My iPhone 6S will not charge to 100%. I have iOS version 10.0.2 right now but I'm pretty sure it has been like this for older versions. It also usually powers off before it reaches 0% (sometimes as high as 30%) so maybe that has something to do with it.
I just downloaded http://www.coconut-flavour.com/coconutbattery to see if that might give any insight.
Is there anything else I can try?