Re: Charging from tow vehicle

On 3/5/2025 7:41 AM, Ted Heise wrote:

So I looked at the manual for the fridge, and it clearly states it
only runs on 12 V if the trailer is connected to a running tow
vehicle.  I guess the next step in better understanding the issue
would be to hook up the trailer and check the battery status while
the fridge is running on 12 V.

  Regardless, it seems pretty clear my Acadia tow vehicle is not
  producing enough juice to help.

 If I want to look into getting a DC-DC charger and cable to back
of my Acadia installed, where would I start--is that a GMC dealer
thing, or an RV shop?  Other?

OK, this is a good place for getting started on what I was suggesting we do to get some consensus on this charging of trailer batteries universal problem for us RVers.
The first place to start would be the 7 prong plug in, on the tow vehicle, and the vehicle being towed.  I am going to take it for granted most of us would be using a 7 prong plug.  Any differences are minimal and easily solved.
So let's start on the tow vehicle.  It might surprise some of us that our plugs don't even have power in them as delivered from the factory. I have an F-150 and was quite surprised myself that I had to install an additional fuse to get my line hot.  It was wired, but not fused and inactive.  With today's CANBUS systems, it can be difficult testing this, and you really have to use a 12 volt test light and not a volt meter.  Mine was wired with 12 gauge wire, and I did install a 12 gauge and a fused link that went from the umbilical directly to the battery on my Toad.  This, however is a regular automotive battery and not a Lithium battery.  This would NOT work for charging a lithium as it needs a converter/charger before the lithium, and even if I did have one, the trailer 7 prong is probably only going to be able to provide between 5-10/12 amps and that would not be enough to fully charge a lithium battery.   I think you need at least 20 amps to ever get a lithium charged.  (This is where having a bigger alternator could be useful, as you could get a 40 amp or bigger charger greatly speeding up charging.) Therefore, a DC-DC charger is the next logical option (setting aside a perfect solar charging install).
The next question is what is available on the trailer.  Ted, for example can plug into shore power, and even use his 7 prong, or so he thinks. So what exactly do you have on the trailer that deals with recharging the house batteries if anything?  Does it already have a charger that would be capable if you could only get it enough power?  When you're on shore power, does it also charge up the lithium batteries?
On mine, for example, the house batteries will charge off the alternator when running, and will get charged off the shore power when plugged in. They also get charged by my solar panel on the roof.  I believe if I am on shore power, there is something in the controller that switches off the solar, but I'm not positive on that.
So, I think it is a certainty you cannot get enough power simply off the 7 prong to do much of anything.  So a DC-DC charger is a perhaps, but a 6 gauge or bigger line for power to the trailer is a must.  Now, you have to figure out what you already have on the trailer that you could use, if anything, and what you will need to get.  Next step is becoming certain of what is on the trailer now.  Would also be a good time to get a better understanding of what that solar plug it has installed actually does.  For example, does it just give a route for power from a solar source and charger/controller to the batteries, or does it actually have any of the hardware in the path?  On mine for example, I have a similar solar plug that I can add additional panels to which I believe are then used by the internal RV controller/charger.
please remember this is all new to me too, and I'm trying to work it out as we go too, so I've probably made some false assumptions.  That said, I think if we go step by methodical step, we can figure out how to properly deal with all this in an informed manner.
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Better Days Ahead!
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