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Ken O'Neill asked

Problem with DC-DC charger installation

I have installed an Orion isolated DC-DC charger 12v-12v 18A. I wired the positive (black) and negative (white) wires from the 7-pin to the input side of the charger. Then I wired the output side to the battery via the positive and negative busbars. See attached diagram. The truck has 10 ga wire with a 30A fuse to the 7- pin. With the trailer connected to the 7 pin but not attached to the truck hitch, the charger is working and putting about 17 A into the batteries. However the trailer running lights and turn signals are not working.

I presume that is because the white (negative) 7 pin wire is now connected to the input side of the charger and I have interrupted the circuit for the trailer running lights. (Perhaps I should have gotten a non-isolated charger...)
But now what? I don't want to rely on the hitch to create a negative contact between trailer and the truck. Can I connect both the negative input and output sides of the isolated charger to the negative bus and also reattach the white (negative) 7 pin wire to the negative busbar?

Charger wiring diagram.pdf

installationorion dc-dc
charger-wiring-diagram.pdf (112.6 KiB)
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Bob K avatar image Bob K commented ·

You need to reconnect both wires to the 7-pin connector. The trailer needs both of them to run various DC functions while you're towing.


The DC charger is supposed to have its own dedicated wires from your truck battery or alternator, and the isolated charger is the correct style for those of us with trailers so that the ground to the tow vehicle is made through a wire rather than through the hitch. However, if you wanted to try to power the DC charger off of the wires on your 7-pin connector (not recommended since they're small gauge wires), then you would have to pigtail off of the +12V and GND wires to the input side of the charger. The output side + and - would go to the + and - busbars within your trailer.

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airstreampictures avatar image airstreampictures Bob K commented ·
Bob K, in my airstream, I’ve installed an Orion 12–12/18 using pigtails from the + and - from the 7-pin, as you suggest. Everything works, but the Orion is being fed by the house batteries when the truck is disconnected. It seems to be feeding back through the 7-pin +, which is still connected to the DC bus bar. Pigtailing off the ground seems essential—the brakes and lights need it to work. But should the + from the 7-pin go only to the Orion, rather than through to the bus bar?
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Bob K avatar image Bob K airstreampictures commented ·

Just to be clear -- because I don't want to give advice that could be dangerous -- if you're powering the DC Charger off of the + and - wires on your 7-pin connector (not recommended), you'd have to be super careful about programming the DC Charger properly so that you don't exceed the ampacity of the small-gauge wire on the 7-pin connector. Hopefully, those wires are fused somewhere on the trailer side so you'd blow fuses rather than burn up wires if the DC Charger tries to pull too much current.

Yes, if you've connected the DC Charger input side + and - wires to the trailer side of your 7-pin connector (whether that's going through a busbar or not), you are creating a direct connection to your house battery. That'll create a charging loop.

When your 7-pin connector is connected to your tow vehicle, your DC Charger will still have a direct connection to your house battery. If your 7-pin + wire is switched on your TV, then you'll only put additional current from the alternator onto that circuit when the TV is running. Otherwise, the DC Charger will continue to pull power from the house battery.

Whether you connect to the 7-pin + wire directly or through a busbar, you're still just tapping into the same circuit at different spots, so I don't think there's a way around your conundrum if you're committed to using the 7-pin wires to power the DC Charger. Grounding the - wire to the - on the trailer or the - only on the TV doesn't matter either, because as soon as you plug that 7-pin connector into your TV, those two sides become the same circuit.

If you are committed to this course (rather than running standalone + and - wires from the TV battery), then perhaps you could use one of the advanced switching options available for the DC Charger. You could use the trailer running lights circuit as your switched trigger. Then, the DC charger would only draw from your 7-pin circuit when the trailer running lights are turned on.

Even that would be less than ideal because the DC Charger would still want to pull power from your house battery when your alternator is outputting lower power (such as when you're at idle). That'll exacerbate your charging loop conundrum.

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airstreampictures avatar image airstreampictures Bob K commented ·
@Bob K. I have replied, but my reply is awaiting moderation. I'm not sure why—no earlier post I've made has required that.
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Kevin Windrem avatar image Kevin Windrem airstreampictures commented ·

You must NOT connect the 7-pin + / - wires to the house battery. Doing so shorts out the DC-DC charger (output back to input). In addition, the power from the TV via the 7-pin + / - wires attempt to change the battery directly rather than passing the power through the DC-DC charger.

