Instructions
Remove the collar from the pet and turn it off. Cut through the glue that holds the retaining disk to the power cap. It will be about 1/16th of an inch from the bottom of the power cap. Unscrew the retaining disk and slide the old R21 battery out.
Replace the old R21 battery out and insert the new one in, positive side first. There will also be a piece of metal that wraps around the inside of the power cap and keeps the R21 battery in place. It may fall out when you are removing the old battery. Just place it back inside the power cap and then insert the new battery inside it.
Screw the retaining disk back into the power cap. Do not worry about re-gluing it. It should hold closed on its own. The glue is more to indicate that the power cap has been opened. Turn the collar back on and place it back onto the pet, making sure to position it tight enough so that the shock plate is close to the pet's neck.
How to Replace the Battery in My R21 Invisible Fence Power Cap
An R21 invisible fence is a model of a wired underground pet fence that keeps your pet inside a certain perimeter. It uses radio frequencies sent along the underground wires and a pet collar that emits an electrical shock when it comes into contact with that frequency. A dead battery in your pet's collar can make your invisible fence useless and lead to your pet roaming the neighborhood. Most batteries need to be replaced every 3 to 6 months.