About how to get the characteristics of lithium batteries in fuel gauge applications

To accurately estimate the remaining charge of a Li+ battery, it is necessary to understand how the battery characteristics change as temperature and load current change. This application note describes a method for obtaining Li+ battery characteristics, discusses how to acquire and process data, and loads the data into the Dallas battery management device evaluation software for use in 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 battery full and empty levels to determine the remaining capacity.

Steps to get Li+ battery characteristics

1. Determine the charge and discharge curves

The best way to get the Li+ battery characteristics is to create an environment that is as similar as possible to the actual application. These include protection circuits, discharge curves (including typical values for effective current and standby current in practical applications), charging curves, and the ambient temperature of the application. It is therefore required to simulate the charging and discharging process of the battery and adjust the operating temperature accordingly. In general, various battery characteristics should be obtained in steps of 10 ° C and in the range of 0 ° C to 40 ° C. At the same time, the temperature point interval required by the evaluation software is also 10 °C.

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

The Active Empty and Stand by Empty in the evaluation software fuel gauge section correspond to the point where the Li+ battery is discharged with an effective current discharge and a standby current to an empty voltage (defined by the user). The empty power point is shown in Figure 1, as described in step 5. Users can define different effective empty points and standby empty points. The point at which the charging circuit fully charges the Li+ battery is defined as the full charge point. For a detailed description of the Dallas battery management device using the built-in fuel gauge, see application note 131: Lithium-Ion Cell Fuel Gauging with Dallas Semiconductor.

2. Calibrate the offset register of the device

Depending on the device data sheet, the Dallas battery management device should be properly tuned to the Li+ battery after calibration. The offset of the device can be easily calibrated with the evaluation software of the selected device. Make sure there is no load connected to the circuit. Then click the Calibrate Offset button in the Meters tab. If you do not use the evaluation software, you can step through the offsets according to application note 224: Calibrating the Offset Register of the DS2761.

3. Start recording data

Data can be easily recorded using the evaluation software. Simply enter 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 required data points can be recorded without generating too large a file. All real-time data will be recorded in the specified file until the Stop Logging Data button is clicked.

4. Activate the battery at room temperature

The battery must first be broken-in. The capacity will fluctuate by a few percents at the beginning of the Li+ battery life. Therefore, it is recommended to pass the 20 full charge and discharge cycles before testing the battery characteristics. There is no need to record data during this process, but if data logging is done, it helps the user to monitor other battery offset parameters for final data analysis.

5. Start calibration from the highest temperature

It is generally recommended to test the characteristics of the battery from the highest temperature because the Li+ battery has the largest capacity and is suitable as a reference point for other data. Set the battery to work at the highest temperature and fully discharge the battery to the standby empty point. Subsequently, the battery is fully charged according to the charging curve required by the actual application, which corresponds to the full power point at that temperature. Thereafter, the battery is fully discharged to the user-defined effective null voltage with an effective current to determine the effective empty point. Finally, the load is changed to standby current and continues to discharge until the voltage drops to the standby empty voltage to determine the standby empty point.

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

6. Repeat at various temperatures

Once you reach the standby empty point at a certain temperature, immediately move to the next temperature and start charging the battery until the battery is fully charged. When the charging is completed, the full power point of the temperature is reached. Then discharge the battery to the effective empty point and the standby empty point. The above operation was repeated at all required temperatures to complete the measurement of the battery characteristics.

Filter key data points from characteristic parameters

The evaluation software records the real-time data in a text file in a format with a tab delimiter for easy import into the spreadsheet. You can then filter out the data you need by sorting or plotting the chart.

7. Find all the data points you need

The user can classify the data in the log file and mark all full power points, effective empty power points, and standby empty power points. An easy way to do this is to browse through all the data, look at the Current column and observe changes in the current readings, and insert "x" into the columns that are not used in the spreadsheet. For example, when the battery changes from the charging state to the discharging state, it is recorded as the full power point; when the battery stops discharging at the effective current. It is recorded as the effective empty point; when the battery changes from the discharging state to the charging state. It is recorded as the standby space power point. Then through the spreadsheet's AutoFilter feature, you can easily view each important marker point.

There’s an example of the important data points marked by the DS2761 when the Li+ battery characteristics are acquired and filtered. In this example, the battery is charged at a constant current of 900mA until the voltage reaches 4.2V. Then continue charging to keep the battery voltage stable at 4.2V until the current gradually drops to 70mA, which is the full powerpoint. The battery is discharged at a current of 350mA until the voltage drops to 3.0 V, which corresponds to the effective empty point. The battery is discharged at a current of 3mA until the voltage drops to 2.7 V, which corresponds to the standby empty point. Battery characteristics were obtained at 40 ° C, 30 ° C, 20 ° C, 10 ° C, and 0 ° C, respectively.

If the data is recorded during the battery activation process in step 4, the empty battery points can be compared to determine whether there is an increase or decrease, thereby determining whether the current value has an offset. Since this activation process is done at a constant temperature, if there are no offsets, all empty points will be identical. If there is an offset, the response to the data should be corrected based on the offset introduced by the ACR column to accurately measure the Li+ battery characteristics.

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