The days after you install a new version of iOS, your iDevice is busy doing all sorts of housekeeping. Practically all of your apps have updates, so iOS is busy downloading and installing them in the background.
Additionally, after you install a new version of iOS, it has to do something called “re-indexing.” During this process, iOS 11 will comb through all of the data on your device. As you can imagine, this takes a lot of processing power and can account for why your hearing stories of iPhone losing half their battery life while sitting idle.
The good news is that both of these things are temporary and, after a few days, you should see battery life return to normal. So in short, do nothing. It will all be ok! ??