Battery Charge from Trailer Connector

[Buzz]

Seems to me there would be no problems charging my 12V camper battery by hooking into the 7-point trailer connector and not having any concerns about disconnecting when the engine is not running.

I am running an F250 diesel with two batteries under the hood.

Am I correct or a little off base?

-Buzz

 

[HERR42]

it will work. power is available only when the engine is running...there is a relay under the hood. <BR><BR>the only problem could be that if your camper battery is really low you might pop the fuse during the initial charge period.

 

[Desert Rancher]

I have been doing that for awhile with no problems. F250 power stroke diesel. Camper battery is mounted in the front right side wheel well, between the truck bed and the outside wall of the Grandby.

 

[pods8]

Only draw back is some stock wiring on those connections isn't always a very big gauge so you might not get the charging you are looking for so you might want to verify that and you can always run a heavier line to that connection as well.

 

[HERR42]

well as the charge current drops off the IR loss will decrease.

this means that as the battery charges, the effects of voltage drop along the wire are reduced.

if all you were doing was trickle charinging to top it up, you could get by with a 16gauge wire. the only reason you need a heavy gauge is due to initial inrush current and the bulk charge period....where most of the recovery is done. bringing the battery up to final voltage doesnt take much.

no problem. i bet thats a 10gauge wire.

 

[Buzz]

My truck is an '07 Diesel Superduty with the factory installed tow package. I'll have to look into the wire gage size. Isn't 10 GA wire the recommended minimum for this application?

-Buzz

 

[Desert Rancher]

My F250 with the factory tow package has, 10awg wiring with a 30amp fuse and a (ignition on) relay. It keeps an old battery charged up with no problem.

 

[John D]

At 12VDC and 30 amps, the losses are significant with only 3 or 4 feet of 10 gauge wire, equal to about one third of a volt. If it takes 20 feet of wire to reach the trailer connector at the back of the truck, plus another 10 feet from the hitch to the battery in the camper, the loss could be a show stopper.

For a 12V battery to accept a charge with a resting (fully charged) voltage of 12.4 volts, it must be in a circuit with a voltage of at least 13.4 volts (This is a critical number.). Assuming a voltage of 14.5 for the charging system under light load, it is easy to see the probability that your camper battery will not be charged more than about half capacity with the arrangement proposed. Practical reports will verify these estimates.

Your camper can work by doing this, unless it is planned to run a microwave, frige, computer, TV set, and a big sound system (exaggeration herein to make the point).

 

[Buzz]

Thanks for your input.

I'll have to do some investigating. I have not observed any issues with the battery installed on the trailer shown in the attached pic.

-Buzz

 

[HERR42]

At 12VDC and 30 amps, the losses are significant with only 3 or 4 feet of 10 gauge wire, equal to about one third of a volt. If it takes 20 feet of wire to reach the trailer connector at the back of the truck, plus another 10 feet from the hitch to the battery in the camper, the loss could be a show stopper.

For a 12V battery to accept a charge with a resting (fully charged) voltage of 12.4 volts, it must be in a circuit with a voltage of at least 13.4 volts (This is a critical number.). Assuming a voltage of 14.5 for the charging system under light load, it is easy to see the probability that your camper battery will not be charged more than about half capacity with the arrangement proposed. Practical reports will verify these estimates.

Your camper can work by doing this, unless it is planned to run a microwave, frige, computer, TV set, and a big sound system (exaggeration herein to make the point).


John, keep this in mind:
those losses you speak of diminish as the battery charges. This is because the current flow drops off.
Please look up "IR" losses and see what happens when the current flow drops to one amp and lower.
Those losses will simply disappear! Eventuall the battery will see the full voltage from the alternator....well almost, they never go aways completely.

10gauge solid copper wire has just under an ohm per thousand feet! thats not much loss at 10 amps and of no consequence at 1 amp.

