My TW2 used to last for a day, but recently it had been losing 10% an hour even with minimum use (Ok Tico: Off, Ambient: On). I tried rebooting and even resetting the watch but to no avail.
However, two days ago I thought of a different solution: I'm not an expert so DO AT YOUR OWN RISK...*********
I used the watch till the battery died. Then I waited a bit and started the watch again. Of course, it shortly died afterwards. Then I turned it on again and it showed a white screen with the text "Android optimising" or something. Only then I placed it on the charger.
This was two days ago so it's early to judge, but it seems that this has fixed it. Took my watch off the charger at 6:40 am, now it's 11:30 and I have 71% despite the more-than-usual notifications.