Same problem here with my iPhone 6 (it is hardly 1 year 10 months old). After spending lot of time and effort with Apple Support - they finally suggested that I need to replace the battery. So I went to the Apple service center who diagnosed my phone and confirmed that the battery is still in good condition and therefore replacing the battery is not going to solve the problem. The problem is bugged ios11.
Apple has screwed up and is now refusing to take responsibility. Right now I am on 11.2.1 - the battery issue has still not been fixed by Apple.
So now I have lost faith in Apple. I have no other option but to trash this device and buy a new one. I am never going to buy Apple device ever again. And I would never recommend Apple to anyone.