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.
Try to contact them, maybe they will give you your money back
You are within the return window. Just do a return and take a bunch of pictures of the product and packaging just in case. I always had great experience with Amazon, and I returned items worth thousands (cameras, TVs, e-scooters) with no questions asked.