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From: Ted Heise <theise@panix.com>
Newsgroups: rec.outdoors.rv-travel
Subject: Re: Charging from tow vehicle
Date: Wed, 5 Mar 2025 20:03:57 -0000 (UTC)
Organization: My own, such as it is
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On Wed, 5 Mar 2025 09:05:19 -0600,
  sticks <wolverine01@charter.net> wrote:
>  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.

Agree this is a good place to start, and you actually anticipated 
my next step.  I've been puzzling over in my head on the lack of 
charging while towing.  The Acadia does have a factory installed 
7-pin connector right next to the receiver.


>  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.  

My thinking was to take a reading from the LiIon battery app with 
the battery connected to the trailer and then with the fridge 
started (I think it may run off the batt for a minute or two).
Then I would connect the 7-pin to the running TV and repeat these 
readings.  Does that seem like a reasonable approach?

I need a block of time to get out to the storage unit for this, so 
in the meantime I figured I'd take some measurements at the 7-pin.
First I connected clips from my multimeter with the TV off.  
Surprisingly, I got some sparking, may have been from touching 
across pins.  But once I had a good connection I got no voltage.

Now this could be from the line not being hot (e.g., because of a 
fuse needed), or because a multimeter won't work.  But I vaguely 
recall having gotten a 12-13 V reading from the pins some time in 
the past.  So maybe the sparking blew the fuse.  I'm trying to run 
down where that would be and will let you know what I find.

It's a pain to prop open the spring loaded connector cover to get 
in there with meter probes, so I'm thinking I might look for a 
7-pin plug with attached wires that would make the connecting for 
measurements a little easier (and cleaner).

Another possible hitch (so to speak) is a small incident about a 
year ago.  I was towing a rental trailer and went over a RR 
crossing dip too fast and the trailer tongue popped off the ball.  
It yanked out the connector and adaptor (4 to 7 pin).  The mount 
bracket for the 7-pin on the TV also got bent upward and toward 
the front of the car, so it probably bottomed out going over the 
dip.  So it's possible some connections got damaged, though 
nothing was cut completely because I've used it with all the 
signals and braking working since then.

In any case, the car goes to the dealer on Tuesday to get an 
antenna repair, so I'm going to ask them to check out the 7-pin 
while I'm there.

>  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).

Yep, that all makes sense.  I'll be digging into that more after I 
get the basics above figured out.


>  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?

Yes, the LiIon battery charges up quickly when on shore power.  
The trailer has a Progressive Dynamics PD4000 Series Power Control 
Center...

https://www.progressivedyn.com/pd4000-series/

According to the manual, it has a setting for lithium ion 
batteries, and I believe I have it set correctly for that.

FWIW, the battery is this...

https://www.litime.com/products/litime-12v-100ah-lithium-lifepo4-battery?srsltid=AfmBOooYbGPTdOaEImeJqF-g0vqKIb27qtceloDoubGXsc1Pk8-UK_C4



>  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.

This all sounds right.  I think it's reasonably clear what's on 
the trailer, but if I missed something, please say so.


>    ...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? 

I'm pretty sure the plug on the side provides only a physical 
connection to the power center.  I'm pretty doubtful the control 
center in the trailer would do anything to manage input from solar 
panels, but maybe I'm missing something.


>  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.

I really appreciate the help, can't say thanks enough!

-- 
Ted Heise      <theise@panix.com>       West Lafayette, IN, USA