iPhone 6s Battery drastically reduced after iOS 11

After iOS 11 the battery backup for iPhone 6s has drastically reduced. My friend recently bought iPhone 7 and she is facing the same issue. The poor battery backup is sure degrading battery lie on our iPhone but we still don't have any response from Apple on this yet. My phone is running on iOS 11.1.1 and it is plugged into charge really for the whole day 😟