My solution was to get a lawn tractor battery and mount it in a box along with an "on board" battery charger. The main disadvantage to car batteries is that they are HEAVY. The lawn tractor battery I have is a Sears Gold 335 CCA. It is about 1/3 the size of a car battery. It has a lot of juice and I have yet to run it out of power even after several days. I run a fan as well as heating elements for the secondary, finder and Telrad for my 20" dob off it. After using it on the weekend I just plug in the charger and it's ready to go for next time. The charger shuts off automatically, so I don't have to worry about overcharging. I think the battery, charger, and misc parts came to about $75 total. It works great. Heck, I've even used it to jump start my car!

JW