This lock can always be opened from the inside regardless of battery life. The lock is operated by a nightlatch on the inside of the door which does not require batteries. On the outside of the lock there are two battery contacts for use with a 9v battery. These provide emergency power in the event the batteries do run flat and allow you to operate the lock.
It is recommend that you change your batteries every 12 months but you should be able to unlock your door up to 10,000 times on one set of 4 x AA batteries.
Jonathan P