This DIY repair guide explains how to replace the back-up battery in a garage door opener. Located in the motor unit, the battery supplies power to the opener if the power goes out. Test the battery yearly according to the instructions in your owner's manual. If the battery status light indicates a low battery charge when you unplug the garage door opener to test the battery, plug the garage door opener back in and see if the battery charges within 48 hours. If the battery hasn't charged after 48 hours, replace it with the manufacturer-approved replacement part.
This basic repair procedure works for common Craftsman, LiftMaster and Chamberlain garage door openers.