I have a GPS-only Series 4 Apple Watch (44mm) running watchOS 6.1.2 (17S796). I bought the Watch new in November 2018.
Prior to updating to updating to 6.1.2 (Jan 28, 2020), I had no problems with the battery. On a typical day using v6.1.1 I would use ~30% by the end of the day. I use the Watch often and keep it on during exercise and sleep.
After updating to 6.1.2, I noticed my Watch would often lose 50% or more of its charge by the end of the work day. This would cause me to have to charge it more than once a day, something I've never had to do prior to the upgrade.
To troubleshoot the problem, I did a reset and unpaired the Watch from my iPhone. I set it up as a new Watch (not a restore from my iCloud backup). Without adding any of my apps back to the Watch, the battery performance was still terrible so it's not an app that's causing the battery drain.
Similar problems are reported on this site and the Apple forums. The only answer provided is to do what I've already done (reset and setup again).
On iOS you can see the battery usage of specific apps in Settings. watchOS doesn't offer this feature, all you can see is storage space and time since last full charge.
Is there any way to see what's killing my battery? I have Xcode so I think I can see the device logs but I wouldn't know what to look for. I'm open to install any apps that would show the same info (I don't want to jailbreak).
Update: Updating to watchOS 6.1.3 hasn't fixed this problem. It hasn't made the problem worse, which is something, I guess.
Best Answer
You can try to log Battery Life using these instructions and this profile - you need an Apple Developer account to do so. It's long to do and you may not get many results (I've never tried with this kind log), but if you have the time to try. If you need help analyzing the results you can post them here.