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24 volt to 12 volt

jimbo66348

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It's the same as putting 4 batteries in parallel. You only draw off the last battery, but all of the others back it up. One won't be overcharged, and the other undercharged. They are connected together.ions will travel from a greater concentration, to lesser concentration. In addition, the lmtv has one 24 volt alternator. No such thing as a dual voltage alternator in anything like this. Look at the battery wiring diagram. It's the same as the 900 series. Two in parallel X two in series. The 12 volts is taken off of one set of batteries.
 

simp5782

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It's the same as putting 4 batteries in parallel. You only draw off the last battery, but all of the others back it up. One won't be overcharged, and the other undercharged. They are connected together.ions will travel from a greater concentration, to lesser concentration. In addition, the lmtv has one 24 volt alternator. No such thing as a dual voltage alternator in anything like this. Look at the battery wiring diagram. It's the same as the 900 series. Two in parallel X two in series. The 12 volts is taken off of one set of batteries.
Well I guess that will be your little secret in your own truck. Ok the lmtv alternator has a dual voltage regulator for it. Either way. You will kill one battery before the other with a bad cell. Run a high amp draw and the battery will not charge back up fast enough. Something as simple as a fan. Install a 12v fan. Drive the truck for 3hrs and I bet you it won't start back up.

But I'm pretty sure the LMTV/FMTV trucks charge both 24 and 12v side off 2 separate outputs

https://www.eriksmilitarysurplus.com/al100ampduvo1.html
 
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74M35A2

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Nothing wrong with adding a high powered DC-DC converter like OP is doing. I like the contactor for auto power off, just make sure it is continuous duty rated (like the one in our PCB for dash power).

On the same token, and a lot of people have a hard time with this one, is that the stock military M939xx wiring configuration is to draw 12v from one battery (pair) only for the heater blower fan low speed. It is even separately switched as a dedicated 12v circuit through the dash switch. I know a lot of people will make several excuse-guesses for why they did it, but there it is, factory designed, approved, and released build configuration, in the wiring diagram and every 939 made. The argument becomes more difficult once done for production.

Personally, I treat batteries as consumables, as I run 2 x new $100 group 31's and would draw as much 12v off one battery as needed. I toss them at the 5 year mark and replace them regardless of how they test or crank the truck anyway, judged by the calendar only. I think it is cheaper to do it this way rather than spend extra $ on a 12v system. To me, it doesn't matter if the 12v from one battery is harming it or not, as it is getting tossed out sooner rather than later anyway. I have done it, and have not noticed any adverse effects.
 

silverstate55

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I just read this thread. Why did you go through all this expense, when you could have just ran a fuse panel off of one battery? With a disconnect, if you are prone to leave things running.
That's putting an awful lot of faith in the older alternators....
 

Iceman3005

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Nothing wrong with adding a high powered DC-DC converter like OP is doing. I like the contactor for auto power off, just make sure it is continuous duty rated (like the one in our PCB for dash power).
Yes the solenoid is a 24 volt rated for continuous duty.


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74M35A2

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My favorite topic. They do it by pulling 12v off one battery (bank). On the 939’s, this 12v circuit is even ran through the dash ignition switch, as to shut the heater blower off when keyed down. Factory, stock, approved design.
 

74M35A2

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That’s where the various opinions come into play. Some say nothing. I’m on that side. Others say it shortens that battery’s life due to inbalance.

If one runs $100 group 31’s, I’d be fine with it, even if shorted life. Decision each makes.
 
161
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Argyle MI
I agree. I purchased a small charger battery maintainer from the farm store. Just 12v. I've left the light switch on just with the dash lights and killed the batteries enough it wouldnt crank. Put that little charger on the first battery 12v bank. Overnight good as new. Every so often I'll put it on just for that battery's health. For that last for months I've ran my truck with no alternator. Mine shorted and fried some wires. Ran the truck all day. Even the fan on low. That little trickle charge overnight. No issues, starts every time.


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bluehorse

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clackamas, Oregon
Maybe this was covered but anytime you pull voltage off a single battery in a bank of batteries it causes an imbalance in the batteries. The imbalance causes some to be overcharged and some to be undercharged which is what kills the batteries. This is also why you should use exact same style, model and age of batteries in battery banks.

You may check into getting a latching solenoid they do not get hot like the continuous on relays. https://www.ebay.com/itm/Intellitec...441567&hash=item3d474b8bab:g:M-oAAOSwX4lZrvm~
 

m16ty

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Pulling 12v off of one battery will shorten the life of your batteries, it's proven science, and any battery manufacturer will tell you this. How long it shortens the life depends on the 12v load. Two or more batteries in parallel will balance themselves out, two batteries in series won't.

There are several ways to build a split 12/24v system, the OP is showing one way.
 
161
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Argyle MI
That may but true. In my experience with batteries it's a crap shoot. I've had some that didnt last a month and others a decade.

Sent from my SM-G892A using Tapatalk
 
161
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Location
Argyle MI
Pulling 12v off of one battery will shorten the life of your batteries, it's proven science, and any battery manufacturer will tell you this. How long it shortens the life depends on the 12v load. Two or more batteries in parallel will balance themselves out, two batteries in series won't.

There are several ways to build a split 12/24v system, the OP is showing one way.
Guess it's good they run two 12v banks parallel and then connect them in series to make 24 then. I imagine it's not an accident. More likely for battery longevity than cranking power. Up until the early 90s john deere ran 6v batteries in series to make 12 in all there tractors. Except an early very poor 24v setup that caused nothing but headaches.

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74M35A2

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Two seperate 6v batteries in series vs one 12v is same. Battery cells are in series. Each cell generates 2V. 12V battery has 6 cells in series, 6v has 3 cells in series.
 

doghead

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Please stop debating this in this thread.

The OP did not ask for it, and you're completely derailing his thread
 
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