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

Wayne

Administrator
Staff member
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?
 

spasm3

Member
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.
 

dnewton3

Moderator
Staff member
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.

 

Wayne

Administrator
Staff member
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.
 

dnewton3

Moderator
Staff member
Interesting!
I'll be curious to see if the Aux is hot with the key off.
My 2006 killed the aux upon key off.
 

Wayne

Administrator
Staff member
For the Chevy 2018 HD the verdict is the 7-way outlet remains hot after the key is shut off.

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dnewton3

Moderator
Staff member
So I guess in theory your RV would charge your truck batteries if the RV is connected to shore power then; right?
 

Wayne

Administrator
Staff member
So I guess in theory your RV would charge your truck batteries if the RV is connected to shore power then; right?
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?
 

dnewton3

Moderator
Staff member
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
 
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