I believe those ideas are doomed to fail. You'd be better off to switching to a 24V alternator and then powering the 12V loads with a 24V to 12V converter.

You seem to be using sloppy terminology; voltage converters are ill-suited for battery charging, albeit, 12V batteries are probably very tolerant. Lionel's suggestion seems to be the cleanest, since it avoids trying to balance the batteries, and maximizes the stored energy.

TTFN (ta ta for now)
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The whole truck is the worst example of parts bin engineering. You should see the ballast resistors so that it can run 12 volt glow plugs.

Sourcing a 24v alternator shouldn't be too challenging and the 24-12 converters are commercially available.

While I've never seen a converter used to charge a battery we do use "power supplies" as chargers successfully. The key is to get the right voltage output. You want 13.6 or 27.2 volts out of the supply to properly charge a flooded lead acid battery system.

This is going to be my fun truck and nothing ruins fun like breaking down. I agree that the 24v alternator is the wisest route. The 24 volt alternator is way more expensive is why I avoided the idea but now I see the step down converter is much less expensive than the step up.

Don't forget a 100A alternator at 12V only produces half the power of a 100A alternator at 24V.

Quote:

I have a truck that has a split 12v and 24v electrical system. It currently uses two 12v alternators, one with isolated ground, to charge each battery individually.

If you only want one 12 Volt alternator to charge two 12 Volt batteries why don't you just charge both with 12 Volt?
You want get more power out of that alternator remaking the power to 24 Volt?



I am assuming that batter 1 12V is for the normal running of the car and that batteri 2 together with batteri 1 is for some contraption "toy" you have on it.

So how the loading will affect the batteries has very much to do with how often you intend to use this toy compared to how much you just intend to drive it.

Lets say the alternator is 100 A and each batteri 100Ah, it will take 2 hours to fully load the batteries if there is no consumption.
If you start of with 2 fully loaded batteries and you are just running the car Batteri 1 (12V) lets say it will go between 60 and 100 Ah (I am just making this numbers up)
Batteri 2 will stay fully loaded.
When you then start using your toy, lets say that you hade bat1 60Ah, bat2 100Ah you will have bat1 80Ah and bat2 80Ah (24V)
So when you have finished with that, both batteries will be loaded simultaneously and depending on how much is left in them the time needed just depends on how much you have taken out.

So loading 12 Volt or 24 Volt isn't so important just how many hours and how many Amps.

“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein

There is no 24V available in that drawing. At least the diode will die a spectacular death so you'll have some fun trying it.

Strange, it is an alternator, but it is marked with DC markings. Depending on the size of the loads/batteries, you might be able to use the AC part to boost the voltage and charge a smaller second 12V battery for the 24V service.
But I can't tell as you have not included enough information.

We do all sorts of voltage changes with AC, and a transformer. Then again, I assume you are using an automotive alternator, which I believe is a three phase/six diodes.

It's an automotive alternator, the 3-phase AC and the rectifier is inside.

I have the new engine in hand now. First glance says it has a Delco Remy 21SI alternator which is available in 24V for a reasonable price so I think I'm going to go with the 24V charge and 12V power supply.

My I initial reasoning behind the 12 volt charge and 24V supply was that most of the electrical load is 12V and the 24V is only an intermittent load for starting.

TBE, since the step-down power supplies are inexpensive do yourself a favor and use 2 of them.
Or buy one that is hell for stout, like this
https://www.trcelectronics.com/View/Mean-Well/RSD-...
They do make smaller ones also.
Or these
https://www.ato.com/dc-dc-buck-converter-24v-to-12...

And the 24V is the most critical part, if it doesn't start then nothing else matters.

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[quote You should see the ballast resistors so that it can run 12 volt glow plugs. [/quote]

Depending on how many cylinders there are, it should be quite possible to configure these into some manner of concatenation that obviates any requirement for ballast resistors, for example any vee six cylinder Diesel, two groups of three glow plugs in parallel on each bank of cylinders could be wired in series, such that each group of three receives 12 volts when 24 volts are applied end to end.

It would also in my view not be all that difficult to wire an unbalance relay with two voltage-actuated coils wired subtractively, such that if any one glow plug burns open a voltage unbalance would appear whenever the plugs were energized; the pick-up of this relay could serve to annunciate as a "glow plug failure" alarm, giving prompt warning of such and minimizing the likelihood of burning out the remaining good glow plugs.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

If it is an M1008 or similar, why not just put a 12 volt starter on it and get rid of a bunch of complication?

For starting only, there are series/parallel relays that connect the batteries in parallel when running and switches them to series for starting.

http://www.gonefcon.com/trucktcom/parallel_sw.htm

https://www.texasindustrialelectric.com/T_114_12_s...

12V loads are connected to battery A.

It is an M1008. As for doing the 12V conversion? I drive a M1008. I like to do things differently. Plus, if I ever want build my own HMI for gauges and such, I'll have 24V already available.

That series parallel relay could work. It would have to cycle off and on a few times with the glow plugs but that should be fine.