… and after 25 hours of normal use I still have 70% left. Power consumption seems to be actually the same, so - as I wrote - the battery level control must have undergone some kind of recalibration. I don’t remember it lasting that long ever. Now even with WiFi and mobile data on, 1% lasts 1h:00 - 1h:05… Magic…
It looks that it is very useful to recharge the battery to 100% once in a while to calibrate it.
I had the following out of sync (much lower than actual power level):
/sys/class/power_supply/battery/charge_counter
and recharging to 100% fixed it.
This counter should reflect the level of charge (in mAh) left in the battery (in case of 10 III it should be eg. ~4500000 for full, ~2250000 for 50%, etc.). If it gets out of sync, I guess that the % indicator gets incorrect, too (decreases faster than the actual power draining).