BQ27421-G1: State of charge is inaccurate after first startup.

Hi,

we are working with the BQ27421-G1 fuel gauge and we could observe a strange behavior regarding the state of charge.

The fuel gauge is used with a custom hardware design that is driven by a rechargeable 450 mAh Li-Ion-Battery. The fuel gauge runs with the default configuration except the value for the design capacity.

The design capacity is configured on the first system startup as described in the Technical Reference Manual (Literature Number: SLUUAC5C June 2013 – Revised August 2016) in chapter  "3.1 Data Memory Parameter Update Example" on page 14.

The state of charge is read out every 10 Minutes. Now we could observe that when the charging process has just finished the state of charge is regarding to the fuel gauge at about 100%. One reading later (10 minutes later at current reading frequency)  the gauge tells us that the state of charge is now only at about 40%.

Then we reset the system. During the system reset the design capacity is written to the fuel gauge again. After the system reset is finished the state of charge read from the gauge is at the expected at about 100%. That means that after a system reset the gauge seems to work as expected. In subsequent readings, the observed behavior no longer occurs. The power consumption of the system is ok so this can not be the cause.

Now we are wondering what causes the observed behavior.

Any ideas? Could there be a problem with the configuration of the fuel gauge?

Regards

Marco