I had the same problem with my S3, my battery suddenly begun to drain fastly with no increase but normal usage. This is what I did and it works for me. First I made a backup of my stuff, and then reset the watch. When it came up again, I realize that there were some app's that needed to be updated, even if I search for them before and said to me that no updates were necessary. Then I downloaded my backup from the cloud and from there I have my watch running at a normal battery drain again. Hope it helps.
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