My phone shuts off at 20% battery

I noticed many times the phone shuts down all by itself at circa 20% battery. When I try to turn it on again it says it's 0%

What is causing this?

Isn't there a way to reset battery stats?

I remember there was a setting in the recovery of my Wildfire S but now in TWRP I can't find anything. My current configuration of the M7 can be found in the below signature.

Thanks to anyone that's going to help!