2x of the Pet Stop Brand Invisible Fence® Replacement Battery. The best solution for keeping your Invisible Fence® Brand system operating safely. It fits snugly into your existing R21, R22, or R51 receiver made by Invisible Fence®, but costs only a fraction of the price of Powercap® batteries. You no longer need to buy expensive batteries! Our compatible replacement batteries actually exceed OEM standards – they last longer and cost less!