Do you disconnect the trailer electrical plug from your tow vehicle overnight?

Wayne

Wayne

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I have always unplugged the trailer from our truck even when boondocking overnight. My point is that I don't want any drain on the truck's batteries. I would rather have dead trailer batteries and be able to start the truck in the morning. I've been wondering if disconnecting is even necessary. Does the 12v supply from the tow vehicle turn off with the ignition key? Possibly truck make/model dependant?
 
My new travel trailer melted/corroded the 12v contact on the plug. It was only 3 months old. The dealer told me not to leave the truck plugged up while i was plugged into power. I had hitched up and left the truck plugged up while plugged into house power.

I have not boondocked more than once, but i'd like to know as well.
 
If I am only staying overnight with an early morning departure the next day, then I don't unhook anything I don't have to.


What you're inquiring about is the Auxiliary circuit power status ...
To the best of my knowledge, I don't have anything in my RV that directly runs from the 12v of the tow vehicle to the 12v system of the RV aux.

Way back in the day, many RVs used to pull power from the tow vehicle's 12v supply, but I don't think that kind of system is prominent today, and hasn't been for a while. Before significant use of inverters and shore power, they needed the 12v supply from the tow vehicle to charge the 12v house battery during your travel time. But that type system isn't common today, at least for RVs.

The 12v supply from the tow vehicle is still present in the 7-pin plug, but I doubt there's any draw on it with the typical RV today. It's just a dormant pin in the plug now; that has become such an industry standard that there's no good reason to move away from what is a completely entrenched design element now. In theory we could use a 6-pin lead for the RV, as Aux really isn't needed. But good luck trying to get the entire RV industry to change over ... not gonna happen.

If you want to know if there's power transfer in your RV Aux circuit, hook up you RV to shore power, then test the Aux lead on the trailer lead side. On mine, there's no power coming from the RV, which means there's nothing hooked to it in the RV because it would "backfeed" power from the battery (supplied from the inverter) if so. Another thing to check would be to test the Aux feed from the vehicle plug, with your vehicle totally "off". I've not checked that; it may well be isolated and only "hot" when the vehicle is turned on; that would also keep the RV from draining the tow vehicle when not running.

At least this is the way I understand it to be. I'm open to other interpretations.

https://www.truckspring.com/trailer-parts/trailer-wiring/wiring-information-diagram.aspx
 
I guess our Airstream my be an exception.

From our Airstream manual:

"The charge in the 12-volt batteries can be replenished,
depending on the tow vehicle, from the tow vehicle
alternator through the 7-way cord. This charge will flow
to the batteries regardless of the battery disconnect
switch position. Likewise, if ON or OFF, the solar panel
will still charge the batteries."

So if the 12v supply in 2018 Chevy stays hot with the key off, then the trailer will slowly drain the battery. Now I have to figure out if the 7-way 12 volt circuit is keyed. I'll have the truck here Thursday and can test.
 
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Interesting!
I'll be curious to see if the Aux is hot with the key off.
My 2006 killed the aux upon key off.
 
For the Chevy 2018 HD the verdict is the 7-way outlet remains hot after the key is shut off.

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So I guess in theory your RV would charge your truck batteries if the RV is connected to shore power then; right?
 
dnewton3 said:
So I guess in theory your RV would charge your truck batteries if the RV is connected to shore power then; right?
Click to expand...
It sure should. And I could get twice the duration on battery, given that the trailer would draw from the truck batteries also. Of course, I would need to carry a portable jump starter capable of starting a diesel. Or, I could just let my generator charge the trailer batteries every morning and save the truck batteries. Maybe upgrade to lithium-ion batteries. See how the sweater starts to unravel?
 
If you really want to know, you could do this:
uplug 7-pin, leave the engine off, measure the voltage at the tow vehicle batteries (should be around 12.5v "ish")
then, with shore power connected at the RV, plug in the 7-pin and again measure volts at the tow vehicle batteries after about an hour
- if it's gone up or stayed the same, that's a good thing
- if it's dropped, it's still being drawn upon and is supplying the house circuit in the RV
But then you'd have to start turning on various circuits in the RV to see if you can alter the draw at the tow vehicle; how much can you consume from the inverter system prior to making the truck give up amperage versus gaining amp storage?
Wait ... that sweater is starting to come apart again ... ;)


To answer you initial question at the start of this Wayne:
- if I am connected to shore power AND I'm only going to be in the site for a quick overnight (late arrival, early departure), then I will leave the 7-pin connected as a matter of convenience (not to mention safety in case I would forget to reconnect it in morning)
- if I am not connected to shore power, then the 7-pin gets pulled always
 
On the Ram trucks the auxiliary power pin is always hot in the connector. I use mine frequently to run the transfer pump for my auxiliary fuel tank. I don’t leave it hooked. Checking it is part of my pre-trip inspection. All lights, tires, doors and brakes when I start rolling.
 
OLEJOE said:
On the Ram trucks the auxiliary power pin is always hot in the connector.
Click to expand...
GM/Chevy HD trucks are the same. I always unhook the trailer umbilical at night when we are still attached. No need for the trailer battery charger to charge the truck as well. Or worse, have the truck batteries be dead in the morning.
 
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When we have an early departure from home I have the truck & trailer ready to roll with shore power attached to run the fridge overnight. Should I unplug the trailer cord overnight?
 
Azjeff said:
When we have an early departure from home I have the truck & trailer ready to roll with shore power attached to run the fridge overnight. Should I unplug the trailer cord overnight?
Click to expand...
I'll second that. If all goes well, your probably over charging the batteries in your truck. If something goes wrong, you could have dead truck batteries in the morning.
 
Wayne said:
I'll second that. If all goes well, your probably over charging the batteries in your truck. If something goes wrong, you could have dead truck batteries in the morning.
Click to expand...
I see no advantage to leaving the trailer plugged in to the truck overnight when parked.
 
Azjeff said:
When we have an early departure from home I have the truck & trailer ready to roll with shore power attached to run the fridge overnight. Should I unplug the trailer cord overnight?
Click to expand...
I do as well.
I had one of the rv connector terminals burn/corrode out , the dealer said it was from keeping the truck plugged to the camper with the trailer on shore power. I think it was the hot terminal of the 7way connector.
 
Wayne said:
I'll second that. If all goes well, your probably over charging the batteries in your truck. If something goes wrong, you could have dead truck batteries in the morning.
Click to expand...
shore power thru the converter probably puts out about 13.5 VDC, so it probably would take forever to overcharge the tow vehicle battery, assuming there were no other parasitic loads.. assuming there are some parasitic loads in a RV and tow vehicle, I suspect it would have a hard time even putting any amount of charge into the tow vehicle.

I leave my trailer plugged in at home, it sits for months sometimes and I very rarely have to add water to the battery setup in the RV..

I've never worried about leaving my truckplugged into the trailer overnight.. long term though I don't want it plugged in..
 
It takes the converter forever to charge the trailer battery, did some checking and there are some brand/models of inverters that have low output, I have one. When we camp where there's no electric I bring my Solar Pro-Logix charger and charge the battery in about 1/3 the time. I'll start unplugging the truck as best practice but I don't think my inverter is going to smoke the truck battery.
 
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