Smartphones, and in particular the iPhone, seem to attract more battery life complaints than any other device out there. Maybe it's because we use the thing so much and don't realize it, or maybe it's that we rely on it so heavily for so many aspects of our lives.
And there's nothing like an iOS update to exacerbate battery problems.
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The first thing that most iPhone owners do at the slightest hint of a battery problem is dive into the Settings app and begin flipping switches to turn stuff off. But this violates the first rule of troubleshooting -- if it's not broken, don't fix it.
As annoying as battery issues can be, my first advice is to just ride them out. New iOS updates can throw the iPhone or iPad's battery calibration settings off, and it can take a few charge/recharge cycles to fix this, and the best thing you can be doing during this period is just using your device normally and just charge it up when it needs charging up.
Another problem might be that it's a bug that you're dealing with, and that there's not much you can do about it until it's fixed. It can be an iOS bug (expect Apple to release a couple of updates to iOS 11.3 over the coming weeks to crush whatever bugs squirmed out at the last minute), or it can be a buggy app that's draining your battery.
Before any switch flipping happens, the first thing you need to do is determine whether you have a battery issue in the first place.