Battery back up

Hi
i have ordered 1 channel opto-isolated IO relay board for a project work.I want install a siren that will be backed up with a 12 VDC car battery(battery backed) so that the siren will sound in the even of power failure.
I Have read that it possible to achieve this by extending the alarm leads to the N/C contacts as there is continuity in NC contacts when the coil is de-energized
.Is it possible to achieve this end?

Yes is the short answer read this....**broken link removed**

Cheers Ian

The setup is going to look like maintained emergency lighting system.