This is in fact the same battery as in the Observer Battery Pack, but without the backplate and O-ring.
It is suitable for the Core series and the Heart Rate Monitor.
Energizer Code: 2032. Lithium 3v.
WEEE Regulations - Please Note: To remind you that old electrical equipment can be recycled, it is now marked with a crossed out wheeled bin symbol. Please do not throw any electrical equipment (including those items marked with the crossed out wheeled bin symbol) in your bin.