Petrol Powered Battery Charger Build

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  • Petrol Powered Battery Charger Build

    Ivé been getting back into a project that first came into conception about 4 yrs ago when I bought an MT45 Engel fridge.

    At the time I didn't have a suitable Wipper Snipper engine for the job (Had a few that needed attension though) to be coupled with a little 45A internally regulated Alternator out of a Suzuki Sierra/Swift.

    Working with the numbers.
    30cc 2 stroke = about 1.5hp = 1.1KW
    1100w / 12V = 91.6A

    Assuming 75% effeciency from the alternator = 68.7A
    But even if only 50% effecient 1100w / 12V x 0.5 = 45.8A

    So at full capacity the little petrol engine should have the balls to drive it but when only used to top batteries up then it is not going to need to go at full noise any way.

    I machined up a clutch drum to go on the alternator & a Spool or Bell Housing to spigot the two together for correct alignment.
    To do it I found a spigot diameter on the alternator from where they had "Jigged it" during manufacture but they had only used shallow dowel holes & not threads. Easy fixed, I just carfully drilled them out, making sure not to break thru into the windings & tapped them with a modified plug tap.

    It's looking like it will be a nice tidy little unit once finished, no bigger than another battery & will weigh heaps less.

    Still need to build a housing for it so I can have a Voltmeter, an Ammeter, Throttle control & on/off switch.

    By having an on/off switch connected to the regulator I will be able to get the little motor fired up & running without load. Then once it is happy, flick the switch to energise the field windings & load her up.

    Waiting on a new regulator to turn up ATM so will be doing an update soon when I get it up to run stage.


  • #2
    Gee your a clever little vegimite

    You have given me a idea I got a old whipper sniper motor sitting in the shed ( can't remember what brand ) but my newer one is a stil ( coz I do a bit of whipper snipping )

    Just to find a alternator


    • #3
      nice one mick, can't wait to see it up and running. will be a noisy little bugger at full noise. thought about insulating the box when it's complete?



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        Edit to add quote (Nuzo jumped in too quick for me )

        Originally posted by 6602steven" post=14510
        will be a noisy little bugger at full noise. thought about insulating the box when it's complete?
        Thinking about building a bigger exhaust box into it too, packed with 2 stroke Motorbike exhaust packing. Wont need to be all that big, just like a double skin or jacket in the base.
        For now though, as a prototype to keep things simple, instead of sitting around camp listening to it.
        It would be the sort of thing I could fire up when going to do something else like cutting fire wood & just let it run until it is out of fuel.

        One advantage of this as apposed to using a generator & charger is that Engel issue a warning never to have the fridge connected to the battery while charging, because most chargers are not regulated very well. Hense when I was using the genny & charger the other week I had to swap the fridge over to 240V off the genny & disconnect the battery to charge it.
        With this set up, the Alternator is regulated to a max of 14V & so is just the same as running the fridge off an auxilliary out let in the vehicle while driving.
        So running the genny & then wandering off to do something else was an issue because if it stopped for some reason, the fridge was left without power until I got back. Not so with this setup because it will remain connected to the battery.

        Cheers, Mick.

    • #4
      Awesome idea!

      just a few notes of caution though.
      - make sure you have the AC to DC Diodes, Alternators generate AC Electricity which the diodes convert to DC for use with charging batteries.
      - Alternators need an external input current for them to start up.(to charge the field windings) i.e. a battery, once started they should be self sustaining.

      a good quick read here;

      Good luck and let us know your progress! this could be an awesome little 12v generator!




      • #5
        looks the goods -- one question though ? will it be like a GM ?, will it hold its revs or just die in the arse without having some inertia like a flywheel.
        still the goods though -- I have seen it done with small four stroke motors and they work fine --but that`s around 3-hp driving an alternator.
        I go with the idea of it being better than a second battery as it can be entirely independent -- Do remember to take petrol though if your cruiser is diesel
        Don’t poke the snake, walk around it and come back later with a double-barrelled shotgun and blow its [email protected]#!ing head off!.

        Australia in future, the outcome is the same, a bloody dictatorship run on the whims of a very few ego-centric pathological elitists.


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          Originally posted by NoFerals" post=14594
          looks the goods -- one question though ? will it be like a GM ?, will it hold its revs or just die in the arse without having some inertia like a flywheel.
          Do remember to take petrol though if your cruiser is diesel
          I'm expecting it to hold it's revs while under load. That will also be an audible signal as to when the battery is full because the regulator will reduce the voltage in the field windings which will reduce the load & the revs should pick up, that's when I will know it is done without having to look at the Ammeter (I hope)

          If I don't have any of that, the Chainsaw will not be very happy either

          BTW, Did those 3hp 4 strokes sound like they had much load on them?
          I would assume they were 60A alternators, maybe bigger.


      • #6
        my only concern would be that, being direct coupled, it will have to run at full throttle to make its rated HP.
        A belt or geared reduction drive would let it belt out, but maintain a reasonable rpm at the alternator whilst also holding onto its power band.

        just sayin'.


        • Straightshooter
          Straightshooter commented
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          I built something similar about 35 years ago using a 2 stroke lawn mower motor with a belt drive to an email alternator. It wasn't a huge success.
          Some potential problems.
          Your arithmetic regarding output power is a little off. The alternator produces an output of 14.4 volts with a normal regulator but you also have to take into account the losses in the internal rectifier so your calculation should use a figure of 15.6 volts times the output current to calculate watts out.
          Further most alternators produce full output at about 6000 rpm so if you are trying to supply a heavy current load your whipper snipper motor might not have enough output at lower rpm to ever get near those rpm.
          Even so I would expect that there would still be enough output to charge batteries or power a small 12 volt fridge.
          Good luck with your project.

      • #7
        its an old idea and they work very well ..they have been around for a while. But Its good to see you building one for yourself....


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          Well This little gem worked a treat.

          Recieved my new Regulator yesterday so I slipped it in & did a test run.

          Battery started out at 10V (stuffed battery)
          Once fired up I energised the Regulator to bring it on load & due to the Very low battery it was working hard at first but still had the balls to do the job nicely.

          I didn't have an Ammeter on it but watching the Volts on my Multimeter I could see that it was only pushing 12V so I'd imagine it was driving heaps of Amps (45A Alternator) Then after only about 60sec the load dropped off (obviously the regulators doing) The voltage came up to 14V & I would say the Amps eased off a bit to the point that the little 23cc motor was just humming away nicely, topping up the battery which would by now be up around 12V
          Another Min or so of run time & I flicked the covers off the battery to see it bubbling away nicely..........Gold!

          About a total of 2.5 - 3 mins run time & it had brought that battery back from flat (10V) to 14V no worries.
          Hooked it up to my 4A charger to see all but the last green light lit up so it well on the way to a full charge.

          Under normal top up conditions (12V & above) the little 2 stroke only needs 1/2 throttle to hum away nicely at 3/4 revs so my numbers worked out just great. Got the balls to drive it at maximum demand which means it is not working hard under normal conditions as a top up charger.

          Now to get mounted in a frame with guages & wiring.
          Might do a vid on it then.


      • #8
        awesome stuff dude!




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          Should have also added.
          That throttle setting in the Vid is only the idle screw wound up a bit, it still has about 2/3rds more throttle to go.

          I'm in the process of making on the drawing board & list of jobs to do is a nice big, knurled Brass throttle nob to make it easier to wind it up to full throttle for use on batteries that demand more than 10 or 12A.
          Screwed into the old cable mount with the shortened cable working the butterfly instead of the idle screw.