Trucks 12 volt power lead to trailer battery

JCK wrote:
Do not know what size the gauge wire size is ..But my old truck 2004 Duramax there was a 40 amp fuse protecting the circuit . Don't think the factory would install a 40 amp fuse if the wiring could not handle it . Going on long trip this year would like to travel 2 days and third night campground to replenish water dump tanks . That should be enough to top of the batteries again if the truck is able to mostly keep up during the day.

The truck will "crank up the Alternator Output" when it see's it's own battery being discharged. But it doesn't actually see your Trailer battery very well through the long, skinny wire: You must either hook the Trailer up to your TV using a better, shorter cable, so that it sees the trailer batteries to be discharged; or you will probably need to do something like my scheme. If you use very little electricity between visits to "plug-in" campgrounds, then it won't matter - but if you do run the batteries down, then you'll need to do something different than simply plugging in the Bargeman and turning on the truck. If you do only that, then it will take many, many hours to recharge a pair of discharged trailer batteries. Are your traveling days > 6 hours long?

New Edit: Also, will you run a fridge on electric while towing? That would be a problem because a 12V fridge heater will discharge Trailer batteries to 65-75% before the Fridge begins to pull significant current from the TV. That leaves you only 15%-25% of total capacity available for actual use before needing to charge again. (I'm assuming "typical" SLA Trailer batteries, "typical" Tow Vehicle and Bargeman wiring, and a 120W 3-way fridge heater unit.)

christopherglenn wrote:

Same issue with charging the trailer batteries. The alternator keeps the truck's battery at 14.x volts....

No, it doesn't do that. But anyway: You used a buck converter with attention and skill, turning it on and off manually. In contrast: When I provide buck-boost power to the Solar Controller in my scheme, the Solar Controller decides whether it it wants to use "a lot of it", or "a bit less of it", or "hardly any" - according to battery SOC. I don't have to worry about anything.

JCK.....presume you know ( as per owners manual for your GMC ) that being in tow/haul or keeping your headlight switch turned on will change truck battery voltage regulator so trailer charge wire continues getting full amperage even though truck battery is topped up.

Using six AWG truck to trailer I managed to get 27 amps starting off using a Hubbell 100-amp rated plug and socket. And soldered terminals. It's a tough row to hoe.

Charging is controlled by voltage and current (amps).

Think of voltage like water pressure in a pipe, and amps (amp hours) live the volume of water you move from point A to point B.

To float the trailer batteries you want somewhere near 13.7 volts, to charge it in a hurry you need closer to 14.4 volts.

Imagine 2 55 gallon barrels 100 yards apart, connected by a good garden hose, lets put the full one at the top of a hill and the empty one at the bottom. you will move a lot of water in a hurry and fill the second one fast. Not imagine them on opposite ends of a football field, connected with 1/4 inch drip line. It is going to take all day to fill the empty one. Say we have a hose keeping the first one full (alternator) and the little drip line moving water to the second one. At first the second battle will seem to fill ok, but as the water level rises there is less and less difference between the height of the water in the two barrels.

Same issue with charging the trailer batteries. The alternator keeps the trucks battery at 14.x volts, but the long, small wire between it and the trailers battery limits the amount of power that can be transferred. As the trailer battery charges, the (driving) voltage difference between the two batteries shrinks, and the amount of current (amps) that passes drops as well.

A severely drained battery (8 or 10 volts) may pop a 30 amp fuse, a mostly charged battery (13+ volts) an amp or two may be all you get.



I have a 20 amp buck/boost converter on my trailer, set at 13.5 volts. No matter what the oncoming voltage from the truck is, it pumps it to 13.5 volts and a max of 20 amps into the trailer. When I was dry camping I set it to 14.2 volts, faster charge but higher possibility of overcharging the trailer batteries.

There is a slightly (?) complex upgrade which can charge Trailer batteries AND run the Fridge on 12VDC at the same time. Under "high-load" testing (Trailer parked with Fridge and all lights "ON" plus extra load through an Inverter), a couple of users have been generating about 300W from the TV - and the Trailer 12V system is running at an "effective" battery charge voltage.

If your Trailer already has a Solar system, with an MPPT controller, then the upgrade is pretty cheap. It works by doubling-up the Voltage on the "Battery Charge" wire from the TV, and feeding that into the Solar Charge Controller as a high powered Solar Panel (25-28V, up to 15A). Parts:

* MPPT Solar Charge Controller (expensive - $170 if you don't already have one)
* A "Voltage Doubler" DC-DC Boost Converter, about $30.

