Thanks, tim292stro, for taking the time for such a detailed post.
The stock M1010 has 2 200A 24V alternators and a device called a DUVAC. The bottom alternator charges the 24V battery bank. The top alternator provides 12V power to the vehicle, and charges the front battery, which is the one connected to ground. I'm told the DUVAC rapidly cycled the top alternator on and off to make it produce 12V-14.4V, even though it's a 24V alternator designed to produce at least 18V. What could possibly go wrong? (Lots!)
Many schemes have been tried to replace this with a more reliable and trouble-free system. The SS
Plan B Mod is the name of the most successful and widely used (I'm told) approach. If you're interested in this topic, I suggest you read that thread. It covers all the details.
My personal M1010 came with a fried top alternator. Parts for an alternator rebuild were $650, and we didn't know if the DUVAC fried too, so we went for the Plan B Mod. This replaces the upper alternator and the DUVAC with a 160A 12V single-wire alternator.
Remember the 12V vehicle power is drawn from the front battery and the alternator that feeds it. If you only used the 24V alternator to charge both batteries in series, and you draw 12V from the front battery, that front battery will quickly become depleted. I found out about this in detail on my 600-mile recovery with a failed top alternator.
I hear what you're saying about the 2 alternators not coordinating their efforts. The point of my earlier post was to document the fact that this dual-independent-alternator approach seems imperfect, at least on my truck. Compared to the 14.4 volt ideal, the front battery charging voltage is about half a volt low, and the rear is about half a volt high. It's also true that the rear battery is only used in starting. It sees no load when the truck is running, while the front battery & its alternator provide all 12V power to the truck. That load difference might account for the half volt.
Folks have tried lots of different approaches, using lots of different devices to replace the DUVAC. I read all their horror stories, and went with the Plan B Mod because folks say it works. I have about 1500 miles on mine -- so far so good.
I do not have a separate house bank. I do have plans to add a separate starter bank, used only for starting, and run the rest of the truck off what you might call the house bank. That would require the addition of a simple battery isolator.
You asked for a wiring diagram. This is what I have in mind for adding a starter bank. I bet you won't ask for a wiring diagram again. ;^) The idea here is to reserve the starter bank for starting only, and run everything else off the "house" bank. A few items essential to starting (Glowplugs, coolant heater, battery heater) live on an Aux Start distribution block. These normally run off the house bank, but if we screw up and run that flat, we can run Aux Start off the starter bank. This is a whole separate topic, though.
I don't have an as-built picture handy, but if you look at the picture linked above, the starter solenoid goes to the 24V Distribution Block, and the isolation relays and starter bank (the upper right portion of the diagram) do not exist.
Back to the original issue, I think that if I adjusted my 12V alternator to output 14.4V, rather than the 13.8-ish it outputs now, then my batteries would receive balanced charges. With my batteries, I don't think that adjustment would make a bit of difference in their performance or longevity. For others running less robust batteries, that difference might be important. That's why I broached this topic in the first place.
Thanks again for your contribution to our dialog.
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I'm curious as to your setup. From my recollection, the M1010 is the only CUCV version with a single 24V alternator, so this dual alternator thing is probably making a mess of your voltages - especially if the alternators are not the same voltage and routed to a specific load.
One of the beautiful things about the GM CUCV electrical system is that each 12V battery had its own dedicated source of 12V power generation. With 12V loads one the lower 12V battery, only the alternator that saw those loads had to deal with them. So few things used 24V on the trucks, that you essentially had a spare alternator in the truck - you could take the high-side battery's alternator and put it on the lower 12V battery, and still have enough juice in the uncharging upper battery to glow your plugs and crank a few times - more than enough to limp home.
When using a 24 Volt alternator to charge a series string of 12V batteries, you don't want a separate charging source on one battery. This is due to the way that the alternator's voltage regulators work - they presume a very simple electrical system where they are the ONLY source or charging power. If your lower 12V battery is being charged by the 12V alternator, the unloaded upper battery may get undercharged as the 24V alternator's regulator would see a higher circuit voltage and attempt to fix that by dropping the output current of the 24V alternator. You would still measure a fairly normal 24V charging voltage because the regulator is correctly cutting the power. The flip side is your 12V battery could see an undercharge if your upper battery was absorbing less charge and allowing the lower battery to receive more of the voltage - this would have the reverse effect of tricking the lower 12V alternator into turning off its field to prevent an over-voltage condition.
Either way, it's inherently unstable to have two alternators working as a team with no knowledge of each other. Think of it like this, you're in a row boat and you have the left oar in your hands and you have to correct the course of the boat with it while propelling the boat forward. Some other guy you've never met is sitting on the other side of a wall going straight down the center of the boat doing the same thing with the right oar. Neither of you knows the others strength or how big your oar is, and you aren't allowed to talk to each other. The only way you can attempt to steer the boat is by watching what the boat is doing and reacting to it. Not an efficient/effective way to master a boat is it?
Enter as the hero here is a device called a Vanner - this is a "simple" device that takes a 12V output and makes it exactly 1/2 in reference to ground of what the 24V input referenced to ground. In this way, the 24V alternator only has to watch it sole job of getting the 24V (28.8-29.5) set-point right, the Vanner takes care of equalizing the batteries. If you draw power from the 12V low-side battery, the Vanner makes up the difference while keeping the voltage exactly half of the 24V reference input. This also means that your 24V series string will act as double the size 12V battery, since the total amps drawn from the 12V supply is made up from the full 24V input (double voltage, half current).
Vanners are cheap, I've see the 100Amp version for less than $200 used in good shape, you can team/parallel some models to get higher capacity (70 series for example). One of the slick things you can also do with this is charge a separate 12V battery from a 24V power source without the need for an additional isolator diode. Refresh my memory, do you have a separate house battery bank in this truck? Can you provide a rough sketch of the main power generation circuit in this truck?