It might be the 3 main reason of iPhone battery drain:
- Hardware issue - resolved by changing the phone.
- iOS software issue - fixed by manipulations with the phone.
- Corrupted file / app issue - fixed by deleting the files / apps and resetting the system.
Log of the fixing iPhone battery drain issue (if previous doesn’t help try the next):
- Restart phone.
- Soft reset phone.
- Change different settings:
- Always On Display
- Background apps activity
- cellular settings
- Siri settings
- etc (a lot of video and articles)
4.Update iOS.
5.Reset phone and restore from iCloud.
6.Reset phone and set from scratch.
7.Reset phone with MacBook/PC.
8.Update iPhone firmware.
After each measure take time (couple of days / weeks) and observe phone behaviour.
If all of this doesn’t help - change the system (iPhone to Android “).
If it’s too doesn’t help - throw smartphone, use dumb phone and be happy ))