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Old 06-18-2019, 05:06 PM   #19
Oneilkeys
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Brand: Still Looking
Model: Travato
State: Florida
Posts: 2,475
THOR #1765
I do not pretend to be a battery expert, however, we have had this discussion previously in this forum. In an RV the coach and starting batteries are separate until engine or generator start or you plug into shore power. At that point, when the voltage of the main battery or batteries being charged (the chassis battery by the alternator or the coach batteries by the generator/shore power) reaches 13.1 charging volts, the Trombetta or BCC opens and the batteries are now in parallel. The following is the best explanation during our last discussions on this topic of what happens next.

“Let’s suppose you have 3 different 12V batteries, wired in parallel to supply 12V power to your RV. They can have different capacities on account of size or age, but the same chemistry (e.g. all flooded lead acid or all AGM). Before you start charging, the voltage across each of them is the same–even if one is fully charged and the others aren’t. Charge will flow from one battery to the other two until they’re balanced. With a lead acid battery bank, the internal resistances are limiting to a point that you don’t have to worry about arcing or your battery cables overheating when you connect them (not the case with lithium-ion banks…).

So when we start charging, all of the battery banks are very close to the same point as far as state of charge. Each battery acts like a resistive load, and current will flow to the battery with the lowest resistance, or highest capacity, more than the rest. All of them will reach the end of bulk charging together, when they’ve all reached the same voltage.”

One thread described this as three pails of water hooked together. As you add water to one, it fills up and then the water flows to the next and the next until all are full. The one you are adding water to does not overflow (as long as the inflow is not larger that the connecting pipe can handle).

In an RV, When the charging source is turned off and the voltage on one of the battery banks drops below 12.8v the Trombetta or BCC opens separating the banks.

I am not sure how it works with a truck and trailer situation or how exactly the batteries are wired to the truck alternator. Whichever way they are wired, it seems to me that it is similar in that both batteries are hooked in parallel when the alternator is running and separate when the engine is off. In that case, since the batteries are hooked in parallel when the alternator is charging them, however the alternator senses when the batteries are fully charged and goes into float mode will not happen until all the batteries are fully charged and you cannot overcharge one or undercharge the other.

It may be possible that in a trailer situation, the truck and trailer batteries are somehow not hooked in parallel when the engine is running (I don’t understand how). However this is easy to check. With the engine running the voltage on both the truck and trailer batteries should be about the same (around 13.5v or 14.1v if the alternator has a boost mode and the batteries are way down.
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