Automatic emergency lights

I was wonder how simple this would be? it's simple enough I hope.. it would work like this, as long as I have mains power the lights would be off, but as soon as the power is off the lights would be on (in this case LED's) now, I had planned on using a 18 Ah SLA battery which would be regulated on charge.

I was going to drive the LED's (3 of them) at 350 mA one thing I'm not too sure is run time.. and I know two of the LED's I have the vf is 3.3v at 350 mA and the white is vf 9V @ 350 mA

What is the SLA voltage?

A small 120V AC relay, which is held in by the AC. Wire the normally closed contacts in series with your LED lights.

Well I don't have the battery on hand but I would think off charge 14.4V then tapering off to around 12.5?