Why does the phone off and on
Why does an iPhone off and on
iPhone 6s Plus, iOS 15
Question marked as
With a phone that old, your battery is most likely at the end if it's life.
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.
iPhone Battery Repair & Replacement - Apple Support
About Lithium Ion Batteries
iPhone Battery and Performance - Apple Support (IN)