Charge Line

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Charge Line

Postby mikeschn » Mon Mar 14, 2005 4:41 pm

I've got a question for you electrical experts.

When I had the passat hitch installed, I had them wire in a charge line. Looking at it now, it's nothing more than a black wire from the battery to pin 4 of the 7 way plug.

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Oh yes, supposedly there is a fuse somewhere in that line too!

Is that right? I thought the line was supposed to come from the alternator, not the battery. Is that puny little line from the battery enough to charge the battery in the teardrop when I am towing it?

Your thoughts?

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Postby SteveH » Mon Mar 14, 2005 5:02 pm

Mike,

That will work. The electrical point is the same, battery or alternator. If you want it to not have the possibility of discharging your car battery when it is connected, you need a battery isolator or a solinoid. The solinoid would be easier and cheaper.
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Postby Arne » Mon Mar 14, 2005 5:03 pm

Mike, excellent question.. was talking about this with my bro yesterday.... putting a wire from van to trailer with a diode in it to charge trailer battery while driving.....

I wonder about the size of the wire if the t/d trailer bat is discharged and the wire isn't very big.. will something melt?

I wonder because of sparks flying when jumping a dead battery in one car with another car's battery and the zap that occurs on the hook-up..
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Postby Woody » Mon Mar 14, 2005 5:07 pm

Mike
Mine comes off the battery as well. The alternator does not care if or what battery is demanding power. it will charge due to it sensing a drop in overall battery voltage. Since both batteries will tend to equalize each other, they both will be brought up together. I would install a switch or battery isolator when not charging just in case one goes dead and kills the other battery. The car battery becomes buffer for the bulk charge in a mutlple battery setup so to speak. Correct me if I am wrong
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Postby SteveH » Mon Mar 14, 2005 5:24 pm

I wonder about the size of the wire if the t/d trailer bat is discharged and the wire isn't very big.. will something melt?
Major RV dealer installations use #10 wire. It will handle what the alternator can put out.
I would install a switch or battery isolator when not charging just in case one goes dead and kills the other battery.

Like i said, if you use a solinoid hooked to the ignition "on" of the tow vehicle, the batteries would be isolated automatically when the tow vehicle ignition is off.
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Postby Woody » Mon Mar 14, 2005 5:26 pm

Well ther you go :thumbsup:
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Can you spell that out

Postby Guy » Mon Mar 14, 2005 10:42 pm

Dear SteveH,

Could you spell that out for us dummies. Precisely how do you hook it up to the ignition? What wires go to what posts?
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Postby SteveH » Mon Mar 14, 2005 10:49 pm

A solinoid is just a high current relay. Put the wire from the alternator or car battery on one high current terminal of the solinoid, and the wire to the trailer battery on the other high current terminal. Then put a wire from any point in the car's electrical system that is hot when the ignition is on to one of the coil terminals. The other coil terminal should be wired to ground.

When the ignition is on, the solinoid is energized and the circuit is made from the car's battery to the trailer battery. When the ignition is off, the circuit is open.
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Postby Big Guy with a Little Guy » Tue Mar 15, 2005 12:01 am

I would think it would be much better to connect to the car battery because it is on the other side of the regulator form the alternator. If you are connected to the alternator, it will constantly charge the tear battery, which could be bad.

Yes, having a disconnect is necessary to prevent unwanted discharge of the car battery. I guess solenoids or astroids would work, but how about unplugging the wiring harness? Ya sorta hafta do that anyways if'n your going anywheres.
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Postby SteveH » Tue Mar 15, 2005 7:39 am

I would think it would be much better to connect to the car battery because it is on the other side of the regulator form the alternator. If you are connected to the alternator, it will constantly charge the tear battery, which could be bad.


Karl,

All alternators that I know of these days have internal regulators which means either place, battery or alternator output is the same place electricaly.
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Postby campadk » Tue Mar 15, 2005 9:23 am

I have always been told that the charge line has to have some kind of isolation device to prevent anything nasty happening to the cars electronics. Maybe its something really simple, but I've heard that it is best to get the dealer to install the manufacturers hardware so you don't void your warranty.

Funny Karl... hee hee... would be funny to go in to the dealership and say "do you have an asteroid for my charge line?" :oops:
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Postby Big Guy with a Little Guy » Tue Mar 15, 2005 10:48 am

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Postby bdosborn » Tue Mar 15, 2005 2:42 pm

campadk wrote:I have always been told that the charge line has to have some kind of isolation device to prevent anything nasty happening to the cars electronics. Maybe its something really simple, but I've heard that it is best to get the dealer to install the manufacturers hardware so you don't void your warranty.


The isolator isn't protecting the car electronics, its protecting the start battery. You might wake up to a dead start battery if you don't isolate the trailer battery from the start battery. An easy way to isolate them is to use a solenoid as mentioned above. Turn the key on, the trailer battery will charge (whether the car is running or not). Turn it off and you can run the trailer battery all the way flat and still start your car. An even easier way to isolate the batteries is to do what Karl said: unplug the trailer from the tow vehicle when you camp. Then plug it back in again when you drive away. ;)
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Re: Can you spell that out

Postby bdosborn » Tue Mar 15, 2005 2:59 pm

Guy wrote:Dear SteveH,

Could you spell that out for us dummies. Precisely how do you hook it up to the ignition? What wires go to what posts?


Attach a red #10 wire and a 30 amp in-line fuse to the + terminal of the battery. From the battery, the wire goes to the solenoid contactor (which will be labeled). Then the red wire goes from the solenoid contactor to the + terminal of the trailer battery (through the plug connecting it to the tow vehicle). There already should be a ground wire to the trailer for the stop/turn lights so you don't need to add another one if its there. Make sure you get a continuous duty solenoid. Starter solenoids are only rated for intermittant use and will fail prematuraly. Then find a wire in the car that is only hot when the key is on. Splice in a #12 wire from the key-on-hot wire to the coil of the solenoid. Make sure the coil is grounded and you're done. Clear as mud, right?
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Here's a picture of a starter solenoid for an example. The wire from the batteries goes to the big terminals on the right and left. The key-on-hot wire goes to one of the little posts in the middle. A ground wire goes to the other little post in the middle. I can post a wiring diagram sketch if anyone wants one.
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Postby asianflava » Tue Mar 15, 2005 4:16 pm

Relays and solenoids are pretty much the same thing. They typically have 4 conncetions, 2 for the electromagnet and 2 that are switched. Some have 3 that ground the electromagnet thru the relay body. What a relay is, is a switch that is activated by an electromagnet. When you turn on the coil wire, it will energize a coil that will magnetically pull a contactor closed. There are SSRs but I won't get into that. Think of it as throwing a switch by remote control.

You may be thinking, "why don't I just use a switch and skip this complexity?" When you use a relay, you can run small gauge wires to where you are doing the switching, which also lets you have shorter large gauge wires. Relays also have higher current capacities than switches.
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