Verifying a Fuel Gauge Has a Valid Relaxation Window

A product that appears asleep can still draw enough intermittent current to prevent a fuel gauge from obtaining the relaxed voltage information its algorithm expects. Screen-off time is not automatically a valid open circuit voltage interval. Review the electrical conditions seen by the gauge.

Read the gauge’s state criteria

The TI BQ27426 technical reference manual describes charge, discharge, and relaxation states controlled by current thresholds and timing. It also provides status indicating when an OCV measurement has been taken in relaxation.

Use the selected gauge’s exact definitions. A status that indicates an OCV measurement is not necessarily evidence that every separate learning or capacity update condition has been satisfied.

Measure battery current during nominal idle

Log current with sufficient time resolution to see radio bursts, housekeeping, memory writes, and periodic sensors. Include all loads that pass through the gauge’s sense path and identify any that bypass it.

Compare the measured current history with the configured thresholds and dwell times. A low average can hide repeated pulses that interrupt a qualifying interval. Conversely, a supported relaxation state may permit a small defined current rather than requiring mathematical zero.

Observe voltage recovery

Record battery voltage after charging or discharge stops. The immediate voltage change and slower relaxation contain different effects. Do not treat the first unloaded reading as the final equilibrium value without the algorithm’s required conditions.

For example, a device that wakes every few seconds for a substantial transmission may never provide the same quiet interval as a laboratory battery disconnected from all loads. Test the real firmware duty cycle when assessing gauge behavior.

Correlate status with the operating sequence

Log current, voltage, temperature, relevant gauge flags, and application state on a common timeline. Check whether expected relaxation and measurement events occur after representative charge and discharge histories.

Do not loosen thresholds simply to force a desired flag. First determine whether the configuration, current measurement, cell model, and operating pattern are appropriate. A falsely qualified voltage measurement can be worse than a clearly unavailable one.

Save the idle current record, threshold configuration, voltage settling, and gauge status transitions. Include power cycling and battery insertion cases if the product supports them. A reliable design provides either the relaxation opportunities needed by the selected algorithm or a documented strategy appropriate to their absence. An accurate state-of-charge display during one bench cycle does not demonstrate that the installed product supplies valid long-term voltage references under its normal sleep behavior.