Trailer - Light Charging Wire

[Spargman]

I have a question regarding the power handling capabilities of my 7 pin connector.

My trailer holds a Sears Platinum Marine (AGM) battery and the truck has a 220amp alternator. When I look at how small the power wire is on the 7 pin, I'm terrified it's going to melt when I hook everything up. When the trailer battery needs a short top off from the truck I'm not worried b/c not much will be pulled by the battery, but when the battery is near dead from all the accessories being on for a few days, I do worry about how much power is going to transfer through that wire.

Am I over thinking this?? The industry standard that I've seen is simply to plug it in and magically it all works...but I can't seem to wrap my head around using such a light wire.

I'm not sure if this matters in the equation, but I do have an isolator (500amp) between the truck and trailer.

 

[OS-Aussie]

I have used something like this prior to control the current

Toad Charge

The key point is that it limits the charge to 10 amps and provides protection to the main vehicle

 

[Titanpat57]

One option I used when I had a bigger travel trailer, was run a separate charge line from the battery, along the frame, with a male/female plug at or near the 7 pin connection. Then I ran a wire through the trailer to battery....IIRC I used a pc. of 8#

I would also use a 30/40 amp breaker on the fenderwell to add some protection. Even if the battery in the trailer was low, it never blew the breaker.

 

[Woods]

The theory is that the length and small gauge of the wire, act as a resister. This then cuts the current to where it will not over heat.

I don't like theory, so I went with a toad charge. LINK

 

[Spargman]

some really good intel everyone, thank you for chiming in!

I'm definitely going to investigate the Toad and look how I can run a larger gauge power wire. With how cheap wire is, it would be nice if OEM and the folks that make trailer connectors just moved up a size or two in gauge.

 

[Spargman]

One option I used when I had a bigger travel trailer, was run a separate charge line from the battery, along the frame, with a male/female plug at or near the 7 pin connection. Then I ran a wire through the trailer to battery....IIRC I used a pc. of 8#

I would also use a 30/40 amp breaker on the fenderwell to add some protection. Even if the battery in the trailer was low, it never blew the breaker.

This was the question I was going to call and ask you about today I ended up getting jammed under the truck all day and didn't want to call you too late...i'll reach out next week to catch up!!

 

[peekay]

no need to worry. The amperage going through that wire to your auxiliary battery will be limited by the voltage that your car allows your alternator to put out. In other words, the voltage at which your alternator is at is controlled by the voltage regulator on your alternator. Therefore, if the battery your voltage regulator is connected to, e.g. probably your main car battery, is not really that depleted, then your alternator is not going to put out that many volts. Consequently, the amperage going over that wire from your car battery/alternator to the auxiliary battery on the trailer is going to be fairly minimal. This is compounded by the voltage drop caused by such thin wire.

I had this problem when trying to charge the battery on my travel trailer. The voltage (as detected on my Scan Gauge) that my alternator is putting out is only briefly 14.0+. As soon as the car battery is refilled within a few minutes, the voltage drops fairly quickly--to about 13.4-13.5. Factor in the voltage drop caused by the thin 12-14 gauge wire on my 7 pin connector, the voltage at the trailer battery was very low- resulting in very little amperage going through. I ended up getting three 10 gauge wires to reduce the voltage drop.

In sum, from the perspective of too much amperage, I wouldn't worry about the wiring melting or anthing. But you may want to upgrade the wiring if you need the trailer battery to charge quicker.

 

[toymaster]

I have a question regarding the power handling capabilities of my 7 pin connector....

Ok, the responses so far have gotten you to the right answer just the theory is not exactly correct. The reason why it is safe and 99% of RVers do this is the charging characteristics of a battery. A battery does not "draw" power as a motor does; you have to force the electricity into it. (insert part of peekay's answer here) This is accomplished by a higher pressure power source. Pressure in an electrical circuit is measured in volts by the way. The amount of power available is limited by the small gauge wire used.

A larger wire would allow for a quicker charge. It is in no danger of melting due to too much power going through it from being used to charge the battery no matter how dead. A dead shorted battery is another issue but rare.

Where there could be a possible problem is if the truck battery was dead and the aux batteries were charged. When you tried to start the truck power would be drawn from the aux batteries to the starter. It is possible for this to draw enough power to cause damage to the conductor. After all the motor (starter) is drawing power from where ever it can. This will usually not cause any problems as you are not on the key cranking for an extended period; the wire may get warm but damage would be unlikely (possible yes but, not enough to cause regulations to be in place for public safety). You should have per regulation a fuse in the charging circuit anyway which would blow before any damage to the wire under what ever circumstance.

 

[skersfan]

I think you are all over worrying about this. Every single travel trailer, 5th wheel use the exact same set up for wires running to the towed equipment. the seven pin plug has been around for as long as I can remember. I run mine directly from the battery with a 15amp fuse in the line and directly to the positive side of the batteries in the trailer. Have never had a problem doing that. Built close to 250 trailers with batteries in them and never had a problem. Now I can be pretty stupid, just burnt up my solar controller. so who knows.

 

[Spargman]

I have three batteries in my entire setup - two in the truck that have a isolator between them and one in the trailer.

My second battery in the truck is exclusively for accessories (lights, fridge, etc). In terms of wiring in my trailer, should I also run an isolator between the second truck battery and trailer?

 

[skersfan]

It would be wise to isolate the trailer battery if you plan on being hooked up long. Only because something left on could run the whole system down. I have three batteries in my set, two in the trailer. I normally disconnect my trailer when ever I stop,, just an old habit.

 

[JKJenn]

Resurrecting an old thread to ask a related question. I have a 2011 JKU, which, I believe has an 140ah alternator. I am showing a charge of about 13ah on the 7 pin connector but the trailer is not charging while I drive as it should, I believe. Is there anything on the JK (like in the computer) that would limit how much the alternator can send to the 7 pin? I have an after-market towing set-up.

 

[elmo_4_vt]

Short answer is no... The charging circuit, even in modern engines, is completely dumb (not counting hybrids). The only thing that causes a battery to "charge" is for there to be a higher voltage being applied to it's terminal. The analogy above would show that it is being forced into the battery that way, just like water flowing down hill. However, I'm not sure how you're reading 13ah (amp-hours). The only thing you can really measure is amps, or volts. The amp-hour measurement, is just amps, times the number of hours those amps are flowing. That aside, as long as the alternator is charging at it's normal voltage, around 14 volts, you should see the same voltage at the 7-pin connector at the back of the truck (with the truck running). Go one step further and you should see the same thing at the positive of the battery lead in the trailer, BEFORE you hook it to the battery. If that's the case, the battery will charge as long as it's a good battery. How fast it charges, the amps, is just a factor of the resistance in the circuit (both the wire and the internal resistance of the battery) between the alternator and the trailer battery. Any other questions or if you need more clarity, don't hesitate.

Don

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[JKJenn]

Thanks for the response, Don. I did mean amps. I measured it with a voltmeter on the 7 pin connector, but that was with it cold. I am sure it climbs up closer to 14 with the truck turned on and running. Both the Jeep and trailer batteries are brand new.

I am fairly sure the problem is with the trailer. I had a bad ground wire on it when I purchased it new and it still has a ground loop hum in the stereo. I am working with the manufacturer to get it worked out but wanted to rule out a problem with my Jeep before I took it to the manufacturer for service.