If your iPhone battery suddenly starts disappearing faster than usual, the Battery section in Settings can often tell you exactly where the power is going.
Open Settings > Battery and scroll down to see app activity over the last 24 hours or several days. The list shows how much battery each app has used, making it easier to spot something behaving differently from normal.
Tap an app or the activity view and you can see whether the power was used on screen or in the background. That distinction is useful because an app can drain battery without being open in front of you.
Background activity does not automatically mean there is a problem. Navigation, music, uploads, location services and other legitimate tasks can all continue after you leave an app. What matters is whether the usage makes sense for how you have been using the phone.
After a major iOS update, Apple also notes that background tasks related to the update can temporarily affect battery life. If the drain begins immediately after updating, it can be worth giving the phone a few days before making drastic changes.
For an unexplained battery problem, checking the Battery screen is usually more useful than randomly disabling features because it gives you evidence about what has actually been consuming power.