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Old 05-31-2014, 10:26 PM   #1
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1965 17' Caravel
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Charging Trailer Battery from Tow Vehicle?

If I want my trailer battery (Interstate Group 24 Deep Cycle) to be charged while we're driving, is it at simple as connecting the charge line from the tow vehicle (2014 Jeep Grande Cherokee with tow package and 7-pin plug) to the positive terminal of the battery? Let's assume that I'm disconnecting the trailer from the Jeep when parked to avoid draining the car battery. I've read posts about having a solenoid and/or fuse inline, but not sure if they're necessary or not in my case?

Thanks, Gary
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Old 05-31-2014, 10:36 PM   #2
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Why don't you go ahead and test that connection with the Jeep running and turned off to see if its hot while running and dead when the Jeep is off? (without it hooked to the trailer). That's the case on my Tundra so I don't have to worry about it.
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Old 05-31-2014, 10:51 PM   #3
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Originally Posted by GaryC View Post
If I want my trailer battery (Interstate Group 24 Deep Cycle) to be charged while we're driving, is it at simple as connecting the charge line from the tow vehicle (2014 Jeep Grande Cherokee with tow package and 7-pin plug) to the positive terminal of the battery? Let's assume that I'm disconnecting the trailer from the Jeep when parked to avoid draining the car battery. I've read posts about having a solenoid and/or fuse inline, but not sure if they're necessary or not in my case?

Thanks, Gary
Hi, Gary.

Oft-asked question.

I rely on charging the trailer battery from the tow vehicle.

The main message I have, for you, is to test your setup and be sure it is in fact charging before you depend upon it. There's plenty to go wrong. I don't know much about the 2014 Jeep Cherokee, but it's probably wired correctly.

You can see whether the charge line is working by measuring the voltage at the trailer battery. It should be higher when the engine is running in the Jeep. Or you can connect an ammeter in series with the charge line.

I've replaced a couple of relays and cleaned connector contacts, several times, to keep my setup working. I check it before every trip where I need it.
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Old 05-31-2014, 11:04 PM   #4
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Charging Trailer Battery from Tow Vehicle?

I recently discovered from this forum that my 2008 Ford f150 does not come with relay or fuse installed for trailer battery charge. In reviewing other forums it seems this is common with some truck manufacturers. It's an easy fix. Couldn't find a comparable relay at auto parts stores, thus had to pay $30 at ford dealership. Goes into a socket in fuse panel.
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Old 06-01-2014, 07:58 AM   #5
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I thought that all vehicles with a factory towing set-up, including the wiring harness, were good to go as far as the battery charge line goes. I, too, learned to promptly disconnect the umbilical from the truck after towing to a campground! Enjoy your '60's Caravel, you lucky dog...
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Old 06-01-2014, 10:10 PM   #6
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I checked the voltage from the Jeep's charge line and this is what I discovered:

With ignition off: 12.83v
With ignition on: 14.38v

Does that tell you anything?
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Old 06-02-2014, 12:45 AM   #7
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When your Caravel electrical is hooked to your tow vehicle, and your tow vehicle is running, the battery in your Caravel may be charging, and maybe not. First, you need to be sure that the chassis of your tow vehicle is well connected to your trailer or you will not have a good ground. Check to see if there is a ground wire in your harness and that it is connected to a good ground on your tow vehicle as well as your trailer.
If the battery on your tow vehicle is fully charged or nearly fully charged, the voltage regulator will shut down the alternator to keep from burning up your car battery. If the battery on your Caravel is down, it will draw from the car battery as they are now connected in parallel. As your car battery goes down and your trailer battery comes up, the regulator will kick the alternator up, but only to the degree that your trailer battery has drawn your car battery down. The regulator will not see the low battery, only the high battery, and just as well or the car battery would get cooked. Bringing a low trailer battery up using your tow vehicle could take some time.
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Old 06-02-2014, 12:54 AM   #8
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My 2012 Grand Cherokee (virtually the same as the 2014) has proper 12 volt wiring all hooked up to the outlet at the back with the tow package. Nothing more to add or connect.

I find that with the trailer battery somewhat discharged I can get as much as 10 to 11 amps of charge to the trailer batteries (tongue mounted). Most every other tow vehicle/trailer combination I have owned maxed out at 6 to 7 amps. I am sure as the system ages the power may decrease. There is simply too much resistance, and too many connections and plugs to get a lot of charge power from the engine alternator to the trailer, especially over a few years of use.
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Old 06-02-2014, 07:30 PM   #9
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There is a Ground pin in the 7-pin harness for connecting the tow vehicle's ground to the trailer ground. I have the harness running up into the front interior and going to a terminal block. From the other side of the terminal block; I've run a 14 ga ground wire to the trailer's steel framework. I've also run a 14 ga wire from the terminal block to the battery. I did notice that there are much heavier gage wires in the harness. Should I be using a heavier gage wire to ground and battery? Thanks!
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Old 06-02-2014, 09:10 PM   #10
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I would recommend #10 for the ground and the +12 volt line to charge the battery to get as much current as possible to everything.
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Old 06-02-2014, 09:19 PM   #11
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We ran a dedicated pair of #10 wires from the battery terminals (both +12Vdc and -12Vdc) to the 7-pin plug on the back of the trailer. We installed a 30 amp circuit breaker in the positive line on the firewall very near the battery This is probably the only way to get as much power to the trailer battery as your vehicle can generate.

Our Dodge wiring harness had #14 wire on the power leads and even smaller wire sizes on the the turn signal and running lights.
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Old 06-03-2014, 10:27 AM   #12
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Originally Posted by GaryC View Post
I checked the voltage from the Jeep's charge line and this is what I discovered:

With ignition off: 12.83v
With ignition on: 14.38v

Does that tell you anything?
Those voltages would be what I would expect if the charge line is working properly. However, since it has battery power when the engine is off, there is the risk of discharging the tow vehicle battery to the point where the tow vehicle won't start, if you leave the trailer connected for a long period (overnight or longer) without the engine running.
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Old 06-03-2014, 10:31 AM   #13
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There is a Ground pin in the 7-pin harness for connecting the tow vehicle's ground to the trailer ground. I have the harness running up into the front interior and going to a terminal block. From the other side of the terminal block; I've run a 14 ga ground wire to the trailer's steel framework. I've also run a 14 ga wire from the terminal block to the battery. I did notice that there are much heavier gage wires in the harness. Should I be using a heavier gage wire to ground and battery? Thanks!
My advice is to measure the charge current (amperage) and see if you're happy with it. If not, consider using larger wire.

I rewired my tow vehicle with a pair of #6 wires from the alternator to the trailer connector, one +12, one ground. The +12 is protected by a circuit breaker. There are short sections of #10 at the connector because that's the largest the connector will accept.
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Old 06-03-2014, 04:25 PM   #14
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This is good stuff, thanks for all the advice, but it brings up another question...

A few of you are mentioning putting a circuit breaker on +12v line between the tow vehicle and the trailer battery:

1. Usually circuit breakers (or fuses) are for a safety reason to prevent overloads, but in this case what could cause an overload? I'm wondering what would be the potential cause of the failure and it's effect. i.e., the car alternator going crazy causing the trailer battery to blow-up?

2. Could this circuit breaker be the same type used for AC? i.e. I have an extra slot in my WFCO Power Center for another circuit breaker, so could I use this for the 12v battery line?

Thanks! Gary
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