Did you update recently? Then wait four days
After any major iOS update, Spotlight rebuilds its index and Photos re-analyses your entire library. Both run hard in the background for a few days, and on a large library that is genuinely heavy work. Battery life during that period looks alarming and is not a fault.
The signal is the trend: if day three is better than day one and day four better again, it is finishing. Leave the phone on power overnight and it gets through the work faster. If it has not settled by day five, keep reading.
Find the actual culprit
Settings → Battery, then switch the view to the Last 10 Days rather than the last 24 hours — one bad day tells you nothing. Each app shows its share of usage, and tapping Show Activity splits it into screen time and background time.
An app with a large share of background activity during hours you were asleep is your answer. That is the specific pattern to look for, and it points at one app rather than a vague sense that the phone is worse.
The settings that actually matter
In rough order of effect: screen brightness, which is the single largest consumer on any phone; Background App Refresh for apps that do not need it (Settings → General → Background App Refresh); location services set to Always rather than While Using (Settings → Privacy & Security → Location Services); and push email on a busy account, which is better set to Fetch.
Turning off 5G, disabling Bluetooth or force-quitting apps are the three most commonly recommended fixes and the three least effective. Force-quitting in particular is counterproductive — relaunching an app from cold costs more than leaving it suspended.
Check the battery itself
Settings → Battery → Battery Health & Charging. Below 80% capacity, the phone genuinely cannot hold what it used to, and no setting compensates for that. If Peak Performance Capability reports that performance management is active, the battery is the whole story.
Our battery health guide explains the numbers, and our replacement guide covers what to do about them.
Poor signal is an underrated drain
A phone in a weak cellular area transmits at much higher power trying to hold a connection, and that is one of the heaviest things it does. A day in a basement office or a rural area can halve battery life on its own.
If your drain is location-specific rather than constant, this is why. Wi-Fi calling, or Airplane Mode plus Wi-Fi where there is no usable signal at all, both help considerably.
When it is a specific app misbehaving
If the Battery screen implicates one app with heavy background use, remove it and watch for two days. If the drain stops, the app was the cause and it is worth checking for an update before reinstalling.
Our battery life guide covers the longer-term settings, and our beta drain guide covers the case where you are running pre-release software, which behaves differently.
Frequently Asked Questions
Why is my iPhone battery draining so fast suddenly?
Most often a recent iOS update reindexing in the background, which settles in two to four days. After that, check Settings → Battery over the last 10 days to find the app responsible.
Does closing apps save battery?
No. Relaunching an app from cold costs more energy than leaving it suspended. Force-quitting is one of the least effective things you can do.
What drains iPhone battery the most?
Screen brightness, then poor cellular signal, then apps with heavy background activity. Location set to Always is another common one.
How do I find which app is draining my battery?
Settings → Battery, switch to Last 10 Days and tap Show Activity. Look for background time during hours you were not using the phone.