACR curve should be minimized.

Figure 2. Relationship between the ACR curve and the 2-point line approximation when charging at 20°C

The easiest way to determine the turning point is to plot the ACR time curve during charging and see where the curve turns. It is recommended to choose the middle temperature of the ACR curve as the turning point, and choose this point as the turning point for all temperatures. The turning point is expressed in mAhrs below full charge. In Figure 2, the turning point is about 100mAhrs below the full charge point.

10. Determine the estimated remaining time for charging at each point.
The FuelPack algorithm requires a series of data, including the turning point, the time from the empty point to the full point at all temperatures, and the time from the turning point to the full point at all temperatures, to achieve an accurate estimate of the remaining charging time. Therefore, it is necessary to analyze the data and filter out all the turning points. You can find all the full points and subtract the mAhrs value of the turning point selected in step 9 to easily find the turning point.

From the data in Table 1, the time to the standby empty point (time to start charging), the time to the turning point, and the time to the full point at the previous temperature point are tabulated as shown in Table 4. Table 4 also gives the ACR value for each point. Then, the time from the empty point to the full point and the time from the turning point to the full point at each temperature can be calculated based on the recorded time, as shown in Table 5. The evaluation software's charge calculation method only allocates enough EEPROM for charging data at three temperature points, so the 0°C, 20°C, and 40°C data are chosen to be written to the device.

Table 4. The time when the standby empty point, turning point, and full power point occur at various temperatures

Table 5. Estimated remaining charging time at various points

Write data to the device and start fuel gauging

11. Write the appropriate data to the device
Write the data in Table 3 and Table 5 to the device through the evaluation board. Manually enter the data into the text box of the Fuel Gauging Data sub-tab in the Pack Info tab, as shown in Figure 3, and click the Write button (not shown in the figure). The data will be written to the device's scratchpad and subsequently copied to the EEPROM.

Figure 3. Importing the required fuel gauge data into the evaluation software.

12. Synchronizing ACR
The final step to accurately measure the Li+ battery charge is to synchronize the device's ACR value with the battery charge. A simple way to accomplish this is to fully charge the Li+ battery according to the actual charging characteristic curve and then set the ACR at that temperature to the full charge point. When using the evaluation software, first click the Start Fuel Gauging button on the Fuel Gauging tab to start fuel gauging, as shown in Figure 4. After charging is complete, click the Full button in the Fuel Gauging tab to synchronize the ACR with the battery charge.

Figure 4. Fuel guging tab of the evaluation software.

For more information on fuel gauge data, see Application Note 131: Lithium-Ion Cell Fuel Gauging with Dallas Semiconductor.

Summarize

Dallas Semiconductor's fuel gauges accurately track the charge of a battery as it charges and discharges. This requires knowledge of the battery's fuel characteristics and how the battery behaves under various load and temperature conditions in the actual application. By measuring and recording the battery's characteristic data using the evaluation software provided by Dallas Semiconductor, the fuel gauge can be used to accurately estimate the remaining charge in the battery.