I have 5 months old IPP 10.5 64 wifi only (not refurbished one and clean installed 12.1). Watching 1080p on youtube the battery seems doing fine (10% / hour , 40% audio volume) until doing basic stuff like browsing on safari, gmail, google keep, drive, etc the battery drains quick and inconsistently 1% / 2-4 minutes (or 30-45 minutes / 10%) on 30-50% brightness. I never experienced this before iOS 12. Everything is turned off (screen time, auto update, app background, location, icloud, siri) default email, apple maps, and several apple's apps are uninstalled.
I thought Procreate (painting app) is the culprit since the app is quite a power hungry, but not the case here. Whether Procreate or basic stuff the battery just draining quickly!
Some time a week ago the battery drop from 70% (5 pm) to 44% (next day at 3 pm) in sleep mode. Without any single app activity in battery usage. Geez!
edit:
Current battery health on iMazing: 94% health, and 64 cycle count.