The internal battery in the Open Sesame door operator is a 12 volt, 1.2Ah sealed lead-acid type, like in a car. We use the MK (brand) ES1.2-12. Typically it will last from 2-5 years before needing to be replaced. Usually at the end of it's life, you will notice that the door opens and closes more slowly, then finally reaching a point where the door fails to open. Of course, if you have had a power outage recently, the battery could just need to be recharged. This should happen in a few hours after the power returns (make sure the on-off switch is in the “off” position to insure no charging problems). Also, make sure that the power transformer is still plugged into the wall outlet and that the green light on it is lit.The battery (or the equivalent of another brand) can be obtained at a Batteries Plus or Interstate Battery store, if you have one locally. If not we will sell one to you for $25, plus $12 shipping (2-day FedEx). To replace the battery, loosen the two Philips head cover screws on the operator (at the top and bottom center) about 1/4" and slide the cover off. You will have to open the door slightly so you can get a screwdriver on the top screw since the arm is in the way when the door is closed. The battery is held in place at two places. There is a small black screw that holds it down, but you do not have to remove the screw. A piece of tape runs down the side of the battery to hold it in place. Peal the tape strip off the side of the battery, then rotate the back of the battery out toward the side of the case. It will come out from under the black screw head and you can then remove it. After pealing off the electrical tape over the battery terminals, pull off the two connectors with the attached orange and black wires. When putting in the new battery be sure that the connectors fit tight on the battery terminals, a loose connection here could cause problems. You can squeeze the terminals gently with a pair of pliers while they are disconnected if necessary to make the connection tighter. Make sure to observe the correct polarity when replacing the wires on the battery terminals. The tape that was over the battery terminals keeps them from scratching through the paint and shorting out on the inside of the cover, so be sure to replace the tape. Put the battery back in the operator by positioning it under the black screw head and swivel the back of the battery inward. Replace the tape strip on the side of the battery to hold it in place. Replace the cover and tighten the two screws (be careful not to strip the threads).