DIY Portable Charger/generator


[Glenn] from The Back Shed has built a lot of windmills and stationary generators over the years, but recently decided to try his hand at building something a bit more portable.

The charger is based of a relatively simple design, employing a 5.2 HP Kubota 4 stroke motor and a 12v car alternator to provide power. While you might be inclined to point out that his charger does exactly what an alternator and motor are built to do, there was a bit more to it than simply slapping the two parts together.

A laser cut adapter plate holds the motor and alternator together, but once [Glenn] wrapped things up and gave the motor a spin, he realized that he was driving the alternator backwards. This would eventually cause the alternator to overheat since the cooling fan was running the wrong way. He removed the fan and reversed the fins with a hammer so that he could get the cooling he needed without having to reinstall the alternator in the opposite orientation.

The whole kit was mounted on a hand truck for portability, and [Glenn] says that the charger/generator only needs to run about 5 minutes before a dead battery has enough juice to crank an engine.

[via HackedGadgets]

26 thoughts on “DIY Portable Charger/generator

  1. Alternators are suppose to be very inefficient. That’s what I’ve always been told when looking at energy generators. He’s probably throwing away most of the effort of that motor. Seems to work for him but I’d try to get a better generator instead of that alternator.

    1. It depends on how fast you spin the alternator.

      Because the voltage regulation is handled by varying the magnetization current with a bit of electronics. The slower you spin it, the more magnetization is needed to keep it at 14.7 odd Volts that it’s supposed to do.

      Normally you’d have the engine run at around 1500-2000 rpm with the belt gearing it up a notch from that. A stationary engine runs much slower, because you don’t want it screaming in your ear all the time, so it’s less efficient.

      Unless you put in an extra set of pulleys and belts to make it spin faster.

  2. Years ago, a friend of mine built something similar using a high-displacement weed-eater motor and a high-output alternator. His intention was to market a device to auto sales lots, which frequently have dead batteries.

    He had some mechanism that I don’t remember that would throttle the motor based on current demand, or maybe voltage. He was a little bit secretive about that part of it, saying he had planned to patent it. Never did, as far as I know.

    He never commercialized it, and now there are small 300W generators on the market, along with those portable battery units, which are often enough to boost a car off unless the battery is totally shot.

    I like the idea, although I’m inclined to think the engine used is a bit over-kill. Given the loading of an alternator, there’s probably a fairly simple formula to back into how much horse power you need to actually make a certain amount of power. I know my open-frame generator has an 11HP Honda motor that will produce 6500W continuous/7500W peak.

    1. Oh, and if it’s a regular 60 Amp alternator, then it’s going to need about 1.2 HP at full tilt.

      So yeah, the engine is a bit oversized, and that makes it less efficient as well becase you can’t get the throttle open very much.

      Ideally, you would want the engine to turn with the throttle wide open somewhere around half its maximum RPM, or wherever the torque peak for the engine is to get maximum efficiency out of it. That means it’s not generating the maximum power, because power depends on the RPM as well, but it’s much more than 1/5th of the maximum.

      1. If you run at peak torque it will have the least pumping losses and will reduce the friction loses. The problem is that you are also running it at the peak combustion pressure so the motor will be close a detonation.
        The ideal setup would have no throttle so that you don’t have the pumping loses of the throttle.
        I wonder if you could make a very efficient motor using fuel injection and instead of throttling the intake you varied the amount of exhaust gas that got recirculated. You could control the power by decreasing the amount of fuel and use the EGR to control the mixture.

      2. Actually though, for the simplicity of control, you would want to run the engine a bit past the peak torque speed.

        Because then, if the load increases and the engine starts to slow down, the engine catches it by increasing the torque quite naturally. When it idles it runs a bit fast, and putting a load on it drops it to where it makes more torque.

        If you’re running it under the peak torque speed, then any significant increase in load will result in decrease of torque and the engine stalls, or dips badly, unless the governor catches up really quickly. And likewise, disconnecting the load will cause the engine to run away.

        Small engines like lawnmower engines usually have a governor that operates on the cooling blower pressure, and it’s really slow to react, which easily causes oscillation. To make them work right you need to add a flywheel to simulate the inertia of the blades, or it will never run right.

