Originally Posted by MarkThyme

It’s got nothing to do with Apple. Lipo batteries are good for a certain number of charge cycles. Under normal use, that means about two years. After that, they do start to run down very quickly and exhibit some odd, unpredictable behaviors.

My iPhone 6 is just over four years old. I replaced the original battery after two years, and replaced that battery just recently, two years after the first one. $29 at an Apple Store or authorized service location. I got the most recent replacement done at Best Buy.