Why is my Apple Watch draining charge so rapidly?

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 I power cycled my Apple Watch and iPhone twice.

There are lots of reasons that could be happening. Start your investigation by going to Settings>Battery and seeing what is using up all the battery.

Also, if you have a weak cellular signal, your battery will drain very fast.