If you are purchasing (or already own) a Supernova, these are the replacement batteries. The Supernova is powered by 2 CR2032 Lithium batteries. After approximately 250 hours of use, the batteries in the Supernova will start to wear out. Make sure you order the replacement batteries here - because most stores will charge 4 to 5 times more than what we charge for these little guys. Supernovas are powered by two of these batteries - so, we sell them in sets of two.