I really appreciate all these replies. I was talking to a Genius Bar person today, one who seemed in charge of all the others and very experienced, and she said it didn't sound like a battery issue. Maybe something to do with the iphone, since everything on the watch goes through there. She talked about different software "builds". My current status: after getting the Series 5 (less than six months old) back from Apple Repair (and even the Genius Bar says that Apple Repair doesn't always communicate very well with them, (and, of course, the watch is the one thing the Apple Store doesn't
repair or service), I had the message "We could not duplicate the problem" (even though they only had it for a day) and "We can't do anything if the watch has more than 80% battery strength - a pretty weird comment. I guess they did their version of diagnostics and I got it back and restored it from a previous version (not from scratch, which hadn't worked before). It seemed like some magical repair had happened. It acted normal. But then, every once in a while, it would stay at 99% all day. I still put it on the charger twice a day. I wake up this a.m. and it's dead, after saying 100% at 10pm. Put it on the charger and it zoomed back up to 100% in 20 minutes.