If you let the batteries completely go dead your in a tough spot.  That being said the lock is designed to alert you when the battery voltage is getting low with an audio tone each time you open it.  When the battery voltage gets to low it will not relock.  Now if you think you might still need a fail safe way to unlock the system then I would get one of our power extension cables and hide the unused end just in case you need to connect an external power source.