This question already has an answer here:
Today I was walking using my iPhone 6S running iOS 9.3 as a pedometer and it shutdown.
When I got home and plugged it in, it said 30% capacity.
That seems strange. I would expect it for 2% or 5% to preserve RAM, but 30%? Maybe there is a setting I don't know about.
My question is: Why does my iPhone power off when the battery is down to 30%?
Edit: this is not a duplicate of the low battery crash as I witnessed the low power shutdown and it was not a crash.