IR drop calcs using 40 feet of 10 gauge solid copper wire:
@ 30 amps the loss is 1.2 volt
@ 10 amps the loss is .4 volt
@ 5 amps the loss is .2 volt
@ 1 amp the loss is .04 volt (float charging is usually less than an amp)


His setup will work just fine, and will fully charge a battery.

He only has one risk issue: popping his fuse during the initial charge, bulk, phase. That why the camper manfs use a 30 amp thermal breaker. Even that fuse popping is a pretty low risk.

 

[Buzz]

Thank you, HERR42.

Common sense told me there should not be a problem. I'm going ahead with the project.

-Buzz

 

[John D]

Buzz, when you turn off the ignition, does the circuit go off too? John D

 

[Buzz]

John D,

I have not installed the charging jumper yet. I do recall reading somewhere (aside from this thread) that there is a cutoff relay in the truck charging system that interrupts the charging circuit to the trailer harness.

The trailer brakes and lights are supposed to be powered by the truck battery in the event of failure of the trailer battery. I did detect 12VDC at one of the truck jack pins with the engine turned off once, but don't recall which pin it was.

The only thing that I plan to run off the camper battery is a computer for photo work. I'll just say that I did had a show-stopping issue with my truck batteries once in AK. Fortunately, I wasn't 250 miles up a dirt road in a foreign country at the time. It would be nice to have a backup battery in case of failure, too.

Thank you for your response.

-Buzz

 

[John D]

Buzz,

In our truck camper, we use a single battery, the truck battery. It powers everything including the camper, just one battery. We minimize using 12VDC battery power when camping. Our old truck had a manual transmission, so we had a chance of a roll start should we accidentally discharge our truck battery and we carry a set of jumper cables.

One evening a couple of years ago, when setting up camp, I failed to switch the refrigerator from 12VDC to propane. About 2 hours later I went inside the camper and noted the fridge was still on 12VDC. The battery was dead and I felt like a dummy. Fortunately, we were traveling in company with another truck. I hated to ask for help out of something stupid, but I asked our buddy for a jump which he was kind enough to provide.

Realizing I could easily make the same mistake when help was not available, I decided to start carrying a full sized spare truck battery. Before leaving home, I pull a good battery out of another vehicle and take it with us. It can be swapped for the truck battery if needed. In addition, we carry a spare fresh alternator. Though I used the fresh alternator on an Alaska trip, we have never needed the backup battery since the above incident. Does anyone see why this would not work?

John D

 

[Buzz]

John D,

You power everything by "...just one battery..." AND propane.

I have used the old push/roll start trick many times. Doesn't work too well with an auto trans, though.

Also, the computer needs enough voltage to support the motor during a push/roll start. I learned that from experience, too.

Your backup plan will serve you well as long as you don't have to depend on a "dead" spare battery to start your truck. And, as long as you have propane to run your refer.

If my starting batteries go bad, I'm screwed; unless, the spare battery in the camper has enough juice to turn the diesel over (and I have my doubts about that). Or, unless I can get assistance.

I want a battery in the camper to run my computer for extended times without having to worry about running my starting batteries down. And, I want the extra battery as a backup to assist with an emergency start.

That's my plan and I'm sticking to it.

-Buzz

 

[Barko1]

I'm too paranoid of failure in remote spots so maybe it is overkill but I have the 2 batteries for the diesel, 2 in the camper, a pretty strong battery charger that I can plug into the generator to boost either set of batteries plus 15w of solar. Guess I have it covered

 

[John D]

Buzz,

AND propane. I'm laughing and you are right. You are also right about needing at least minimal voltage to push off a vehicle that has a stick shift.

I have envisioned a rope start on my Chevy, like an outboard motor. The engine needs a magneto for that.

Honestly, you should have more than one vehicle when going into really remote places.

John

 

[Buzz]

My motto has long been: "Better safe than sorry."

It's surprising, though, that even at my age I still run off and do REALLY stupid things.

-Buzz

 

[Desert Rancher]

The F250 trailer charge wire has a relay in the circuit that will only connect when the key is on.