Apple Watch battery draining quickly after updating iPhone to iOS 18.3.1

It will take 24-48 hours for the new operating system to finish it house keeping, as well as resync things it wants from the iPhone. Power cycle the Apple Watch and your iPhone (at least once, but maybe once or twice a day until the Apple Watch battery consumption returns to normal. The MacObserver.com How To Fix WatchOS 10.3 Battery Drain Issue on Apple Watch https://www.macobserver.com/tips/how-to/fix-apple-watch-battery-drain-issue-after-watchos-10-3-update/ Also see: https://discussions.apple.com/docs/DOC-250007078 For some steps that are not in the MacObserver article. iPhone -> "Watch" app -> General -> Reset -> Reset Sync Data Again it may take a day or two for the rsync'ing of data to settle down. As a point of reference, my Apple Watch Series 4 with 90% battery health maximum capacity, was back to normal after 24 hours (watchOS 10.5). I did 1 mid-day recharge, and https://discussions.apple.com/docs/DOC-250007078

NOTE: The Series 'n' Apple Watches are rated for 18 hours. The fact that they might last longer is so that after 6 years of service it will still give 18 hours. The Ultra is rated for 36. In either case, you should still be charging your Apple Watch everyday, and only stretch it for longer in rare situations where you cannot get to your charger because "life" got in the way. Repeatedly pushing the battery until it is dry will quickly destroy a Li-Ion battery.

It is not even lasting 24 hours now.