I had the very same issue, I had a series 0 SS 38mm that was fine for me for a year or so, I would regularly get 18 hours+ out of it. I then gave it to my mum when i upgraded to the series 2, she could barely get 5 hours out of it. It got sent to Apple for them to test, they came back saying it was fine. We had tried the usual pair and re-pair with the iPhone and it was still the same. She then gave it to my brother in law and for some strange reason it works perfectly fine. I’ve read somewhere it could be a issue with the paired phone not the actual watch. If possible pair it to a different phone and see what it’s like. If it works okay there is your issue.