Iso 15 15.1 update
iPhone 6s - since update iPhone seems to shut down several times when battery has been charged and still decent battery.
Also this update seems to indicate false or drain battery issues ?
iPhone 6s, iOS 15
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Having the battery replaced by Apple or an Apple Authorized Service Providers may resolve the issue.
Have an iPhone 6S here, in normal use it's fine, but it got really hot only during the last update to iOS 15.
Also be aware that the iPhone is 6 years old, which also affects the battery life.
Have the battery replaced by an Apple Authorized Service Providers, because now it is still possible.
For older iPhone type is a bit trickier, I don't see any older iPhone type in the list, see:
https://support.apple.com/iphone/repair/service/battery-power
If you are going to replace the battery, read this Apple article:
Get your iPhone, iPad, or iPod touch ready for service
https://support.apple.com/en-us/HT201557