Ran into some strange background battery drain on an iPhone SE 2nd Generation - it was noticeably above room temperature at all times (but not warm/hot to the touch), indicating a lack of deep sleep, and the battery chart showed a continuous decline, even during periods of no notifications or screen use. Additionally I received notifications suspiciously quickly - within a second of the event occurring, rather than the usual 2-5.
I say 'unreported' in the title as there was absolutely no indication that background drain was taking place on the Battery screen, other than in the percentage chart. 'Screen Off' did not exceed 30 minutes per day, and most of that was believably attributed to WhatsApp. This led me to search for system services as a cause, specifically anything I may have done that is out of the ordinary that may cause a system service to go haywire.
I was unable to narrow down exactly what the fix was, but after these steps I no longer get the issue or any of the symptoms above, and SoT has increased from 3-5 hours to 6-8 hours - a fantastic result!
- Stopped using iMazing Mini for automatic backups over WiFi. I was suspicious of how it could effortlessly read battery info etc while the phone was asleep. I suspect whatever iTunes-imitation connection this uses causes the phone to remain permanently awake, even while not backing up! To keep the benefit of automatic backups, I moved iMazing to a virtual machine that I automatically suspend during the day, so that it only connects overnight while the phone is on charge.
- Killed the Pebble smartwatch app (by swiping it away in the app switcher), as it is unnecessary for notification delivery, which is the only smart feature I use my watch for that requires a phone connection. I also ensured that I was connected to the Pebble by Bluetooth LE only, which you can do by adding your watch as a Pebble Time Round, no matter what model it actually is.
- My email account contains an insane volume of logging/reporting, and many huge old folders that I unsubscribe from on Thunderbird. iOS Mail does not support unsubscribing from folders, so to stop iOS seeing and indexing these useless emails, I moved them all to a separate account, which I will not add to iOS. I then deleted my account on iOS, rebooted, and finally re-added. In total I slimmed the iOS exposure to email messages by 75% in terms of quantity and 50% in terms of disk size.
- Updated to iOS 13.5 (no relevant changes reported, so unlikely to be this)