The HTC One (M8) is a good phone except that it is a sealed box and you cannot replace the battery. My battery has been acting up – it goes from 30+% charge to zero in seconds. I had this once before and fixed it for a couple of months by doing this:
To recalibrate battery and HTC charger when battery rapidly or erratically discharges, this procedure clears all battery stats, coordinates and normalizes charging.
Turn off Fast Boot in settings.1 Power off phone.
Plug phone into HTC charger and charge for two minutes or more
While charging, hold down volume up+volume down+power button and continue holding
Phone will turn on and off repeatedly every 15 seconds or so while continuing to hold all three buttons
Keep this going for 2 minutes, then release buttons when phone is ON
Now, let phone charge fully normally (with phone either on or off–doesn’t matter) and battery level reporting, charging and battery life should be normalized.
I got this from XDA and am posting it here to make it easier to find again since I may have to do it every month….
Note that there is no fast boot option in Marshmallow, so don’t freak if you cannot find it. ↩