My IPhone 6s Plus keep turning on and off and when charging still wouldn’t add percentage
My iPhone 6s Plus keeps turning on and off every second. And when I plug it. It wouldn’t charge
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Your battery is probably at the end of its 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. This is an approximation, that is why it may be plus or minus a few percent on a month to month basis.
Apple recommends replacing it when it reaches 80%. To check your battery health, go to Settings > Battery > Battery Health.
iPhone Battery Replacement.
iPhone Battery Service - Apple Support
About Lithium Ion Batteries
iPhone battery and performance - Apple Support (IN)