Why is my Apple Watch charging to only 80-81%?

Thank you Ingo. That's a really interesting thread, especially the importance of location to optimize logic.

Changing the location where you sleep and charge the watch temporarily disables optimized charge limits, seemingly for about ten days.

I have been at my current location for some time, more than 10 days. Previously my watch has always been charging to 100%. This is probably because I'm very rarely in the same place continuously for that long.

I'll try disabling Optimized Charging tonight and see what happens, if that resets it. Perhaps I should just shut it off permanently as it really seems to impact the battery performance very badly. Previously the watch would last from full charge to about 40% from 8pm to 3pm the following day, approximately 18 hours. With optimized, the performance from 80% to 35% lasts approximately 11 hours.

Thanks again.

Thanks. I'll try that and see how it goes.