I have a little bit unusual case of Android draining the battery. Mobile: LG40, Android: Kitkat 4.4.2
Mobile phone was never connected to any data network (wifi or GSM). No updates, no new apps, no GPS, no Bluetooth. No playing games or using other apps. Just a couple of calls and SMS. Battery could live in this for 10 days nonstop. After 2 years battery changed (for original piece). New battery again could live for 10 days.
Until now. After 6 months after battery change, something is wrong with the process "Android OS". It consumes more than 80 % of the battery, which drains it in 24-30 hours. Even without any call.
Sometimes the restart can help for few days, but after that it starts again draining. Storage is ok, more than 1GB free, SD card is completely free (2GB). I have never done a factory reset on this phone, it is about 2,5 years from the buy.