There are two solutions: 

1) Physical Keys 

Physical keys are provided together with every Deadbolt 2S which ensures that you will still be able to enter the house mechanically even if its batteries run out. It is strongly recommended to keep at least one spare key on you at all times for such situations. 

2) 9V jumpstart
For Deadbolt 2S, there is an alternative solution, which would be to use the 9V jumpstart to provide the lock with enough power for a PIN code unlock. 

To use this feature, you have to touch and hold the battery contacts against the 9V jumpstart pin on the lock until you hear a series of beeps. 

While holding the 9V battery to the jumpstart, key in your PIN code on the keypad followed by ‘#’ to unlock