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One of the most useful things I've built is a portable battery charger. Basically a car alternator driven by a small petrol motor, its great for charging a flat battery quickly.
Its got me out of trouble a few times. Examples of where I used the charger are...
1. Flat battery in the car. Or a car that's hard to start after some repairs. The portable charger can re-charge a car battery in a few minutes.
2. Power Failure. I have an inverter to power a few appliances from my small battery bank, but I don't have a lot of battery capacity, so the portable charger can keep the batteries topped up.
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You can use just about any small petrol, or if you can find one, diesel motor for a portable charger. Lawn mower motors are a good choice, 4 stroke is preferred, though 2 stroke can work. If you took the cutting plate off, and now find the motor hard to start, you will need to add a flywheel. A large steel pulley will work.
Weed Eater or Whipper Snipper motors? Forget it, your going to need at least 4 or more horse power to drive a alternator at full power. |
 This is my first build of the portable charger. The motor is a Kubota 5.2hp 200cc 4 stroke I bought off ebay a few years back. The alternator is a 75amp 12v Bosch with internal regulator. At the time I had access to a laser cutter, so I lasercut the adapter plate from 3mm mild steel. It worked pretty good, but wasn't very portable.
Since then I've mounted the charger to a old trolley, see the photo above. Now its portable.The wheels are from an golf buggy.
After building the charger I discovered the motor runs in the opposite direction to the direction the alternator was designed to run. I should have mounted the alternator out the front of the motor, facing the other way, but too late now, and besides I like the compact layout. The only problem with running a alternator backward is the cooling fan wont work as good as the alternator needs it to. If you only intend to use the charger for a few minutes, on light loads, you can get away with it. But if you want a bit more reliability, then you need to reverse the fan.
This is the factory fan. Its designed to run clockwise looking at the front, but I've got it running anticlockwise. |
So I took the fan off and belted it flat with a hammer. To take the fan off you'll need a rattle gun, or rip the whole alternator apart and grab the armature in a vice, that front nut is tight! If you do pull the alternator apart, be careful when you put it back together not to break the brushes. |
Then with the help of a vise and hammer I bent the fins the other way. Its pretty easy, except for the last fin, a bit fiddly. |
And back on the alternator. Its now OK to run anti-clockwise. |
While I was at it, I decided to give the rusty old adapter plate a clean up and paint. |
All back together with belt fitted. |
I also added a guard to stop little fingers and dog noses loosing skin on the belt or fan. |
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