If the power is out you want to keep your doors locked. That's where backup batteries come into play. Sometimes the building provides them but typically they are installed in combination with every larger access control system as an extra layer on top. Typically the batteries will make sure to provide extra 24 hours of backup power to keep everything safe and secure while there is an outage going on. 

Altronix backup battery

Low voltage supplier Altronix provides the most used backup batteries in the access control market. They get continuously charged but when supply stops, they can hold power for up to 24 hours.

Kisi's opinion: While it's great to have a backup battery in theory, in real life it can also be a big burden especially if the system is not designed well. You need to make sure the doors can still unlock even when there is only backup power running.
‍‍Altronix backup battery