DIAGNOSIS The sealed lead-acid battery inside the backup unit is failing and needs replacement. The system beeps to tell you it can't hold a charge, which means it won't work in a power outage.
FIX You'll need a replacement 12V 5Ah battery (UB5 style). 1) Unplug the opener from the ceiling outlet. 2) Locate the battery compartment on the side of the motor unit. 3) Remove the cover and disconnect the old battery's red (positive) and black (negative) clips. 4) Pull the old battery out and put the new one in. Connect red to red, black to black. 5) Replace the cover and plug the opener back in. The beeping should stop immediately.
IF THAT DIDN'T WORK If the beeping continues with a new battery, the backup module's charging circuit on the main board might be faulty. In that case, the entire logic board assembly would need replacement.
HEADS UP These batteries typically last 3-5 years. When you replace it, write the date on the new battery with a marker. And always recycle the old one—don't just toss it in the trash.