  3. and in less something has changed I believe that an alternator (out of a car) requires power to make power. So if your battery is stone cold dead an alternator will not energize to charge it…

      1. Then again, if your battery is stone cold dead, then no current source will revive it because it’s dead.

        0 Volts at the terminals means it’s gone for good, at least for lead acid.

    1. Here, though, he’s using an already-self-sufficient motor to run an alternator, and it would be quite simple to create a small magnetization circuit to get the alternator cranking.

      Keep in mind that a lot of smaller alternators (that would be sufficient to charge a battery) also have a permanent magnet, instead of using a charged winding like what you’re referencing.

  4. I have been doing some research into alternators recently. They are typically 55 to 65 percent efficient. So generally every 30A of current requires about 1 HP to make.

    According to this paper most of the loss in efficiency is due to copper loss in the coils followed by diode losses

    You can bump this efficiency by about 20% By using active rectification with mosfets. Designing a controller to sense and time turning on the mosfets is the tricky part. I’m working on this currently. (hint its cheapest and easiest not using a micro controller)

    You could also put some more efficient diodes with a lower forward voltage drop in parallel to the existing diodes. Diodes inc SBR4040CTFP might be a good choice

    1. You’re going to need a much larger engine than the one used in this project, and a different generator/alternator. I always wanted take a generator from a portable welder, and connect it to a geared up hydraulic motor, powered by a PTO attached to the engine in the truck it was to be used on, in order to avoid the annoying noise of a true portable welder. I thought of this when I used to use various bucket trucks, some of which used Onan generators to power the boom, and others used PTO. The PTO trucks were much, much more enjoyable to work around…

      1. Interesting. Have you given any thought how you would about fitting the existing welders automatic throttle or some sort of governor to the vehicle engine? In the past my ability to home brew or a or electric facsimile of a governor a satisfactory automatic throttle thwarted my dad’s and I effort to fit an aircraft generator to to a car engine to build a portable welder. No doubt others solved that problem, but this was decades before popular access to a WWW. While they aren’t particularly noisy hydraulic motors aren’t particularly quit either.

  5. I have been thinking about a similar project. I scored a Xantrex 24V input 120v 1800watt output true sine wave inverter. (pretty pricey so a good score) I was planning on using a small motor with a 24v alternator going into a battery bank of 2 12v gel style marine deepcycle batteries. I figure the battery bank will be a nice buffer, and I can pull 12v, 24v DC power, or 120v ac current for a variety of things. The inverter has a built in low voltage shut off, so I would only need to charge the batteries intermittently, and not have to worry about them going flat. Only issue is going to be the weight of the whole thing.

  6. I like Glenn’s write up. No so much that an automotive alternator was anymore efficient that the automotive generators at the time. Just that an alternator could be driven at speeds to provide output at lower vehicle speeds, and survive mechanically.

    For those using the older GM alternators with an engine that turns in the wrong direction, from what I read the tip is to use a fan of a Corvair alternator.

    Reads as is the decision was to use an engine that was on hand. Theoretically the engine is capable of creating more power if ran at full bore, but who wants to be around a one plunger running at full bore for very long? There is an advantage of using an engine larger than the calculated need in this application. That the engine can be “geared” to drive the alternator in a fashion so full output from the alternator be realized at lower engine speeds.

    Here’s a PDF file of a similar project that appeared in QST some years back

    http://www.other used to have a page on a similar project that included a template to build a bracket as seen in this project, but I can’t find that there now.

  7. Hi guys.

    I think some of you are missing the point of this(my) project. The car alternator isn’t the most efficient beast, but its what I had lying around. The motor is a little over sized, but its what I had lying around! This is a throw together project that cost almost nothing, and has been a very handy tool, its got me out of trouble many times. Dont over analyze it, because if you over analyze everything, nothing is worth doing. If its raining and the car has a flat battery and you need to jump start it, you dont care how efficient the jumper cables are, do you?

    Other questions. I was always aware the cooling fins needed reversing, but for short duration running its OK, they still move some air. I reversed them to finish the job, as it were. The small stationary motors run at about 3000rpm, the alternator is running at about 4000rpm, and it can supply over 50 amps when needed. The motors have a built in governor, they throttle up under load automatically. Yes, the battery needs some charge to fire up the field coil of the alternator, but not much. I’ve used it ok on a battery that was down to 6 volts and would barely light the light bulb. If the battery was dead flat, chances are it wont take a charge anyway.


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