Updated to iOS 18 recently, and I've noticed the battery now barely lasts until evening instead of two days. I turned off background refresh and set brightness to minimum. Is this a system bug or just my phone reacting that way?
Usually after a major update the system spends a few days "reindexing" itself in the background - cache is being reprocessed, system processes are optimizing themselves. If your battery dropped that sharply, that's most likely what's happening. Give your phone a couple of days to work in a calm mode, don't use it heavily at night, and see if the drain goes back to normal.
If after a week or so the battery still hasn't returned to normal, the problem could be deeper. Try restarting your phone, check in settings which app or service is consuming the most energy - sometimes after an update some process starts hanging and hammering the battery. If that doesn't help, you should try resetting your network settings or even reinstalling the system through restoring from a backup.
Also keep in mind that the iPhone battery itself degrades over time - if you've had the phone for about two years already, it's quite possible that the update just revealed its actual capacity, which the system was somehow compensating for before. Check the "Battery Health" percentage - if it's dropped below 80%, a replacement is due.
Give your battery another day or two to settle down - after a major update, the iPhone really does put a heavy load on the processor while it's reindexing and optimizing everything in the background. If the drain doesn't normalize, check in settings which apps are eating the most power (Battery Health & Device Management) - sometimes after an update some app starts running in a loop and causes damage. You can also try doing a full restart of the phone, not just turning it off, but an actual hard reset using the power button.
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