Automatic disconnect of the tow vehicle to trailer charging circuit

[Mustang65]

I know that some of the members here learned the hard way that their TV’s charging circuit is supplying 12 volts 24/7 to their trailer and if not disconnected can drain the TV’s battery along with the TT’s battery(s). Some members like to keep their TT’s 7-Way pigtail connected to their TV while camping. I can only speak for the Ford 12Volt supply to the TT and it is only supplying 12Volts when the ignition is on, unlike some other manufactures.

So, is there an inexpensive way of automatically disconnecting the charging circuit while leaving the 7-Way pigtail connected to the TV? Well, for less than $10 you can make your own automatic TV charging circuit disconnect switch.

All it takes is a 12Volt 30 or 40 Amp Automotive relay, which can be purchased at any auto parts store or on Amazon. It can be either a SPDT or a DPDT normally open relay. You do not have to cut any of the TT’s wires as the JAYCO 7-Way pigtail wires terminate (in wire nuts) in a 4x4 electrical box somewhere on the frame near the battery(s). All the connections you need are in that 4x4 electrical box. You will only have to supply a few small pieces of wire. Some of the Automotive relays have a pigtail on it so that would make the project a even easier (all needed wire is included).

Now you are probably asking… “How does it work?”.. When the relay is not activated, the charging circuit is open (no connection to the TV). The activation coil in the relay is activated when you turn on the TV's lights. The relay receives its 12 volts from the TT’s marker/running lights circuit. The only requirement is that when you are towing the TT, you keep your TV’s lights on to keep the charging circuit active. It is recommended that you travel with your lights on when towing anyway. When you stop for fuel or at the end of the day and you shut your TV lights off, the circuit opens disconnecting the TV from the TT’s 12 Volt system.

This is not a difficult circuit to make, if you have a little electronics experience. I tried to make the diagram as easy to understand as possible.

Don

[tylersdad]

I'm totally confused. You have a Ford. You stated that, when the ignition is off, the TV doesn't charge the RV and the RV doesn't kill the TV battery.

So are these instructions for non-Ford owners?

[Mustang65]

Originally Posted by tylersdad

I'm totally confused. You have a Ford. You stated that, when the ignition is off, the TV doesn't charge the RV and the RV doesn't kill the TV battery.

So are these instructions for non-Ford owners?

Yes, for non Ford trucks.. I have no clue about how GM or Dodge 12 Volt supply is handled.
Don

[ttavasc]

My 2014 RAM 1500 w/factory tow package has 12v on pin 4 of the 7-pin connector all the time. I verified it the ignition off and a multi-meter. So this might be a handy little project. Thanks for posting..

[tylersdad]

Ok. So I'm safe to leave my RV plugged into my F-150?

[Mustang65]

Originally Posted by tylersdad

Ok. So I'm safe to leave my RV plugged into my F-150?

Electrically speaking, yes. I do it when we are staying at CG's overnight while traveling.

Don

[tylersdad]

Originally Posted by Mustang65

Electrically speaking, yes. I do it when we are staying at CG's overnight while traveling.

Don

Thanks! Yeah, overnight is the only time I might keep it plugged in. Even then, I usually have to unhook to get the trailer perfectly level front to back.

[Swampy]

If you are plugged into shore power for the overnight, your 120V - 12V system will charge the TV battery as well as keep the TT batteries charged, but if you are not plugged into shore power, your TV battery is acting in parallel with the TT batteries and you should disconnect between the two else you risk not being able to start the TV in the morning. The auto disconnect would eliminate the need for manually disconnecting the umbilical.

[MzLiz]

Originally Posted by Mustang65

Electrically speaking, yes. I do it when we are staying at CG's overnight while traveling.

Don

Is that true for an older (1999) F250 too? Asking because on my way down to Vegas this year I stayed overnight in a WalMart parking lot, forgot to unplug, remembered after going to bed, got worried my truck wouldn't start in the morning (it was about 10 degrees out and of course I had the heater running) and had to get up and go out in the cold to unplug it. I could have just left it without fear?

[norty1]

I had a 97 F350 that had separate fuses and was de-activated when the engine was not running.

Looks like you got up for nothing but I would have done the same thing.

[Swampy]

Originally Posted by MzLiz

Is that true for an older (1999) F250 too? Asking because on my way down to Vegas this year I stayed overnight in a WalMart parking lot, forgot to unplug, remembered after going to bed, got worried my truck wouldn't start in the morning (it was about 10 degrees out and of course I had the heater running) and had to get up and go out in the cold to unplug it. I could have just left it without fear?

If you want to know for sure, with your TV parked, not running and nothing plugged into the connector, measure between pin 2 and pin1 if you read 12VDC, then you need to disconnect the TT, if it reads 0VDC, the TT can remain connected without fear of killing the TV battery.

[Mnwelsh]

I have always lived under the principle, "When In Doubt, Don't ". Things change and systems fail. I always disconnected my Trailer from the truck when I was going to RON. My truck was a daily driver, I knew it would start and get me on the road again. My RV often was in storage for several months and who really knows what has changed within it until a few days down the trail. Just my 2 cents worth.