Diodes, Dudes! - Fiberglass RV


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Old 01-12-2006, 11:12 AM   #1
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I'm curious about diodes in the tow/trailer charging circuit. If electrical stuff is boring to you, please pass on by. We've already established I'm a geek so let me be. Today I'm a militant geek. I just want to learn something.

I'm imagining a trailer with a low battery. I decide to hook the trailer up to the tow vehicle.

First I'm imagining the wiring as looking like shown in my simple sketch. (In my drawing, I just threw a light bulb into the trailer circuit. It's not even turned on.)
The tow vehicle is hooked up to the trailer. When the engine is running, Mr. V. Oltage Regulator is eyeballin' the tow battery. He's got one hand on the alternator dialing in enough amps to keep that battery pretty much filled. Note that the tow vehicle battery doesn't have much capacity. Its 13.6 volts tall, but it drains pretty quickly.

And keep in mind that the regulator doesn't have much to work with. You're not pedaling that tow vehicle very fast. At a fast idle, or even highway cruising speeds your alternator is probably only putting out maybe half its capacity and you're using up most of it just running the tow vehicle let alone recharging anything.

By the way, I picked 13.6v as full because I was too lazy to differentiate between a full battery and a charging battery.

The way I have it drawn, the voltage in the tow vehicle battery (13.6) is higher than the depleted trailer battery (12.0). Amps are going to flow from the tow battery back to the trailer battery. If the engine is running, then likely Mr. Regulator will bump up his amperage level just enough to make up for whatever is being actively consumed by the trailer.

As long as there is no additional drain on the trailer battery, eventually, slowly, bit by bit, the trailer battery will refill. Not quickly because the regulator isn't seeing the problem directly, but rather filtered through the tow vehicle's battery.

If you're putting a significant electric drain on the trailer battery, I doubt the alternator will ever recharge the trailer battery. The regulator is just replacing what is lost to the tow battery, probably only as fast as it goes out. No faster.

Turn off the engine and the levels of the two batteries will seek to level out pretty much evenly. There's nothing to stop the tow vehicle battery from draining into the trailer battery. Further, because the trailer battery has so many more amp-hours in it than the tow vehicle battery, the resultant voltage in both batteries will be closer to 12.0 volts than it will be to 13.6 volts.

Many people have this wiring and they handle it by unplugging the trailer when they stop for anything longer than a few minutes.
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Old 01-12-2006, 11:13 AM   #2
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My Explorer has the positive line to the trailer on a relay tied in to the ignition key. Turn off the ignition, then the relay opens and no current flows from the tow to the trailer. My Explorer doesn't charge the trailer battery any more or less than the first example. However, with the engine off the trailer won't drain the tow battery.
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Old 01-12-2006, 11:14 AM   #3
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Some people install diodes to more or less duplicate what the relay does for me. I wondered where the diode goes to keep the tow battery from draining. Putting it like this doesn't do any good. Put the diode there and all it does is keep the trailer battery from charging the tow battery.

Not necessarily a trivial issue. The trailer might be plugged into shore power, the converter putting 13.6 vdc into the trailer battery and I might have the tow vehicle connected but not running and current might try to recharge the tow vehicle battery in this situation. (But not on the Explorer if I have the key off and the relay is open.)
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Old 01-12-2006, 11:15 AM   #4
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This is all I can come up with. This location allows the alternator to provide current to the trailer and the tow battery. How much the regulator tells the alternator to send must be some sort of "average" of the tow and the trailer voltages. Plus, what arrives at the trailer battery is reduced a significant amount by the electrical resistance in the long wiring run back to the trailer battery.

Turn off the engine, which shuts off the alternator, and the diode keeps juice from going from the tow battery to the trailer battery. This means you'd have to interrupt the charge line from the alternator to the tow vehicle battery to install the diode, then splice in the trailer charge line "upstream" of the diode.

So For those of you who have done this, is this how you wire an aftermarket diode?
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Old 01-12-2006, 11:27 AM   #5
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Actually, Steve, the diode shown in your third post prevents the trailer from recharging the tow vehicle battery.

The diode in the fourth post prevents the tow vehicle battery from discharging to the trailer.

