Power for Car Living
I bought a 12Volt, deep cycle, Marine battery for my van. Its main use is to power two fans--- which keep me cool during summers in the South. The fans are ~4.5 inch brushless fans... the kind used in computers... and can be bought at Radio Shack. I wired them (in series) to the extra battery. At night, I put the fans in opposite windows (one in the back right, one in the front left)... one is used for intake, the other blows out warm air. These fans draw very little power... but even in July- in Georgia -they quickly cool the sleeping area (took one hour at most).
In stealth conditions, I wedge the fans between the front seats and the car body... and crack the front windows several inches. The fans are pointed to the openings... and though not directly in the window, they still cool the car rapidly. As a result, my car is far more comfortable than many of the airless guesthouses I have stayed in - in both India and Thailand. Also, the fans provide "white noise" that drowns out external sounds.
Another possible use for the battery is to power a 12V electric blanket (or heater) in the winter. Georgia winters are mild so I never investigated this option.. though I plan to when I move to Vermont!!
Beyond such basic necessities, the auxillary battery (or batteries!) could be used for a whole host of appliances. Check out the link to "Chagnon's Power Essentials" for the full range of possibilities. Though I'm essentially a minimalist, I admit that blending fresh smoothies in my van has a certain decadent appeal (Chagnon's carries 12v blenders)!
Last summer I re-charged the Marine Battery using a battery charger. This is the type that plugs into a normal 110 outlet. I was parking at a friends house at the time so that worked great. However, I plan to wire the battery to my car this summer so I can re-charge it while I drive. It's quite easy to do (thanks Chris for the wiring instructions).
Simply wire the deep-cycle battery to your car battery in parallel (+) to (+) and (-) to (-). You can put an off/on switch in the (+) to (+) line in order to disconnect the extra battery once its charged. Use large guage wire (jumper cable type wire would be great) for these connections if possible. Try to keep the Marine battery fully charged, as leaving it in a partially discharged state will decrease its life considerably. I recommend good ventilation when charging batteries, as they can give off hydrogen gas when overcharged (crack the windows and charge the batteries while driving around).
Also, it is important that the Marine battery be dis-connected from the car battery/electrical system WHENEVER THE CAR IS NOT RUNNING.... otherwise you may reduce the life of your car battery. Installing an on/off switch is the easiest way to control this.
Also note that if you connect two or more Marine batteries in parallel, they should be of the same type and in similiar condition (both new, for example).
Finally, when using auxillary batteries, a multi-meter is recommended (volt/ohm/amp)... you can use it to test your batteries and to test the power draw of various appliances.