iOS 11.1.1 , iPhone 7 Plus battery is draining so fast !

Hi zamir,
Thanks for using the Apple Support Communities. If you are having issues with the battery life on your iPhone after a recent update, you may find the information and troubleshooting steps outlined in the following article helpful. In particular, you may want to check to see if any specific app or service is consuming an unusual amount of battery time (under View Battery Usage):
Batteries - Maximizing Performance - Apple
Regards.

Hi

If you are using YouTube application be sure you updated to latest version of YouTube. Because Youtube wasn t optimized for ios11 so that was that massive battery drain. But after update your battery should be again normal.

Ok, I hore I did help you, If not sorry. Say If that worked for you.