My battery gets full then restarts then ether

so my iPhone 6 has been strange,

so I charge my phone it then it restarts with battery at 10%,

now once it turned out to be a visual glitch.

but the rest turns out to be 10% for real, but sometimes it restarts for a 2nd time and the battery is normal again.

why is my phone like this, if you have answers please let me know, also i don't have a sim card or plan on it if that helps