Acquiring Lithium Battery Characteristics for Fuel Gauging Applications

To accurately estimate the remaining charge in a Li+ battery, it is necessary to understand how battery characteristics change with temperature and load current. This application note introduces a method to acquire Li+ battery characteristics, discusses how to collect and process data, and how to load the data into Dallas Battery Management IC’s evaluation software for fuel gauge applications. 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 charge and empty charge points to determine the remaining capacity.

Steps to Acquire Li+ Battery Characteristics
Determine Charge and Discharge Curves

The best way to acquire Li+ battery characteristics is to create an environment as close as possible to actual usage conditions. This includes protection circuits, discharge curves (including typical values for effective current and standby current in actual applications), charge curves, and the surrounding ambient temperature of the application. Therefore, it is required to simulate the battery charging and discharging processes, adjusting the operating temperature accordingly. Typically, measurements should be taken in 10°C steps within a 0°C to 40°C range, as the evaluation software also requires temperature points to be in 10°C intervals.

Effective Current refers to the typical output current of the Li+ battery during the user’s use.
Standby Current refers to the typical output current of the Li+ battery during idle states.
In the evaluation software’s fuel gauge section, Active Empty and Standby Empty correspond to the points where the Li+ battery discharges to the empty voltage (user-defined) with effective current and standby current, respectively. The empty capacity points are shown in Figure 1, with relevant details in Step 5. The user can define different effective empty capacity points and standby empty capacity points. The charging circuit defines the full charge point as the point where the Li+ battery is fully charged. For further information on using Dallas Battery Management ICs with built-in fuel gauges, refer to application note 131: “Lithium-Ion Cell Fuel Gauging with Dallas Semiconductor.”

Figure 1. Relationship between Voltage and Current during Stepwise Discharge

Calibrate the Offset Register of the Device

After correctly connecting the Dallas Battery Management IC to the Li+ battery, the offset of the device must be calibrated. The evaluation software for the selected device can easily calibrate the offset. Confirm that no load is connected to the circuit, then click the “Calibrate Offset” button in the Meters tab. If the evaluation software is not used, refer to application note 224: “Calibrating the Offset Register of the DS2761” to step through the calibration process.

Start Recording Data

Data can easily be recorded using 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 ensures that all necessary data points are recorded without generating excessively large files. All real-time data will be recorded in the designated file until the “Stop Logging Data” button is clicked.

Activate the Battery at Room Temperature

First, the battery must be activated (break-in). Typically, there will be a slight fluctuation in the capacity during the initial phase of the Li+ battery’s lifespan. Therefore, it is recommended to complete 20 full charge-discharge cycles before testing the battery characteristics. Data recording is not required during this process, but it can help monitor other battery offset parameters for final data analysis.

Start Calibration from the Highest Temperature

It is generally recommended to begin testing the battery characteristics at the highest temperature because the Li+ battery’s capacity is maximal at this temperature, making it an ideal reference point for other data. Set the battery to operate at the highest temperature, and discharge it fully to the standby empty point. Then, fully charge the battery according to the required charge curve, which corresponds to the full charge point at that temperature. Afterward, fully discharge the battery using effective current to the user-defined effective empty voltage to determine the effective empty capacity point. Finally, reduce the current to the standby current and continue discharging until the voltage drops to the standby empty voltage to determine the standby empty capacity point.

To speed up the process, the user can gradually decrease the current from effective current to standby current. As shown in Figure 1, set the effective current to 200mA, the standby current to 5mA, and both empty voltage points to 3.3V. Discharge the battery with 200mA current until it reaches 3.3V, then after a few seconds, discharge with 100mA current until it reaches the same empty voltage point. Gradually decrease the discharge current from 50mA, 20mA, 10mA to 5mA until the battery voltage stabilizes at the empty voltage. This will quickly reach the same empty capacity point without a long 5mA discharge process.

Repeat the Process at Different Temperatures

Once the standby empty capacity point for a given temperature is reached, immediately move to the next temperature and begin charging the battery until it is fully charged. Once charging is complete, the full charge point for that temperature is reached. Then, discharge the battery to both the effective and standby empty capacity points. Repeat the process for all required temperatures to complete the battery characteristic measurements.

Filter Key Data Points from the Characteristic Parameters
The evaluation software records real-time data in a tab-delimited format, making it easy to import into a spreadsheet. The data can then be categorized or plotted in charts to extract the necessary data.

Find All Necessary Data Points

The user can categorize the data in the recording file and mark all full charge points, effective empty capacity points, and standby empty capacity points. A simple way to do this is by browsing all the data, checking the Current column, and observing changes in the current readings. Insert “x” in the columns not used in the spreadsheet. For example, when the battery switches from charging to discharging, it is marked as the full charge point; when the battery stops discharging with effective current, it is marked as the effective empty capacity point; when the battery switches from discharging to charging, it is marked as the standby empty capacity point. The AutoFilter function in spreadsheets can then be used to easily view the important marked points.

Table 1 shows an example of key data points after filtering the recorded data from the DS2761 during Li+ battery characteristic acquisition. In this example, the battery is charged with a constant current of 900mA until the voltage reaches 4.2V. Charging continues until the current decreases to 70mA, which corresponds to the full charge point. The battery is discharged with 350mA current until the voltage drops to 3.0V, corresponding to the effective empty capacity point. Finally, the battery is discharged with 3mA current until the voltage reaches 2.7V, which corresponds to the standby empty capacity point. These characteristics are recorded 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 capacity points can be compared to see if they have increased or decreased, allowing the user to assess if there is any offset in the current values. Since the activation process is completed under constant temperature conditions, if there is no offset, all empty capacity points will be identical. If there is an offset, the data should be corrected based on the offset introduced by the ACR column, ensuring accurate measurement of the Li+ battery characteristics.