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Thread: running dual charging systems 12V/24V

  1. #1

    Default running dual charging systems 12V/24V

    Here is the question I am posing. I am in the process of replacing the 230 with a 350 sb. I want to keep the 24v lights because I have acquired the 24V LED MRAP lights to replace the headlights and I am wondering if I can run my 24V generator for the lights and jump capabilty for other military vehicles or 24V diesels on the original wiring harness and run a 12V alternator for the sbc and wire for that. I have modified the bracket assembly from the old A/C mount for the 24V generator(which works out quite nicely btw). Is there any isolation things I need to do or just keep the wiring seperate? any input is greatly appreciated
    Jeff Pall
    1967 M715 W/Winch

  2. #2
    Join Date
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    Lindale, Texas
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    I did my truck that way (12 & 24) and ran it for 3 or 4 years with no problems.
    It's been a few years and my truck is all 12 volt now. Memory is foggy about it now just how I did it. The only things I had were 12 volt was the starter and HEI. I do remember having 3 batterys. As I recall the 12 volt stuff was a different wire harness alongside the 24 volt stuff and the 3rd battery was behind the right seat. I also used a toggle switch for the ign. and a push button switch for the starter..........Al

  3. #3
    Join Date
    Aug 2013
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    Others will add their actual experience, But for sure you need to isolate your grounds.

    Especially on the 12 volt alternator bracket... You have to isolate the alternator from actually grounding to the engine... With rubber bushings.
    ( You dont want the 12 volt alternator electrically touching the engine, to make a "ground" )
    Float Your 12 volt grounds directly off the alternator body, (separate from the block and body ) and You will need to run 12 volt ground wires directly to each appliance, as well as the POS + 12 volt input wire.

    For Sure Fuse everything.
    ( I even fused my grounds, to be above safe )

    For sure, Isolate all of your 12 volt grounds, and any "appliance" connected in the 12 volt circuit, should NOT be attached ( electrically ) directly to the body of the vehicle.

    You will also need a battery, dedicated to Your 12 volt system and its associated loads, connected to the alternator, so it will energize its field, and the alternator will sense the battery and charge it as needed .

    there are a lot of write ups here on that subject, and a lot of guys have done that...
    So I'm hoping others will write in as to their experiences.

    this is just off the top of my head in 5 minutes.

    The easiest way and safest way is to just install a 24 volt to 12 volt converter box ( electronic ) and run Your 12 volt appliances off of that box...
    not emp proof BTW...
    Just sayin'
    Last edited by Blitz; November 23rd, 2014 at 08:17 PM.
    hostis est intra portas tuas

  4. #4
    Join Date
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    I understand the grounds need to be seperate between the 2 but to what does one ground the 12V stuff if the 24V stuff is grounded to the motor and body? Just to the battery or alternator body itself?

    How do the M10xx trucks do it with 2 batteries and 1 alternator and dual voltage?
    Lord send your Holy Ghost into our hearts and make the desire of our hearts Your Will.

    Pro-choice, that's a LIE, babies don't choose to die!!

  5. #5
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    Quote Originally Posted by brute4c View Post
    I understand the grounds need to be seperate between the 2 but to what does one ground the 12V stuff if the 24V stuff is grounded to the motor and body? Just to the battery or alternator body itself?

    How do the M10xx trucks do it with 2 batteries and 1 alternator and dual voltage?
    YES.

    Because the truck is "stock" at 24 volts, and everything "works" as designed at 24 volts...
    let the Truck "have" that Ground, that exists stock , and at 24 volts.
    Anything "grounded to the truck, motor, chassis, body will be "grounded" on the 24 volt circuit... Just as designed.


    When we go to add the 12 volt system, with an alternator and 3rd battery dedicated to ONLY 12 volts...

    you have to keep all that "grounding" separate for the two voltages.

    So any 12 volt stuff you install, must be insulated from the body and chassis and motor and frame "electrically", so you are not incorporating the 24 volts that is already running around the truck.

    The easiest way is to INSULATE the Alternator (12 volt ) from the motor with the rubber bushings, so it does not make electrical contact anywhere ( including the bolts) with the existing 24 volt grounds. (any metal on the truck )

    Once the alternator is isolated, run a ground wire (BIG) from the 12 volt alternator chassis itself ( Isolated from the other metal of the truck, and the brackets ) , directly to the dedicated 12 volt battery (and do NOT ground this battery to the truck frame, chassis, body, etc... )

    The HOT wire from the 12 volt alternator, also goes directly to this 12 volt dedicated battery.

    everything is Floating now... on the 12 volt side.

    So when You want to install say a driving light.... that is 12 volts...
    If the "frame" of the driving light is used as a ground, and there is one wire coming out of the driving light....
    You will have to run (install ) a dedicated 12 volt ground wire to this "frame" of the light, AND INSULATE this frame of the light FROM the truck chassis, body, motor, etc... So it does not contact the 24 volt ground system.
    Like with a piece of gasket cork, rubber or wood.

    Does that make sense?

    Another example is that if you want to install a driving light that has TWO wires... One ground and one hot ( 12 volts ) You can then USE the "frame " of the light to directly attach to the body of the truck...( because its only a frame and not an electrical connection)
    But you still have to run Your two wires to the light. A 12 NEG and a 12 POS.

    So long story short,
    You will have to run TWO wires to every appliance on the 12 volt system, no matter what...
    And all of these wires originate FROM the 12 volt battery itself, and the 12 volt alternator only charges the 12 volt battery.

