You don't need a car battery, any (about) 12V battery would do just fine. My old 20W DIY-halogen light used 12V NiMh battery which I built from 10 cells.
But anyway, halogen is ancient history, just forget them. LEDs are what you should be looking at. And no, not the stupid and crappy halogen bulb replacements, but something like Cree XM-L etc. All the info you need to build your own led light is available on this forum. It may seem a bit complicated at first, but in the end really is quite simple. Yes, really, it is.