Here is a picture of my battery-backup board that has 2 supply sources, a 12VDC battery circuit (2 6VDC 260Ahr in series... overkill, I know, but recycled them from my RV) and a 12VDC output from a 110VAC>>12VDC power supply. When the 110VAC power is lost, there is uninterrupted 12VDC that is supplied by the 12 battery (thanks to the 2 capacitors). There are 3 10K pots on the board I designed . The battery backup board supplies power to my Energy Management System (BrulTech GEM/DashBox) that monitors my energy usage and stores the data on a memory card. It also will provide battery backup for my Home Automation System (Universal Devices ISY994 and Polisy). It also provides power to a WiFi Router that supplies Internet connectivity (when there is AC power for the ISP modem).
The WiFi router's DHCP provides IP addresses and allows the ISY to keep talking with my DashBox in the event of a 110VAC power outage. This setup prevents both systems from having to reboot every time that there is a power interruption, which happens once every few weeks.... what a pain.