Fido Fence Brand Invisible Fence&Atilde;&sbquo;&Acirc; Compatible Battery by Mightypets Replacement battery for Invisible Fence R21 and R51 Works with all R21 & R51 Fence Systems This does go with "compatible" collars and for Invisible Fence Brand & Dog Watch Replaces battery inside invisible fence brand name collars Batteries are guaranteed to work as well or better