* 6 Relays - 5 of them are "cheap" 12V automotive, available anywhere. The lasst one is also cheap, but hard to find: A relay with a 24VDC coil. (That one allows the Trailer to "see" that the Bargeman wire is running at high Voltage: It orders the other two Relays in the Trailer to disconnect the Bargeman from the 12VDC "power distribution panel", and connect it to the MPPT Solar Controller input instead. (The other standard Relay, switched at the same moment, disconnects the "Genuine" Solar panels in favor of the "Fake" Solar Panel connection.) Three "standard" Relays are present in the Tow Vehicle, allowing it to be switched from "12V Trailer Charge Mode" to "24V Trailer Charge Mode".

* A switch for the TV dashboard - controlling "Trailer Charge Mode". You need the switch in order to tow other Trailers. (The Trailer switches automatically.)

* A new Fuse/Circuit Breaker for the TV, and (good idea) another for the input lead to the 24V "Trailer Charge Wire" within the Trailer. It's no longer provided by the 12VDC power distribution center.

* A bunch of wire: Mostly #10, plus a little bit of #8 under the hood (Where the Boost Converter pulls _double_ amps at 13-14.5V, to create "half" the Amps at 26-29V).

Is anyone interested in the details and wiring diagrams?
BTW, JCK: Tow Vehicles typically use "chassis wiring" rules for wiring to the "Trailer Charge Bargeman terminal, they are often undersized (by MY standards). For example: My 4Runner is fused at 30A, and you'd expect to see #10: but the actual wiring was #14.

Do not know what size the gauge wire size is ..But my old truck 2004 Duramax there was a 40 amp fuse protecting the circuit . Don't think the factory would install a 40 amp fuse if the wiring could not handle it . Going on long trip this year would like to travel 2 days and third night campground to replenish water dump tanks . That should be enough to top of the batteries again if the truck is able to mostly keep up during the day. That's what I'm looking for someone who has experience doing this and there experience. In reply to previous post yes we have Led lights no need for coffee or toast just run a inverter for the TV at night.

What gauge wire between tow vehicle battery and RV battery and how long is it?

RoyB wrote:
Some folks have beefed up the truck start battery lead back to the 7-way trailer connector to get more DC amps back to the trailer...

It is not that simple any more ! About 10-15 years ago, most auto manufactures went to "smart" charging systems. The short version of this, you are likely to have insufficient voltage at your TT batteries to completely recharge them. They probably will not discharge further, but don't expect them to be "topped off".

With lead acid batteries you need greater than 13.2V AT THE BATTERY for it to charge. The ECM will want to keep the voltage at 13-14V (the voltage immediately after you start the engine will be higher, but it will drop withing a few minutes) and after the voltage drop through the wiring to the TT battery you will be lucky to get that 13.2V !

Expecting your vehicle charging system to do most of the bulk charging after using your inverter to make your morning coffee and toast your bagel, even after 8 hours on the road, is going to leave you disappointed.

There are some DC-DC chargers out there, but make sure that they will work down to about 10V. For best results, make sure it does a 3 stage charge.

X2 on 6-8AMPS... It is at this DC CURRENT mostly because of the size of the small wire between the Alternator and the 7-way connector with the trailer plugged into it...

This is almost like a trickle charge since you will be having a parasitic drain from the trailer to contend with all the time...

When driving you have to leave the trailer 12V Battery connected to satisfy the DOT Safety requirement for emergency breakaway feature to apply Brakes in the event the trailer becomes disconnected while being towed. If you could disconnect all of the parasitic loads then the battery could be charged up faster...

I clamped-on the lead once with my Sears CLAMP-ON DC current meter with everything hooked up and the truck motor running and read right at 8AMPS DC Current.

In my case i beefed up on additional batteries and beefed up cables from my on-board converter/charger unit so I can get to my destination points without dropping my batteries to their 50% charge state point... Then I just look at the 6-8 DC amps coming from the truck start battery system when in tow mode as icing on the cake only...

Some folks have beefed up the truck start battery lead back to the 7-way trailer connector to get more DC amps back to the trailer...

Again in my case I try not to do hardly anything to the truck start battery setup for fear I will kill something on the truck side as this is the only way to get back home...

Roy ken