There is a problem with using a diode in either application, however, and that is the voltage drop across the diode. A silicon diode drops about 0.6 volts when current is flowing in the forward direction. If Mr V is watching the wrong side of the diode, he'll charge when he shouldn't or not charge when he should because he sees the battery voltage minus 0.6 volts. In your example, he sees 13.0 volts instead of the 13.6 volts at the battery. Therefore he'll say, "lets get this dude charged up."
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Old 01-12-2006, 11:30 AM   #6
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The configuration in your last post is pretty much the way with a diode isolator. Here's a bit more information.

http://www.bcae1.com/battiso.htm
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Old 01-12-2006, 12:14 PM   #7
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To continue what Morgan has explained, one diode can keep the trailer from draining the tow battery, and another can keep the tow vehicle from draining the trailer battery, but you need two diodes to do the whole job. I believe that the typical diode isolator, such as the one linked by Byron, has the two diodes, with one common connection, and thus has those visible three high-current terminals, with a configuration as shown in the this drawing crudely adapted from steve's last one:

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To address the problem of control which Morgan mentions, some isolators have that fourth terminal which may be used by some alternators to "sense" the voltage on the battery side of the diodes, so that the desired full battery voltage is reached.

I use the relay approach in my Sienna to supply my Boler, mostly due to the voltage drop issue, and to avoid interfering with factory wiring with an intervening diode.
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Old 01-12-2006, 12:25 PM   #8
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I prefer the relay approach myself.
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Old 01-12-2006, 12:38 PM   #9
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Steve,

Byron's link hits the nail on the head. It doesn't show the two diodes necessary to make this work. On the diagram shown there, they sit "back to back" astraddle the junction with the alternator. In other words, one diode "points" away from the juction to the tow battery, the other diode, on the other side of the junction, "points" toward the RV battery. Thus, neither battery sees the other and the alternator only deals with the current draw, whatever the source, at the junction.

Morgan's point about the voltage drop across the diode is important. Isolators for some GMC electrical systems have an auxillary line to the sense input of the alternator that effectively fools the alternator into putting out a higher voltage than 13.6 ... namely 14.2 volts or so. (There's nothing to be done about this voltage drop. It's built into semiconductor physics and is pretty much the same for any diode, large or small.)

I've only fiddled with GMC so don't know how, or if, isolators for other makes compensate for the voltage drop. Given that 1/2 volt represents a significant amount of capacity in the discharge curve of a deep cycle battery, it needs to be dealt with.

IMHO, without compensation for the diode voltage drop, the ignition relay approach is better, despite not being as elegant as diodes.
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Old 01-12-2006, 01:07 PM   #10
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I have installed a hybrid system of what you have here. In my system I use a relay to turn power on & off to the trailer connection.

At the trailer I have a Y connection inside the closet where the power enters the RV. One leg goes to a diode > battery charging. The other leg of the Y connection goes directly to the refrigerator via a 10 gauge wire.

When the Tahoe is running it is providing power to the refrigerator. If I have the refrigerator on 12 volts, it operates it. When we are towing the RV battery is being charged to some extent.

Now when the Tahoe is turned off (ie at a restaurant) there is no bleed back from the Tahoe because of the relay, and NO bleed back from the RV battery because of the diode.

This has worked out very well on long trips keeping the refrigerator cold and not having to remember to unplug or turn anything off when we stop. We also dont need to run the refrigerator on propane to keep it cold.

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Old 01-12-2006, 02:35 PM   #11
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Go to JC Whitney and purchase a battery issolator. It will charge the camper battery and prevent your vehicle battery from discharging should you keep things connected up whild camping.
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Old 01-12-2006, 02:45 PM   #12
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Go to JC Whitney and purchase a battery issolator. It will charge the camper battery and prevent your vehicle battery from discharging should you keep things connected up whild camping.

An isolator is just a couple of diodes encased in potting and terminals to make connections to. The working part is still diodes with .5 to .7 volt drop across each diode.
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Old 01-12-2006, 02:58 PM   #13
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An isolator is just a couple of diodes encased in potting and terminals to make connections to. The working part is still diodes with .5 to .7 volt drop across each diode.
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Old 01-12-2006, 03:29 PM   #14
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I can only speak to my experience with a '93 GMC 2500. I bought it used and it had a Surepower isolator mounted in the engine compartment on left fender. It was not hooked up. Thinking this was cool, as I bought the truck to tow with, I endeavored to hook it up. After doing some research, I learned I needed a special adapter for the alternator sensing connection AND I would have to physically modify the wiring at the connector AND the diagrams included with the adapter kit didn't match anything I had. This went from a trivially simple, in principle, idea to something more, in practice.

I passed.

The screw terminals on the isolator proved handy to hook wires up to the straight-through connection to my 7-pin trailer connection at the back. Not real sophisticated but worked fine for several years.

I'm sure there's a large population of diode isolator users out there. My old tow vehicle and I just aren't members.
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