I regularly turn my phone off at night. It is always at 100% charged when turning off, but when I turn on in morning it is between 17-30% charged. why is the battery discharging when its switched off. The battery last well during the day while phone is on so assuming the battery is ok.
iPhone SE (2nd generation)
Here is what battery expert Lawrence Finch has to say:
There is never a time when any smartphone is doing nothing, so it will use energy 24/7, and the amount used will vary from day to day and minute to minute. For that reason alone you should charge your phone overnight, every night. In addition, the absolute best way to get maximum use on a charge, as well as slow the decline of battery capacity long term is to enable Optimized Battery Charging (Settings/Battery/Battery Health) and charge the device overnight. The battery will fast charge to 80%, then pause. During the nighttime pause the phone will use mains power instead of battery power, allowing the battery to “rest”, and thus reducing the need to charge the battery quite as often. The phone will resume charging to reach 100% when you are ready to use your phone; it will “learn” your usage pattern. If you enable iCloud Backup (Settings/[your name]/iCloud - iCloud Backup) the phone will back up overnight also, assuring that you can never lose more than the current day’s updates.
Here's more information→About Optimized Battery Charging on your iPhone - Apple Support
Battery discharging when phone turned off
