I believe that real car batteries will not survive long if you just take them out of the car and discharge them. They usually can only stand being empty a couple of times before they are shot.
Here's a quick list of power supplies that you could probably use, I'm sure you could find cheaper ones though:
http://www.universal-radio.com/catalog/hamps.html
I think the MFJ ones are probably the most cost-effective. Remember that you can always use a 12V power supply around the shop for other things too, so it's a good idea to have one. If you had a power supply and a battery (like a robot battery), you could rig it up so that you could just switch power sources when you were "mobile." I definitely wouldn't charge the battery with the power supply though
Good Luck
