How to build your own generator capable of 12VDC at 60AMPS
This is a really cheap project to build. My whole unit cost about $160 Canadian dollars (only because I didn't have an old motor for it). I really like it because I can use it for camping for my 26 foot travel trailer, or even to boost batteries around the yard. I also use it for backup to charge my battery bank for power for the house when needed.
What you need
I used a vertical shaft motor such as a lawn mower engine, they are cheap and available. Mine cost brand new on sale at a price of $99 for 3.8 horsepower. It is also complete with tank, air cleaner, exhaust, pull starter. If you go with a horizontal shaft you might have to purchase these extra.
You need an alternator such as from a Chevy from the wreckers. Make sure it has an internal regulator. Make sure it has the pulley on it also. I took mine off a wrecked car I have out back.
You also need a 4 inch pulley to fit the engine with the same belt type as the alternator. Preferred V due to price.
Then the last thing you need is something to bolt everything too.
I also purchased a rather large exhaust so I could keep my unit very quite so the neighbors would not complain. This was also on sale for $20.
You will also need something to mount the alternator and the motor to. I found a small cart looking thing with wheels on it in the garbage. It also had a handle on it which is actually nice to drag around.
How to Build
Mount the motor and the alternator to whatever you choose to
use. I have seen on the internet people just using an old lawn mower
and cutting holes into the back of the mower for the belt to run
through. Then they mount the alternator towards the back.
once you get everything mounted find a pulley for the lawn
mower engine. I have found that the pulley I am using at 3.35 inches
in diameter seems to be the right one that will spin the alternator fast
enough, but also allow my 3.8 horse power motor to keep up.
find the right size of belt that will fit snug between the
motor and the alternator. Put it on. I used an idler pulley 
wire for your ground to the chassis of the alternator. Take all other wires and tie them together from the plug of the alternator, also connecting the thick wire on the one post on the alternator. Then run another off here to wherever you want your power for positive. This will activate the field in the alternator, but if you leave it connected to a battery without the alternator, it will drain it. Even better if you have a wire that has an "F". If so that is the wire that has to be wired to the battery positive post on the alternator.
NOTES
I have found that I can charge at about 30 AMPS if I let the lawn mower engine idle, but it seems to labor quite a bit. So when I charge I am usually at half throttle. This will give me 60 AMPS charging but the motor seems allot smoother.
This setup will give me approx 500 watts at 110 Volts AC with a inverter such as Stat power. If you work out the figures, house power is way cheaper at 6-10 cents an hour where this setup is about 55 cents, based on the amount of fuel I usually burn. But as a backup generator it works great. especially when you are camping, and trailers are allot more efficient with the power use also so it seems to go farther.
I have tried to keep instructions on this page as complete as possible with keeping in mind not to be complicated. It's enough information to build an working unit yourself.
Have fun!
10/22/2005
I have made some changes to my unit, it is much much smaller so I will put some pictures on this site as soon as I get them. It works even better and I have also built it without any additional pulleys, and it made it run without additional noise. It works great.
My 12 Volts DC to 117 Volts AC Backup setup
OK, now that you have read this, I have started building and using my Permanent magnet alternators. They put out so much more power, no regulation, but twice the power! My battery bank is large enough that I don't miss having it regulated, and also, its nice to be able to overcharge a bit to equalize my battery bank.
Once converted to AC with a larger inverter, it can supply almost 1000 watts AC and keep up. I tested it with a 900 watt oil filled space heater and the unit still keeps ahead. That being said, I don't run the unit more than 45 amps charging continuous due to the heat on the alternator. I try never to go over 170 degrees Fahrenheit to preserve the permanent magnets.
If you can, build an alternator with PM's. If not check this site out as he has some really nice units. The 12Vdc model is good for a 5hp motor and will put out 60amps plus, my gauge stops at 60. The new system. I am using a Honda motor now days and it works very well. It is way better on fuel also, with a larger fuel tank on it (very easy on the Honda's) it will run on about 1 gallon for 8-10 hours. I have the throttle turned way down to keep it 35-45 amps of charging. The first time I tested this, I started it at 6:00 at night, but when I left for work the next day, I was surprised to hear it still running!
