12v car battery stuff

Due to our culture's reliance and fixation upon automobiles and the economics of mass production, the rechargable 12 Volt car battery is your best bargain in terms of power per dollar.

This makes it ideal for powering your Burning Man campsite or installation.

Also: Silent operation. I really hate generators. Really really hate them. Noisy and smelly and icky. Also: Wide variety of appliances and gear available to run on 12 Volts, sold for cars, boats, and RV's. This includes lights, dvd players, audio equipment, LCD tv screens, fans. It's all very easy to hook up.
A typical (*) battery charging arrangement.
Use RED consistently for the positive terminals, and black for the negative. Paint a RED plus sign near the positive battery terminal. Use red wires. Most gadgets will follow this convention. (But house wiring does not.)
Why use a fuse?
A car battery doesn't have enough voltage to shock you much, but it can be very dangerous if you accidentally short some metal across the power contacts. They are designed to start automobiles, so they can deliver lots of power when needed. If you short metal across the terminals, it will heat to the melting point. This includes jewelry you might be wearing, falling appliances, and bits of copper sculptures.

Metal rings have been known to amputate and cauterize fingers, accidentally.

A fuse is designed to burn out if the load looks "unusually high".

Almost any size fuse is better than none; I've used "10 amp" fuses throughout, for no particular reason.

Insulate the positive terminal of the battery and all wiring that happens from there until the master fuse.

Put the master fuse as close to the battery as possible. (An inline fuseholder is great for this.)

And if you do not use a fuse, please be careful! No, strike that, just freaking use a fuse.

What Kind Of Wire?
There's various standard charts which map Amps and Volts and wire gauge against length, heat and voltage drops. But let me skip ahead to the answer: Use 14 gauge or heavier, keep the wire length for each circuit down to 5 feet or less, and use a 10-amp or smaller fuse.

If your wire is too thin, or runs too long, then -- if you accidentally short something -- the wire might heat up and melt instead of the fuse. This probably won't be dangerous, nothing like a ring biting off your finger with 120 cranking Amps, but will be more annoying to fix than a fuse.
How long will your battery last?
Batteries are rated in "amp-hours". Your basic car battery has about 100 amp-hours on a full charge. (You want to get marine "deep cycle" batteries, which are designed to run appliances and then get recharged.)

Stuff you find for cars or boats run on 12 volts, directly off your battery. Their power needs will be rated in either Watts or Amps. 12 Watts = 1 Amp. (For a 12 volt system, that is.)

So, your basic car battery has about 1200 Watt-hours on a full charge.

If something uses 1 Amp, your car battery can run it for 100 hours. If something uses 2.3 Amps, your car battery can run it for 100/2.3 = 43.47 hours. If something uses 20 Watts, your car battery can run it for 1200/20 = 60 hours.

Sometimes devices are rated in mA. Those are milliamps. 1000 milliamps is 1 Amp. 600 milliAmps is 0.6 Amps.

My Burning Man Subway Station installation used about .5 amps for the stereo, and .3 amps for the Cool Neon lighting. So it could run for 100/.8 = 125 hours. I left it on continuously, and swapped in a second fully charged battery about halfway through the event.

(By the way, I highly recommend www.coolneon.com for your EL Wire needs. Benny does an enthusiastic business out of Oakland, and loves to help burners get their art together. Web or phone order, or drop-in.)
How To Build It

Take a look at the pictures above. I found that putting everything onto a piece of thin plywood was very helpful. It provides, you know, structure. The terminal strips and four-banger fuse holder and automotive switches all came from radio shack. Use hot-melt glue or epoxy to anchor things down. Choose boxy switches that have a face you can glue down, or make a simple front panel face plate like I did.

One of my four switched circuits is connected directly to that dashboard-lamp, and also to the voltage meter. It's handy to be able to flip the switch and see something work, and see if your battery is putting out some volts. The lamp draws half and amp, though, so I don't leave it on.

This was built before I learned that you really should insulate the positive terminal more safely than I did.

Special thanks to Matthew, Bobeson, Dolphin, and Space Cadet, and others, for their help and wisdom on this.