my iphone constantly restarting

It's probably time to replace your battery.

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)

leni51 Said:

"my iphone constantly restarting: my phone constantly restarting even while in charging"

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Being Stuck in a Boot Loop:

Your iPhone is stuck in a boot loop. This typically happens when you unsuccessfully download something or an update stalls.  So, refer to my User Tip: How to Fix an iPhone in a Boot Loop - Apple Support