SLUA857A December   2017  – September 2022 BQ27426

 

  1.   bq27426 Gauging Design for Applications With 2-S Li-Ion Battery
  2.   Trademarks
  3. 1Introduction
  4. 2Implementation
  5. 3Voltage Calibration and Accuracy
  6. 4bq27426 Configuration and Learning Process
  7. 5Cell Balancing Consideration
  8. 6Revision History

3 Voltage Calibration and Accuracy

The internal voltage divider in the bq27426 is factory-calibrated to have a 1-mV accuracy. The external voltage divider requires additional calibration.

For calibration, a calibrated voltage meter with resolution better than 1 mV is used. Apply a constant 8 V to the positive input of the voltage divider and read the voltage measured by the bq27426 gauge. The expected voltage should be 4 V. The difference is the offset error, and it can be stored in data flash inside the gauge.

To further investigate the impact of temperature and input voltage on the voltage divider offset and gain, a bq27426 for 2-S reference design evaluation module is built with this voltage divider circuit. Test is implemented with voltage input from 8.4 V down to 6 V with 200-mV increments at –10°C, 25°C and 50°C. The voltage measurement error across the temperature and voltage range after calibration is shown in the following table. The maximum voltage error due to the voltage divider is in the 1- to 2-mV range after calibration.

Table 3-1 Voltage Divider Accuracy After Calibration
–10°C 25°C 50°C
Pack Voltage Divided Voltage Abs. Error After Cal. Error Pack Voltage Divided Voltage Abs. Error After Cal. Error Pack Voltage Divided Voltage Abs. Error After Cal. Error
8.401 4.193 0.015 –0.001 8.401 4.192 0.017 0.001 8.401 4.192 0.017 0.001
8.201 4.093 0.015 –0.001 8.201 4.092 0.017 0.001 8.201 4.092 0.017 0.001
8.000 3.992 0.016 0.000 8.000 3.992 0.016 0.000 8.000 3.992 0.016 0.000
7.800 3.893 0.014 –0.002 7.800 3.892 0.016 0.000 7.800 3.892 0.016 0.000
7.600 3.792 0.016 0.000 7.600 3.792 0.016 0.000 7.600 3.792 0.016 0.000
7.400 3.693 0.014 –0.002 7.400 3.692 0.016 0.000 7.400 3.692 0.016 0.000
7.200 3.592 0.016 0.000 7.200 3.592 0.016 0.000 7.200 3.592 0.016 0.000
7.000 3.492 0.016 0.000 7.000 3.492 0.016 0.000 7.000 3.492 0.016 0.000
6.800 3.392 0.016 0.000 6.800 3.392 0.016 0.000 6.800 3.392 0.016 0.000
6.600 3.292 0.016 0.000 6.600 3.292 0.016 0.000 6.600 3.292 0.016 0.000
6.400 3.192 0.016 0.000 6.400 3.192 0.016 0.000 6.400 3.192 0.016 0.000
6.200 3.093 0.014 –0.002 6.200 3.093 0.014 –0.002 6.200 3.092 0.016 0.000
6.000 2.993 0.014 –0.002 6.000 2.993 0.014 –0.002 6.000 2.992 0.016 0.000
Unit: volts