"The different series have different batteries which are not interchangeable as the connector interfaces are very different.
Series 1 - 205 mAh (38mm) 246 mAh (42mm)
Series 2 - 273 mAh (38mm) 334 mAh (42mm)
Series 3 - 279 mAh (38mm) 352 mAh (42mm)
The space the battery sits is larger in the Series 2 & 3 watches so the battery within them is larger unlike the older Series 1 which has a thicker logic board so the space the battery is limited."
