Shooting from the hip I had planned on 2 2/0 wires.
If I recall correctly, Fred (DiploStrat) actually used a pair of 2/0 for positive and another pair for negative on the big battery bank / dual alternators of his previous camper. He's a belt and suspenders kind of guy. Or at least he was. However, even with a pair of 125a alternators in parallel, and pairs of ginormous wire in parallel, again IIRC, he rarely saw over 150a charge current, and for the most part, significantly less than that, so I doubt he's so much into overkill on his current truck setup.
But, if I understand what you two are saying here correctly then, in the case that the alternator is connected to the main starting battery, and a cable from that same connection also runs directly to the 'camper' batteries - so you more or less have a series of parallell battery circuits - then why would the alternator be unable to 'see' the lower state of the camper batteries? I suppose maybe it depends on where the ground circuit is located for the charging regulator? I may need to poke around with the good old multimeter to actually see what I'm thinking in the noggin though.
This also makes me consider, again if I understand correctly, that it is absolutely imperative to make sure that the wire run from the alternator to the camper batteries has the same, if not less, overall resistance than the alternator to the primary starting battery. If this is done, without having to use massive 3/0 welding cable), then would the regulator see the camper batteries first?, or at the very least allow an equal charge to the camper batteries and the vehicle's main batteries?
Unfortunately, speaking in generalities, as Fred was doing in that bit you quoted, does have its drawbacks. It can get simple ideas across quickly, but breaks down under deeper analysis.
When the batteries are tied via ACR or dumb solenoid (either would be what is technically known as a "split-charge relay"), then it's just one big system. I call it "the 12v bus". The voltage regulator monitors the voltage of the bus at some place, and then reacts to changes in the voltage that it is monitoring. It reacts by applying voltage to energize the "clutch" in the alternator, known as the "field coil" to turn the alternator on, or cutting the voltage to the field coil to turn the alternator off. This happens very quickly.
Lead-acid batteries reach a state called "surface charge" where electrons build up on the lead plates at the plate to electrolyte interface. This surface charge has several effects, the one that concerns us in this instance is that the "apparent voltage" of the battery is falsely inflated. Stop charging and take a reading across the terminals of the battery and it might read 14v, even if the battery is less than half charged. (This is why you can't get an accurate read on the battery SoC with voltage, unless you stop charge/discharge and wait long enough for the surface charge to dissipate and then read the battery's true voltage, called the "resting voltage".)
Okay, so both batteries are connected to the bus (thus becoming loads on the bus rather than supplies to the bus). The voltage regulator is reading bus voltage somewhere. The bus voltage potential when the alternator is switched on is higher than the batteries' resting voltage potential, so power flows from the bus to the batteries. Each battery absorbs at a rate determined by its particular situation - resistance of the battery, resistance of the wire, battery voltage potential vs. bus potential. First of all, you don't get power flow from one battery to the other, because the alternator has a lower resistance/higher potential than either battery, so if either battery is going to absorb power, that power will flow from the alternator.
Now the engine battery, having been drawn down very little to start the truck, will very quickly reach a point where the SoC/resistance/surface charge prevents pretty much any power from flowing through it.
The aux battery, which we can assume is at a much lower SoC, will take longer to get to that point. Say, an hour. But it will get to that point. So power essentially stops flowing through the aux battery, the bus voltage is steady and high, and the voltage regulator spends the majority of its time with the alternator clutch switched off. But because the aux battery does have a low SoC, it will dissipate the surface charge somewhat quickly, it's voltage/resistance will fall some, it will absorb power from the bus, the bus voltage will fall, and the voltage regulator will switch the alternator back on.
As the aux battery reaches a higher SoC, it takes longer for the surface charge to dissipate, and the resistance is higher, so the amount of time the voltage regulator spends with the alternator switched on is reduced, and the amount of current flowing to the aux battery is steadily reduced over time (tapers off) as the SoC rises. (This is why lead-acid battery chargers "taper off" the charge current as the battery approaches full - it's not a smart feature of a well designed charger, as the marketing droids would have us believe, it's just a basic fact of how the physics/chemistry works in charging lead-acid batteries.)
This is unavoidable and will happen. It can however, be made worse.
One potential problem is if the wiring to the aux battery isn't adequate, then the resistance of the aux battery + wiring will further reduce the amp flow beyond what it is already being reduced by the battery's surface charge and SoC/resistance. At least, it will in the beginning of the charging cycle, when the amp flow is highest (assuming the battery wasn't completely dead, which would also have a very high resistance and cause reduced amp flow until the battery SoC came up some). As the battery approaches a higher (near fully charged) SoC, less amps will flow no matter if the wire is the size of a telephone pole, and the amps will taper off naturally. Reducing the amp flow, reduces the load on the wiring, which reduces the voltage drop induced in the wiring, which reduces the need for oversized wire.
Another potential problem is if the bus voltage is low (such as the previously mentioned Toyota 13.9v example, or an alternator/voltage regulator that puts out less at idle (as most do)). The lower bus voltage represents a lower difference in potential between the bus and the battery, which will reduce amp flow.
So if the voltage regulator holds a decently high voltage (I believe you said yours does 14.4v, which is decently high), then the only mitigation strategy needed is larger wire. But that only helps during the heavy amp flow part of the charging cycle (known as the "bulk stage"). Once the SoC reaches a certain point, the need for oversize wire becomes less and less. So my normal recommendation is to ignore such things as "voltage drop calculators" and instead size the wire to safely handle the max expected amperage flow. So if you have a 130a alternator, you don't really need wire/fuse sized to handle more than that. Sized to handle 150a would be adequate. Yes, you might see a slightly reduced charge rate (amp flow) during the initial bulk charge phase, but that effect will go away as the battery surface charge and SoC rise, and then for the
majority of the
hours long charge cycle, your wire sized to handle 150a would be grossly oversized. Not having oversized wire might add a few minutes or perhaps even an hour to a charge cycle that is going to take 6-8 hours (or more), depending. Oh, when designing solar systems for FBOs, that extra time might be a deal-breaker, but for charging camper batteries, it's not really an issue. Not driving enough, or not having enough solar, or drawing down the batteries too low on a regular basis is much more important than whether a full charge cycle takes 8 hours or 9 hours (or 15 hours or 16 hours).*
If however, the voltage regulator doesn't hold a decently high voltage, or the vehicle spends a lot of time idling (reduced bus voltage), then a B2B can help, by A) drawing down the bus voltage, thereby forcing the voltage regulator to keep the alternator switched on, and B) by bumping up (in DC electronic terms, "boost converting") the incoming voltage to a higher output voltage, thereby creating a higher supply vs. load difference in potential, and causing more steady continuous amperage flow through the battery.
* Note that this applies to sizing wire to charge lead-acid batteries. Sizing wire to feed a load
from a battery - such as an inverter - is a different matter; in that situation voltage drop matters because it gets worse over time, instead of getting better. Sizing wire to charge big lithium batteries is a different matter - you might
want to restrict the amp flow with a big Li battery.
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And all the other information on this thread and others.