    Its probably easiest to make a fuse panel or 12 volt marine breaker switch panel OFF of the 12 volt battery POS side, with one big wire to the panel, then smaller wires to each appliance for the 12 volt POS + wire "feed in ".

    That way everything is fused or breaker ed on the 12 volt power in side, or fused... as it should be, and these can also be Your on off switches, if You get a MARINE 12 volt re settable Breaker Panel , with push to reset switches.
    ( they also act as a normal ON /Off Switch , until they trip at 15 amps and have to be re set )

    As a Final Precaution,
    I would suggest also fusing the grounds.( Just for Murphys Law, and to do everything possible to keep from burning down the truck )
    A blind crossed wire, between the two 12 and 24 volt systems "could" happen on the ground wires as well, so just fuse them too.

    Once again, just a fuse block that holds normal blade type fuses ( 5 or 10 should be adequate in number of fuses )
    And run one BIG Ground wire, directly from the battery to this fuse block for the NEG side of the 12 volt circuit.
    Then as before, run the smaller diameter wires to each appliance for the NEG ground for the circuit.


    Its a lot easier in reality than it can be explained in type.

    The rule is.... Isolate everything... Hots and Grounds, from the "stock" truck grounds and charging system...
    Like the truck 24 volt system is antimatter, and the 12 volt stuff is Matter...

    The two cannot mix.

    How they do it on the M1XX series trucks...
    Is unknown to me.

    It has to be the same as above... Isolated grounds and Isolated hots.

    I cant see how it could be any different... and share a common ground.

    I know this is wordy, but I'm trying to be concise.
    Last edited by Blitz; November 24th, 2014 at 10:08 AM.
    hostis est intra portas tuas

  6. #6
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    Those trucks, I am speculating here, must have a converter somewhere as they have 2 batteries, 1 alternator and both 12 and 24 volt items...but do the 12 volt items then need separated grounds too?
    Lord send your Holy Ghost into our hearts and make the desire of our hearts Your Will.

    Pro-choice, that's a LIE, babies don't choose to die!!

  7. #7
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    Honestly,
    I do not know.

    I'm not familiar with the M1xx series Trucks ( Yet ).

    I can speculate, that the alternator has both 12 and 24 volt outputs on it?

    If so facto,

    I will "assume" that there must be a separate ground system for the 12 and 24 volt circuits as well.

    Maybe they do it "electronically" with some sort of isolater "box", or electronic switching gizmo that can sense the different voltages?

    I would "assume" that would be possible to do in some sort of "smart box", that does the same thing as running separate wires,
    but I do not know how they would do that, and I'm pretty weak in electronic engineering.

    I'm really curious now though...
    As this knowledge will help me, If I could understand it.


    Hopefully Barrman will show back up and explain it, as I'm sure he has one of there M1xx series trucks, or at minimum, understands the principle of the circuits with a dual output.

    Its really interesting...

    I did not know those trucks ( M1xx serries )had such a dual system, until i read it this morning.

    Now you lit a fuse... : )
    I'm gonna have to find this out.

    I hope someone will just explain it, though.

    As looking at wiring diagrams is not rocket science, and i can do it and understand it, But my old eyes are getting weaker, and that spaghetti gets confusing on paper.

    Some sort of blocking diode that senses 24 volts or 12 volts, on the NEGATIVE side, and lets the 24 volts go to a separate ground, while the 12 volts goes to another ground?

    I'm sure is do-able electronically... with such a smart switch.

    Kinda like splitting the sine wave of 120 volts into direct current...
    Its done ... electronically... and I think with blocking diodes...
    But to do that on DC only...

    I just do not know !!!

    A little help here please?

    hostis est intra portas tuas

  8. #8
    Join Date
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    Al, nice to see you posting. I thought you had a box of some kind on yours as well? I remember you showing it to me when we were there painting your truck and that I was supposed to take it home but forgot it.

    The CUCV trucks are 24 volt start/glow plug/slave system. Everything else is 12 volt. 1 alternator isolated ground and the other is just regular. Super easy system to understand once you study the diagrams. I wired my M715 up like them when I put the 6.2 in. You are trying to go the other way though.

    I would suggest making everything on your truck 24 volt including the starter. Then use a convertor for the ignition. Much easier than the extra alternator/wiring/controls/switches. Cheaper too.
    Remember if you didn't build it you can't call it yours.

    6.2 powered M715, 5 M1009's, M416, 2 M101's, 2 M105's, 3 M35's, M1007 6.5 turbo Suburban project called Cowdog.

    https://www.youtube.com/channel/UCCz...HGkBCfhXZ5iuaw

  9. #9
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    I must be missing something, Why can the grounds not be bonded together on a dual voltage system?

    If the dual batteries that make up 24 volts have the negative bonded to the chassis and the 12 volt system has the battery negative bonded to the same point how is that a problem?

    Now if you were trying to do positive ground mixed with negative ground I could see a problem.

    In theory electrons flow from positive to negative. If both systems have the negatives bonded together I don't see where the problem is.

  10. #10
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    Are you asking CUCV wise or just theory wise? I really don't want to take over his thread writing CUCV stuff if he isn't going to use it.
    Remember if you didn't build it you can't call it yours.

    6.2 powered M715, 5 M1009's, M416, 2 M101's, 2 M105's, 3 M35's, M1007 6.5 turbo Suburban project called Cowdog.

    https://www.youtube.com/channel/UCCz...HGkBCfhXZ5iuaw

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