I found a battery, 3.7 volt at 165mAh in the same style, should be close enough but I do not have the size of the factory battery, if anyone has taken theirs apart, can you post up the measurements in mm? thanks

Looking for this battery as well; mine lasted exactly 1 year. 7 days now down to hours, the surge did the same thing, maybe my last Fitbit watch ever! Disappointed to say the least.