Picture of your Battery Box?

[Awesomeness]

I diveded the 24V and 12V circuit. I wanted to get rid of the 12V drop of between the two batteries.
The charging of this system works without problems. And I never unload the 24V system with the light or radio.

That doesn't seem like a very good idea. Can you explain more?

 

[gemini25]

That doesn't seem like a very good idea. Can you explain more?

Why shouldn't it be a good idea? The M1078A1 has an alternator with 24V and 12V. So with the 24V I charge the two big batteries (2x12V serial) and with the 12V I charge the small batterie. And for the truck it isn't a problem neither. Both systems are getting charged without a problem.
The reason to do this was, I wanted to get rid of the mitlitary style batteries. These batteries are now standard truck and car batteries. You can get them everywhere.

 

[coachgeo]

Why shouldn't it be a good idea? The M1078A1 has an alternator with 24V and 12V....

you probably know this.. But just in case.... all versions of the FMTV are this way.

Does the alternator's regulator works as he is thinking? Aka- like it's two regulators in one? I suspect not

 

[Awesomeness]

Why shouldn't it be a good idea? The M1078A1 has an alternator with 24V and 12V. So with the 24V I charge the two big batteries (2x12V serial) and with the 12V I charge the small batterie. And for the truck it isn't a problem neither. Both systems are getting charged without a problem.
The reason to do this was, I wanted to get rid of the mitlitary style batteries. These batteries are now standard truck and car batteries. You can get them everywhere.

You met your original goal of "wanting to get rid of the 12V drop", but that goal wasn't an improvement (besides using COtS batteries, which would have been electrically better with 4, or even 2). Can you explain why the "12V drop" is an issue?

 

[gemini25]

That doesn't seem like a very good idea. Can you explain more?

Why shouldn't it be a good idea? The M1078A1 has an alternator with 24V and 12V. So with the 24V I charge the two big batteries (2x12V serial) and with the 12V I charge the small batterie. And for the truck it isn't a problem neither. Both systems are getting charged without a problem.
The reason to do this was, I wanted to get rid of the mitlitary style batteries. These batteries are now standard truck and car batteries. You can get them everywhere.

you probably know this.. But just in case.... all versions of the FMTV are this way.

Does the alternator's regulator works as he is thinking? Aka- like it's two regulators in one? I suspect not

The A1 is equipped with the Niehoff N2003 Load & Battery Control Device (I am not sure if the A0 has it too) and the charging works without any problems.

 

[gemini25]

You met your original goal of "wanting to get rid of the 12V drop", but that goal wasn't an improvement (besides using COtS batteries, which would have been electrically better with 4, or even 2). Can you explain why the "12V drop" is an issue?

Normally 2 batteries are parallel to 2 batteries in serial (I hope you can understand it). The 12V worn down between it. So 2 batteries are getting more discharged then the other two's. I know the truck is equipped with the Niehoff Battery Control device. But in my opinion (and the opinion of my mechanic) this is not good for the livetime of the batteries. Basically the state of charge of the batteries is different.
And I can let the lights and radio run till the 12V batterie dies. The truck just need the 24V to get started.
(Hope you can understand my explanation. Speaking english is easier as writing )

 

[Awesomeness]

Normally 2 batteries are parallel to 2 batteries in serial (I hope you can understand it). The 12V worn down between it. So 2 batteries are getting more discharged then the other two's. I know the truck is equipped with the Niehoff Battery Control device. But in my opinion (and the opinion of my mechanic) this is not good for the livetime of the batteries. Basically the state of charge of the batteries is different.
And I can let the lights and radio run till the 12V batterie dies. The truck just need the 24V to get started.
(Hope you can understand my explanation. Speaking english is easier as writing )

In short, what you did is worse in almost every measurable way, electrically. It will probably be good enough for your needs, since the system is admittedly overkill to begin with, but hopefully people don't read your post and think "That guy said the 12V drop was bad, so I need to mimic his 'solution' to get away from it." You came up with something cheaper and easier to source, but worse.

It's better to draw (and charge) across more batteries, not less. I'm not sure why your mechanic would suggest otherwise. They set the batteries up correctly in the original wiring layout, and splitting the two circuits results in more imbalance, and more potential problems.

