WatchOS 9.whatever the latest. I downgraded from a S6 about 4 months ago because the S6 was having issues and all my local store could offer was a SE2 and $75 so i took it.
SE2 here - I went out of town this past weekend and didn’t take a watch charger with me. I wore it in my usual pattern (7am to 10pm) and had maps going while I was driving. I purposely charged it to 100% the night before I left (usually have optimized charging on).
The battery was at 57% when I took it off and 50% when I put it back on the next morning. When I took it off the next night after I got home, it was at 29%.
The optimized charging decided to charge to 100% overnight at home. I’m sure it will return to a 75-80% charge tonight.