What drains the AA batteries on an CO-Series lock?

Causes for AA battery drain in CO-Series LOCKS - Possible Cause Fix/Corrective Actions 

1. One of the four AA batteries was inserted reversed and the reserved battery drains the other batteries.
• Always make sure the polarity is correct on all AA by measuring the voltage of 6.5 – 6.4 for new AA four pack or a visual inspection of battery polarity markings.
2. The card reader is loose and not seated properly on the escutcheon.
• Make sure the seal between the reader and the escutcheon plate is seated in the groove and fitting well to the reader and escutcheon plate surface.
• See  the "Install Reader" step of the installation instructions. Proper installation of the seal is necessary.
3. The card reader is loose and not making contact or pin contacts in the plugs are bent and not making good contact. 
• Check to make sure pins are not bent. Make sure the lock is screwed tight to the door and reader is screwed tight through the lock with a seal in place.
4. Battery wire plug is smashed under the cover and damaging the AA battery.
• Make sure the plug is positioned up and away and not being pinched between the cover and the batteries.
5. The battery wring is pinched, and shorted causing current drain.
•  Locate the battery wires and inspect to be sure they are correct and not pinched.
6. Ribbon cable for the reader is pinched between the door and the  lock draining the battery.
• Locate the pinched wire. The ribbon cable is sometimes  pinched under the reader and outside escutcheon plate during filed re-handing for installation.
7. The outside of the lock, reader included, has gotten very wet casing battery current drain.
• Is the lock under an awning or protected by an overhang?   b) If the lock is loose on the door water can get behind the reader and lock base during high winds.
8. 8)    Follow the proper procedure when changing the coin cell battery.
• Remove/unplug the AA batteries, removing power from the lock before you remove and replace a coin cell battery. This avoids damage and allows the proper power up sequence and initialization to occur.