There is no back up if you forget to change the batteries other than using an alternate door. As well as there is no way to open the lock if you happen to leave your phone at home or as I have done many times, left my phone inside while walking a friend out or taking the trash out and the door auto locks. A trip through the garage is often needed in our case..lol The lock does its job wonderfully. In hindsight, for our family, I should have bought a lock with an outside back up entry method. I will be ordering the bluetooth accessory keypad to remedy our issues with being locked out of our home.