Yes, with your layout you can run the 12V stuff until it dies, but with the original layout you could run it 2x as long without dying. Also, every time you run a lead-acid battery "dead" (below a couple volts on each cell), you cut roughly 15% off its total capacity/life. Deep cycle batteries use special materials to try to combat that, but it's still really bad for the battery.

AGM batteries, like the original military batteries, are significantly better technology. They have lower self-discharge rates, lower internal resistance, higher current capacity, extreme shock resistance, and significantly longer lifespans. Despite how expensive they seem, even buying the actual military batteries will nearly pay for itself from the extended life alone. If you're going to swap away from them, a set of 4 smaller COtS AGM batteries (e.g. Optima "red tops") would have been a better solution (though still a significant reduction in capability).

 

[tennmogger]

I see the merit of wiring like that. In the original military setup the batteries in series, call them the 0-12 batteries and the 12-24 batteries, DO get charged differently. I have verified that on the four trucks here and others. The reason is the Niehoff voltage regulators (some models at least) set the 0-12 to a fixed voltage using SCR's (read that somewhere) and the 12-24 set of batteries get the difference between the 12 v regulated voltage and the 24v output. Those voltages are seldom the same. Verify that if you doubt it. Measure your voltages with truck running.

Many users rotate their batteries for that reason, swapping the 0-12 with the 12-24 occasionally. Lots of work but an excellent idea.

The way Gemini25 wired the batteries, the 24v pair are not unbalanced by the 12v load and charge purely in series so they will not become charged differently, that is they stay balanced. The 12v battery is in-effect maintained by it's own regulator so is not in an unbalanced pair. Paralleled batteries and unbalanced series pairs are killers on batteries.

Anytime two batteries are in parallel they will suffer. No two batteries are the same. The 'worse' battery will draw down it's partner and both will discharge prematurely. Not a problem on batteries used a lot, but let them sit and the batteries go bad. Note the condition we almost always find the batteries on newly purchased trucks.

Gemini25's idea eliminates that parasitic draw. Obviously I like this idea. However, I would have used another large battery for the 12v. The 24v pair need the high CCA for cranking. The 12v battery needs the higher AH rating.

 

[Awesomeness]

I see the merit of wiring like that. In the original military setup the batteries in series, call them the 0-12 batteries and the 12-24 batteries, DO get charged differently. I have verified that on the four trucks here and others. The reason is the Niehoff voltage regulators (some models at least) set the 0-12 to a fixed voltage using SCR's (read that somewhere) and the 12-24 set of batteries get the difference between the 12 v regulated voltage and the 24v output. Those voltages are seldom the same. Verify that if you doubt it. Measure your voltages with truck running.

Many users rotate their batteries for that reason, swapping the 0-12 with the 12-24 occasionally. Lots of work but an excellent idea.

The way Gemini25 wired the batteries, the 24v pair are not unbalanced by the 12v load and charge purely in series so they will not become charged differently, that is they stay balanced. The 12v battery is in-effect maintained by it's own regulator so is not in an unbalanced pair. Paralleled batteries and unbalanced series pairs are killers on batteries.

Anytime two batteries are in parallel they will suffer. No two batteries are the same. The 'worse' battery will draw down it's partner and both will discharge prematurely. Not a problem on batteries used a lot, but let them sit and the batteries go bad. Note the condition we almost always find the batteries on newly purchased trucks.

Gemini25's idea eliminates that parasitic draw. Obviously I like this idea. However, I would have used another large battery for the 12v. The 24v pair need the high CCA for cranking. The 12v battery needs the higher AH rating.

Yes, there is a parasitic draw when batteries are in parallel, but it's not drastic, and the AGMs would handle it better (lower internal resistance). Taking away 1/2 your battery capacity to get rid of it is a really silly solution. If it bothers you that much, adding an isolator solenoid/switch would be a better idea, or add a charger/Solargizer.

Similarly, splitting the 12V and 24V circuits just unevenly loads the alternator because it was designed to charge a 12V circuit, and then also charge a 24V circuit that the 12V circuit was inside of. (That's why, as you describe, the 24V "gets the difference" because they assume the 12V is not separate.) And you lose the capacity and redundancy of the doubled-up battery circuit.

Except in unusual situations, this is not a practical improvement.