Chip memory is kept using power from main battery (negligible) or bupbat (the same one that keeps clock). It requires much less power than real-time clock, so even in the cases where you lose hour/date instantly on removing battery, your calibration data will persists for a ~10-20 seconds longer, allowing to replace batteries without need to re-calibrate.

Removing battery for longer (without working bupbat) results in chip losing data, and resetting itself to default 2048 mAh full charge capacity.

Device must be powered on for calibration to take effect. In fact, shutting it down for too long/too often during single discharge cycle, invalidate VDQ (valid discharge quality) flag, resulting in NOT taking that discharge cycle into account for calibration. Charging to full sets VDQ flag to 1, again.

/Estel