My series 5 Watch battery indicator was stuck at 100% for up to 16 hours, then started declining and went into power reserve mode shortly after reaching 53%. The Apple Store ran a diagnostic and didn't observe a problem with the watch. I decided I needed concrete evidence of the history of the battery indicator and went to the App Store to see if there was an app for that. There was: Battery Grapher! I left my watch on the charger for 2.5 hours to guarantee the battery was fully charged (remember, the indicator couldn't be trusted) and then started using the watch normally with BG recording the battery indicator value once an hour. I took screen shots along the way, because BG doesn't show a full 24 hours on one screen, and used them to create a little Notes document describing the problem with the watch. That document, with the BG graphs, convinced the genius at the Apple Store that my problem was real and they sent the watch to the service center.
I was not very happy when the service center sent it back with a note saying they couldn't reproduce the problem. It turned out, though, that the first thing they did before even looking at the watch was reinstall the firmware and OS software, something I could not do (nor could the Apple Store). That actually fixed the problem!
Having to monitor my watch so closely taught me something I didn't know before. You know, Apple advertises 18 hour battery life. Well, my series 5 watch runs (with my normal use) 22+ hours before it has to be recharged.