Home-Built 12-Volt Electrical Generator

Build your own home made generator and battery charger

Summary: How to build a 12 volt generator on a budget using a lawn mower engine and automotive alternator.

You may want to check out our Selected Books section under the DIY and Projects category for books covering boat projects and renovations that you can do yourself.

I wish I had photos of this, but I built this project quite a few years ago, didn't think of taking pictures at the time, and sold the generator with my last boat.

When I was young and cash-strapped, and planning a 3-month summer trip aboard my first boat "At Last", I needed to find a way to charge my battery. My electrical needs were very minimal with that boat - just lights at night, VHF radio, cell phone, and stereo. No fridge (no icebox either!), GPS, auto pilot, computer, etc, etc. But I didn't have any solar panels and there was no engine alternator on my outboard either.

It happened that I was visiting my Uncle Claudio's place one day while he was cleaning out his shed, and found that he was discarding an old Lawn-Boy lawn mower that hadn't been in working condition for a few years. He kindly gave it to me to tinker with, and after taking it apart, cleaning out the fuel system, replacing the condenser and points, and fixing one of the reed valves (for intake I think, but it was 15 years or so ago!), I got it running.

It turned out to be a perfect motor for an electrical generator (apart from the thunderous lawnmower noise) because it used a centrifugally- actuated governor to control the throttle, giving it a constant RPM.

I removed the motor from the lawnmower chassis, removed the blades, and attached a pulley to the power shaft.

A visit to an auto wrecker produced a cheap GM alternator of the sort that was self-exciting - no need to feed power to the field coils. I can't remember the ampere rating on it - 40 or 60 amps most likely.

I built a chassis out of plywood and angle iron to mount the motor and alternator, connecting the two of them with a V-belt. I glued some thick rubber pads to the bottom of it to absorb some of the vibration and keep it from rubbing off the deck's gel coat.

For electrical connections, I set up a positive and negative stud on either side of the chassis (so they wouldn't be accidentally shorted), and set up a series of resistors with an alligator clip to the alternator's regulator so that I could intervene manually with it to keep the output at a reasonably high rate. Doing that meant having to keep an eye on the battery's state while charging, but I had to be around anyhow to keep an eye on the generator. Basically, the more resistors I put between the battery and the regulator (using the alligator clip), the lower the battery voltage the regulator thought it saw, and the higher the charge rate it wanted to put out.

To charge the boat's batteries, I just fired up the generator and hooked the positive and negative terminals of the battery to the positive and negative posts on the generator, and away it went.

Total cost of the setup at the time was something like $25 for the engine parts, $10 for pulley and V-belt. The rest of the materials I had laying around.

Of course, it was LOUD. I always made sure to move to an out-of-the-way place to run it so as not to disturb others at anchor.

The generator served well through 3 boats, and was still working nicely when I sold it with my previous boat "Belle Argo", although it was seeing less use at that time since the Belle's engine (Volvo MD2B diesel) could charge the batteries, and I had two solar panels as well.