Phone will not stay on unless it’s plugged into a charger
Why won’t my iPhone stay on unless it’s plugged into a charger
iPhone 8, iOS 16
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Best reply
Your battery is most likely dead.
Batteries degrade over time with usage due to chemical aging, that's just how they work.
You can expect about a 1% drop per month with average use on an iPhone.
Apple recommends replacing it when it reaches 80%. To check your battery health, go to Settings > Battery > Battery Health.
iPhone Battery Replacement.
https://support.apple.com/iphone/repair/service/battery-power
About Lithium Ion Batteries
https://support.apple.com/en-in/HT208387