I have a refurbished Apple Watch 8 and I’ve only had it a week. The battery capacity is at 100% and I turned off optimised charging after it only charged to 80% a few days ago and left me with a dead watch. It was on 35% when I put it on charge at 9pm and at 8am it is on 60%, and looking at the battery screen it has slowly lost power overnight despite being on charge. I have recently (and regrettably) updated to WatchOS 10, would this cause it? My charger is an apple-branded one.
Unfortunately I got it from Amazon. I know they can be dodgy but I have always had a good experience with them.