@WarnerCrocker Not sure which watch exactly he has but my partner was having similar problems. He turned off a lot of background processes and the smart charging where it doesn't charge all the way to 100%. That didn't help at first so he did a hard power cycle on the watch and his phone and now it's behaving normally. Not sure exactly which step actually is the fix!
It was crazy, he'd lose like 30% in the morning & end up getting a low battery alert by dinner.