When Low Power Mode is On and you charge the iPhone & wait until the battery is fully charged (100%), the Low Power Mode will turned off automatically.
See this Apple article: Use Low Power Mode to save battery life on your iPhone
Use Low Power Mode to save battery life on your iPhone or iPad - Apple Support
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When Low Power Mode is on, your iPhone or iPad will last longer before you need to charge it, but some features might take longer to update or complete. Also, some tasks might not work until you turn off Low Power Mode, or until you charge your iPhone or iPad to 80% or higher.
Low Power Mode reduces or affects these features:
- 5G (except for video streaming) on iPhone 12 models1
- Auto-Lock (defaults to 30 seconds)
- Display brightness
- Display refresh rate (limited up to 60 Hz) on iPhone and iPad models with ProMotion display2
- Some visual effects
- iCloud Photos (temporarily paused)
- Automatic downloads
- Email fetch
- Background app refresh