I put my charger in my phone when it was at 13%, when I did this the screen suddenly turned black so I turned it on again and the screen went black again, I then turned it on again and it seemed to be fine as it said it was charging. I then checked it a couple of minutes later and it was at 10%?
I didn't find the right solution from the Internet.
References:https://forums.imore.com/iphone-7/387281-my-iphone-7-battery-dodgy.html