Sudden battery drainage every few minutes

My iPhone 6 plus runs on 11.1.1. After the last update I noticed it is draining battery every few minutes. It has been exactly 3 years since I bought the phone, but even couple of weeks back I was getting good battery life. I checked background app refresh, nothing is enabled. I am not a heavy user per se. I occasionally check WhatsApp, FB, use Apple news and Flipboard. I used Safari much recently for couple of days to read some reviews, and I saw that Safari is using the battery most, more than 90%.

Any advice please ?

Thanks.