How to Build a 12Volt Portable Generator

If you have ever been interested in creating a generator for your DC shop power use, battery charging, or to have available in emergencies or out camping - then this ehow is for you. Described here is a method for creating your own high current 12 volt DC charging system that is very effective for charging batteries and supplying DC power for short periods of time using a gas-powered lawn mower engine, and a car alternator.

    The purpose for building this generator is to provide a cheap but effective source for creating constant 12 volt DC power. This unit can be used on its own, or in conjunction with already existing 12 Volt power plants such as wind or solar power. The most reasonable applications of use for this generator would be to charge car or boat batteries, to maintain power to a battery bank on an alternative energy source, to help jump-start a car, or even to use while camping or in the event of emergencies - for short periods of use (with an AC converter).

    The first thing to do is to find an old lawn mower on Craigslist or free-cycle that has a working engine. I would recommend that you find a 3-5 horse power Briggs and Stratton motor (3hp is very common). A lawnmower engine is a vertical shaft mounted engine that is easy to use, maintain, and replace if needed. If you have to buy the whole lawnmower, look for one that is old because we are going to be getting rid of the body and just keeping the engine. NOTE: You can use a horizontal shaft engine as well, like what you would use for a go-kart. My first 12 volt generator was actually made using a Horizontal shaft Briggs and Statton 5hp motor which was being used on an old go-kart. I first played with the idea to make the go-cart generate its own electricity and charge its own batteries - and viola - a portable generator idea was born. Again, you can use a horizontal shaft engine as well, but the only difference you will have using this engine is in the configuration of your alternator and pulley setup.

    Horizontal Shaft Engine

    Here is a picture or a horizontal shaft engine for your reference.

    GM Alternator with internal regulator

    The next thing you need to find is an automobile alternator with an internal regulator. All alternators on the market today have internal regulators - so they will not be hard to find. Go to a junk yard, Murrays or AutoZone, and you will be sure to find an alternator. To save some money, get a rebuilt unit or get one from a junk yard. It does not matter what model you use as long as it has two wires coming out of it. The cheapest one I found was a rebuilt unit at Auto Zone - model number: Delco-Remy 1100934 37A, 3D10 12VNEG.

    12 volt battery cables

    Now pick up some red and black automotive battery cables, also available at Murrays, AutoZone, or your local hardware store. Get at least three feet, and make sure that you have the terminals on the battery cables. You can use the 2AWG12 type.

    Great! Now that you have the engine and the alternator, you are 70% done putting this together. I recommend that you place the motor on the floor of your garage, and also place the alternator next to itonthe floor of the garage. When setting this up, you want to find a large 2x6 and mount the engine to it. If you used a vertical shaft engine, just drill a hole for the shaft sit down through. If you used a horizontal shaft engine, mount the engine in place on one side of the board with the shaft pointing outward. (see picture for reference)

    Once you have mounted the engine in place, place the alternator next to it. The space in between the engine shaft and the alternator will be filled with a belt to help transfer the power of the engine into the alternator. The belt we want to look for is called a V-Belt. These are really easy to find, so do not worry. Go to your hardware store (not Home Depot) more like True Value, Ace, or even a garden equipment shop and they will have a lot of different sized V-Belts available. You want to get a 4 foot A Style Industrial V-Belt. Keep in mind that the belt length should be kept to a minimum to reduce belt slap and associated wear - the size of the belt really determines on your setup, but use my picture as a guide. It is important to note that I did not use a V-Belt from an automotive store because they just were not as hardy as the industrial style belt.

    Cast Iron Pulley

    While at the hardware store, purchase one cast iron pulley wheel. This will also be found in the garden equipment section normally. Look for the Chicago Brand Pulleys. You want a pulley with a 3/4bore (interior hole) with a lock nut, and a 4" - 5" overall diameter. They are relatively inexpensive, and I found mine at True Value Hardware.

    Now connect the cast iron pulley to the lawnmower shaft at a distance that will help you attach the v-belt to the alternator. I will add pictures later for reference.

    Simply slide the alternator away from the engine on the 2"x6" board until the pulley is nice and tight. Set some screws down to hold the alternator in place. Now remove the alternator and the pulley and re-install the alternator a few centimeters past the screw holes you used to hold it down away from the engine. This will help to add tension to the pulley that you could not add with your hands alone.

    Re-Mount the alternator. Use a Prybar to help you get the pulley onto the cast iron pulley (flywheel) on the lawnmower engine, and onto the flywheel of the alternator.

    Great - you are almost done - 92% of the way there! Now just run the wire leads from the alternator to an automotive or marine deep cycle battery.

    Connect a DC to AV converter on the battery, and you are ready to power your accessories. You can use this generator as it is without the converter to power 12 volt accessories, or use the converter to power things like small televisions, lights, and radios when out camping.


    • Vertical shaft mowers have the shaft and blade that sticks out straight down the bottom of the engine. Horizontal shafts stick out the side. Most vertical shaft lawnmower engines have a shaft that is 7/8" with a 3/16" key way and a threaded hole in the bottom of the shaft, once the blades are removed.

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