The eXtreme Dog Fence replacement battery is for use with Extreme Active, Perimeter Invisible Fence 700 Series compatible and Perimeter Comfort Contact fencing systems. These 6-volt batteries have a lifespan of about 2-4 months. Keeping a few packs of batteries on hand ensures you'll never find yourself unable to rely on the fence system due to a dead collar battery. The batteries have a 6-year shelf life so storing a few spare packs won't affect the length of battery life.