Trailer Plug Wiring 12 Volt Line?

[k1hog]

When I picked up the new Cargo Craft, the tech checked to see if the inside light worked, which it did, he stated the trailer plug, had the 12 volt wire connected from the truck. The light switch in the trailer had two wires, one to the light and one coming from the trailer wiring. I plan on running the inside lights from the batteries in the trailer.

The question: The 12 volt wire from the truck through the trailer plug, which did power the inside light, can that wire be connected to the batteries in the trailer and thus charge to trailer batteries when the truck motor is running?

Thanks

Johnnie

[crumbruiser]

If you have a typical 7 place TT plug, then the tow vehicle does charge the TT's battery while driving. Also, when parked, the tow vehicles battery and the TT's batterys both get drained at the same time. If you park overnight and want to use the TT, disconnect the plug so you don't have a dead battery in your tow vehicle in the morning.
Frank

[eamarquardt]

Usually there is a lead to the radio that shuts off power to the radio when the key is turned off. If you tap into that lead to run a relay you can "automatically" disconnect the trailer battery from the tow vehicle when you turn off the ignition preventing the discharge of the vehicle's battery by the trailer.

Also, ensure that there is a fuse in the line from the tow vehicle's battery/alternator (as close to the battery as practical) so that if there is a short along the way that you don't start a fire.

Hope this helps.

Cheers,

Gus

[pete42]

Johnnie check out this site http://rvbasics.com/techtips/rv-travel-trailer-plug-wiring.html
it should show you what you need.
some trucks have a relay that prevents the trailer from discharging the trucks battery when the key is in the off position.
on my F150 I had to buy a relay and a fuse in order for the trailer battery charging circuit to work.

pete

[smatthew]

just remember, you will get little to no battery charging through the tow connector, unless you've significantly beefed up the wires. It'll run lights and stuff, but batteries need a voltage of 13.8 or so to really charge.

if you go ahead and run larger gauge wire from the battery (fused of course) to the trailer plug, then it will work better. Make sure the trailer is very will grounded to the vehicle otherwise that will limit the charge rate.

If you're worried about killing your vehicle battery, i'd recommend something like the blue sea Automatic Charging Relay relay. It connects when the vehicle is running, and disconnects when the engine is stopped. [/url]

[eamarquardt]

just remember, you will get little to no battery charging through the tow connector, unless you've significantly beefed up the wires. It'll run lights and stuff, but batteries need a voltage of 13.8 or so to really charge.

if you go ahead and run larger gauge wire from the battery (fused of course) to the trailer plug, then it will work better. Make sure the trailer is very will grounded to the vehicle otherwise that will limit the charge rate.

[/url]

I am "not entirely convinced" that a wire that is slightly smaller than optimum will prevent the battery from becoming fully charged. True when the current charging the trailer battery is high there will be some voltage loss and the battery won't charge as fast as it would if a larger wire was used. However, as the battery comes up to voltage/charge, the current will drop, and the associated voltage drop will be reduced eventually permitting the battery voltage to be virtually the same as the alternator voltage. I used 12 gauge wire and haven't had a problem getting the trailer battery fully charged.

Cheers,

Gus

[smatthew]

you're right - the battery will eventually charge - given enough driving time and assuming there aren't any loads on in the trailer.

The problem is that you never are able to develop high charging currents over the long wires from the vehicle alternator to the trailer battery. As more amperage tries to travel, the voltage loss increases.

Doesn't hurt to do at all though. It's "free" battery charging. I just wouldn't recommend using it as your only battery charging.

[pete42]

Johnnie
you said that the "inside light worked when hooked to your tow vehicle"

some things would be useful to know:
was the tow vehicle running?
if it was not running was the key turned on?
if it was not running was the tow vehicle's running lights turned on?
do you already have a trailer battery installed?

the running lights may feed the inside light terminal 3 of the 7-way plug
the trailer battery is charged from the tow vehicle to the trailer through terminal 4 of the 7-way plug
it may or may-not be connected if you don't have a battery on the trailer.

I have towed trailers since the sixty's the supplied wiring has served me well.
no need to add a wire larger than the one already there but you may need to added a relay and fuse for the charging circuit
in my F150 the relay socket and fuse socket were located under the hood I just plugged them into the marked slots.

Pete

[k1hog]

pete42, and all, I am not positive what condition the truck was in when the way through was done. I am going to hook the truck back up to the trailer and test the line with the various conditions of the truck.

I will post with the results.

Thanks to all for the replys

Johnnie

[BC_Explorer]

you're right - the battery will eventually charge - given enough driving time and assuming there aren't any loads on in the trailer.

The problem is that you never are able to develop high charging currents over the long wires from the vehicle alternator to the trailer battery. As more amperage tries to travel, the voltage loss increases.

Doesn't hurt to do at all though. It's "free" battery charging. I just wouldn't recommend using it as your only battery charging.

Coincidentally, this past weekend, I discussed this very topic with a major Travel Trailer manufacturer factory rep at the annual RV show.

In summary, the longest pickup truck lengths (for example Dodge Ram Crewcab @ around 17.5 feet) have no problems charging a trailer battery(s.) As long as the proper sized wiring is used to the connectors, the trailer battery will charge at a nominal rate. Of course there are many variables to this including state of battery health/age, state of actual discharge before charging, state of TV and trailer electrical system, ambient weather temps etc.

After all, there are hundreds of thousands of trailers out there, of which, a large majority do not have generators, solar or any other methods of charging the trailer battery other then when connected to the TV.

[smatthew]

After all, there are hundreds of thousands of trailers out there, of which, a large majority do not have generators, solar or any other methods of charging the trailer battery other then when connected to the TV.

I've never seen a trailer with a battery that did not have a converter. They're cruddy battery chargers usually, but they are there. They also provide that 12v power when plugged in.