Is your battery draining faster than usual? Is your iPhone not making it through the day? Chances are there’s an app that’s gone rogue and is running in the background.
First, go to Settings > Battery and look at the Battery Usage list. Tap anywhere on the list to change it from showing percentages to also showing you a breakdown of how much screen time and background time the running apps are taking.
Remember that while some apps — the Music app for example — are designed to work in the background, most apps are not and could be the cause of the problem.
If you notice an app with unexplained high background usage, then you may have solved your problem. Go into Settings > General > Background App Refresh and turn off background refresh for that specific app.
Also look to see if you have No Cell Coverage on the list. If this is responsible for high battery usage, then you’ve found your problem — being out of cell coverage or in an area with poor coverage. If this figure is high, try putting the iPhone into Airplane Mode when cell coverage is poor (you can still turn on Wi-Fi and Bluetooth independently) to see if that helps.