Thanks SethGeckoXDA for the super fast reply. I tried what you said - regarding the airplane mode I have tested it before and that effectively prevents battery drain (~80% battery left after a day without much usage) thus I would assume the drainage is caused by any of the connections to the outside - Wifi, Bluetooth, mobile service, you name it. Today I set the phone to 2G only but that did not help. Tonight the phone is pretty much dead (2% battery left). Again I would be happy to help, as it is currently the only phone that I own and is working.To troubleshoot the battery drain, you can do the following:
- activate aeroplane mode over night and see if it stops the battery drain.
- if yes: go to settings - Network and Internet - Mobile Network - Preferred network type and choose 2G. Again, let it sleep over night and see if the battery drain occurs.
If the battery drain only occurs when the preferred network type is set to 3G or LTE, you can think about leaving the option set to 2G and only switch manually to LTE when needed. Of course this is only viable if you don't need highspeed internet all the time. For Whatsapp and background activity of apps, 2G is enough.