My iPhone SE’s battery performance had begun to drop dramatically, for example it went from fully charge to 53% in six hours doing nothing but being on stand-by, no apps were running.
I arranged for replacement of the battery, that cost AU$75 & was done today. The Genius Bar fellow says Apple considers a battery should be OK up to 500 charges, mine had undergone 698 charge cycles. The Genius man said most of the battery replacements he saw were done between 700 & 800 cycles, so mine was just about into that area.
A terrific Mac app called Coconut Battery can display the battery & storage levels of any iOS jigger that’s connected to a Mac via USB cable.