Excessive Watch 5 battery drain with Watch OS 8.5 update?

I found something you can do that has helped me thus far. I did a forced reset, i.e. rebooted my watch, and it seemed to calm things down. I'm not sure that this has the same effect as powering it off and back on, I actually did both.

Full story. I left my watch on the charger all night Wednesday night. It did the same rapid discharge Thursday morning and was down in the mid 40s for charge level after about 2 hours, completely dead in less than 3. I was doing a 500 mile drive Thursday, so I had time to experiment. After charged it back up to almost full, I did a reboot: hold down both the crown and the side button for about 10 seconds until the Apple logo reappears. After I did this, battery consumption seems about normal. About six hours or so later, my watch is at 47%, which is acceptable.

So this appears to me that my watch had some sort of process in a race state that was gobbling resources at a fierce rate, and correspondingly also gobbling battery power. So reboot your watch after charging it, see what happens. I think I'm going to make a reboot a standard thing to do after OS updates from now on, just to make sure things are in a clean state.

I'm surprised that the update process doesn't do a full reboot as clearly it has to restart the watch. Something odd there.

It took a couple of days to confirm, but it looks like the 8.5.1 has stabilized in my Watch 5 as far as battery drain is concerned. Drain is as good as it was before, if not perhaps better.

Now I just have to figure out if my iPhone Mini 13 is still borking my hearing aids! And then there's the battery drain issue with my iPad. But one thing at a time, the hearing aids and watch were the most critical, and I had an easy workaround for the former: I switched back to my iPhone 8!

I'm sorry it's happening to you, but glad it potentially confirms something going on in the update. I put mine on about 90 minutes ago and it's down to 64%.