I replaced my own battery. After a couple days, I'm coming home from work (12hrs) with over 50% battery life. Just make sure you buy a separate kit to open the watch as the screwdrivers that come with the batteries suck.
Interesting... my Galaxy S3 Classic LTE has been fine since that update, lasting 2-3 days between charges even with constant auto HR on. Have you tried a factory reset? If the battery drain occured suddenly after the update, perhaps a glitch happened during the update causing a incomplete install to drain the battery. If the battery life between charges has happened gradually while owning the watch, the watch is getting old by electronic standards (being built in 2017) and the battery may need replaced.
First, minimize data loss. Use the Galaxy Wearable app to backup your S3. Connect your S3 to your phone via bluetooth and open the Galaxy Wearable app. Scroll to the bottom of the screen and tap Account and Backup. Tap Backup and Restore, then Backup. This will backup several data points in your S3.
If you use Samsung Health on your watch I recommend syncing this as well which is separate. Open Samsung health on your phone, tap on the screen and drag down. You'll see near the top of the screen that it is syncing with your S3. Let this finish.
Once the above steps are complete, press and hold your S3's power button until it shows the Galaxy S3 logo with restarting written below. Quickly press the power 3 times while this screen shows, a menu should appear. Use quick power button presses to select recovery, then hold the power button to select. The S3 should reset. After the watch restarts a few times, it will be ready to pair.
Pair it with your phone through the Galaxy Wearable app. The Wearable app should offer to restore the backup automatically. If not, scroll to accounts and backup, and select restore this time.
Unfortunately some data on the S3 will be lost in this process which is a limitation of Samsung's backup software, much of it in the Samsung Health app on the S3. The data will still be available on your phone. Some of the widgets will need moved to your preference, which I personally think should be part of the backup process.
Hopefully this restores the battery life you are uses to. Again, if the battery life between charges has happened gradually while owning the watch, the watch is getting old by electronic standards (being built in 2017) and the battery may need replaced. I'm not looking forward to that process.
Prior to the May One UI update, watch typically had 72-73% of charge left after wearing for about 15-16 hours. Plus, could use the Smart Caddie app for Golf GPS and have 30-40% of charge left after playing 18 holes.
After the One UI update, usually end up with around 50-55% of charge after 15-16 hours of normal watch use - and Smart Caddie is only able to run for around 9 holes before the watch warns charge has dropped to 15% and the watch should start using low battery mode.
We have another Gear S3 that has had similar battery performance - which has not been updated to the One UI update - and is still working great.
Ran a test a few weeks ago - and was able to get 2.5 days of use of the watch. Every time it reported low charge and shut down, would put it on the charger, note that the watch now reported there was still charge left in the battery - and then rebooted and resumed use of the watch.
So the battery still appears to have close to the original charge - just that the software isn't properly detecting the actual charge, and that the charge detection seems to be better when the watch is off - usually showing more charge than the One UI update is showing.
Samsung needs to fix this OR provide a discount for upgrading to a new watch that works.
Before the update I could work a 12 to 14 hour day and have 20 to 40% battery life when I got home. Charge over night and repeat. I tried the reboot, took off the charger it said 92% charge, it died in 1 minute. Reboot fail... I'm going to replace the battery myself but find it odd that this happened immideatley following the update. I'll let you know how the new battery goes.
I Have the same problem with my 46mm. Whatever you do don't send it in for warranty repair like I did. They will tell you a week later that it has water and LCD damage and want $458 dollars to get it back. Customer service if you can ever get through is useless. Really sad when it was just fine when I sent it to them and still owe 15 payments on my carriers installment plan. Have to pay that off to shut off the phone service to the watch. That Kitchen appliance suite the wife wants won't be from this company nor my next phone.
Im having the same battery draining issue. Its been going on the last month. I bought this watch new May 2018. The watch use to last 3-4 days. Now im only getting 24hrs. Ive uninstalled everything down to media downloads. Still no solution. So anyone have a solution for this problem. Ive researched and many many customers are experiencing the same issue. I gain the love of Samsung when i bought the watch.
Same concern for me. About 2 weeks ago (so very late June or so), normal use allowing 2 days per charge was reduced to 6-8 hours only. Deleted apps, deleted 3rd party faces, and power cycled. After that didn't help, I did a reset, though I dont know if it fully reset....it did not go back to factory settings. Rather, it just cleared my personal data and settings. Very frustrating. I have experienced this a couple of times since buying the watch 1.5 years ago, but those times, I could resolve it. Nothing helps. I would love to hear from @SamsungMod23 on this.
I finally had time to install a new battery today. The good news is that the watch hasn't died like it used to. I'll ware it for a couple days and report back. Still find it interesting, as soon as I updated in May this began. Perhaps the new update required a stronger battery. Not one close to the end of its life.