Connect the 7-pin + and - wires ONLY to the input of the DC-DC charger. Then connect the output of the DC-DC charger to the house battery (via the bus bars or however you choose to wire it.) Don't forget fuses at the battery end of the + wire or you will burn up that wire should it short to RV chassis.

I have this very setup in my travel trailer and the DC-DC charger's logic that is based on incoming voltage was not usable due to voltage drop in the lines from the battery, through the 7-pin and on to the battery at the back of my trailer. As Bob K suggested, I used the running light circuit in the trailer to turn the DC-DC charger on and off. This of course means the TV won't charge the RV if your lights are off but I always travel with my lights on so that works for me. Otherwise, you may need to route a switched on/off circuit from the ignition switch to the trailer. All pins in the 7-pin connector have assigned functions so this would require a separate wire and connector at the back of your TV.

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Bob K avatar image Bob K Kevin Windrem commented ·

"You must NOT connect the 7-pin + / - wires to the house battery. Doing so shorts out the DC-DC charger (output back to input)."

I think you're mixing a couple of concepts here. You'll find that the 7-pin + and - wires are usually connected directly to the house battery. That's how the tow vehicle is able to send some charging current to the house battery when the TV is turned on. Whether you probe the female terminals on your 7-pin connector or probe where the house batteries are connected to the busbars or probe somewhere along the 7-pin connector wires, it's all the same circuit. They're all connected to the house battery. That's how the entire DC side of the trailer gets its power.

Also, feeding the DC Charger input from the 7-pin + and - wires (and putting the DC output back into the house batteries...which are connected to the 7-pin + and - wires) doesn't create an electrical short. Rather, it creates a charging loop: The house batteries feed the input side of the DC Charger through the 7-pin + and - wires. The DC Charger then bucks/boosts the voltage (along with some efficiency loss) and sends it back to the house battery through whatever + and - connections you wish to make. It's a complete waste, but it's not a "short". The same thing happens when someone wires an aftermarket inverter into his RV. Batteries feed the inverter which outputs AC power to the onboard charger/converter, which converts that AC back into DC and sends it to the battery. That's a wasteful "charging loop". To prevent this, the user has to turn off the charger/converter circuit breaker when the aftermarket inverter is in use.

In other words, it doesn't matter where you tap into the + and - side of your trailer in order to feed the input side of the DC Charger and it doesn't matter where you feed the output side of the DC Charger back into the + and - side of your trailer. It's all the same circuit (assuming no weird switches and proper fusing, of course.)

This creates a de facto charging loop, though, and is definitely not the recommended way to use a DC Charger. The only way to get this to work is to do what you did: Use one of the DC Charger's voltage trigger functions so that it only comes on when the TV is running.

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Kevin Windrem avatar image Kevin Windrem Bob K commented ·
We are saying the same thing. By "short" I meant that the DC-DC charger's output is connected to its input: your "charging loop".
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Bob K avatar image Bob K Kevin Windrem commented ·
O.k. Be advised, though, that this "short" or "charging loop" exists in this setup regardless of where one taps into the + and - side of the trailer to feed the input side of the DC Charger, and regardless of where the output side of the DC Charger feeds back into the + and - side of the trailer.


That's why the separate voltage trigger is necessary. (You're using the running lights circuit). That keeps the charging loop at bay unless you turn on the trigger (ostensibly once the TV is running and providing a net rise in voltage to the house battery).

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Kevin Windrem avatar image Kevin Windrem Bob K commented ·
For this to work at all, you MUST disconnect the 7-pin + and - wires from the house battery / bus bars ! The whole idea of the DC-DC charger is that it raises the voltage from the TV to charge the house batteries and that can't happen if in and out are connected together. Since the DC-DC charger is essentially a current source, no damage will occur but nothing positive will come of it and the house batteries won't charge better than the 7-pin only.


The 7-pin + / - connect to the DC-DC charger input and the DC-DC charger output then connects to the house battery / bus bars.

You can connect other devices to the 7-pin + / - wires, for example the breakaway emergency break battery, but again these must not connect to the house battery. Or you can connect these kind of things to the house battery but not to both.

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Bob K avatar image Bob K Kevin Windrem commented ·
You keep saying not to connect the 7-pin + and - wires to the house battery.


But they are, by design, connected to the house battery. Go ahead and probe the trailer's 7-pin + and - wires anywhere along their length. You will see your house battery's voltage.

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Kevin Windrem avatar image Kevin Windrem Bob K commented ·
What you are saying it probably true for most RVs when shipped from the factory but in order to use a DC-DC charger, that connection needs to be broken. The DC-DC charger input and output must not be connected together.
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Bob K avatar image Bob K Kevin Windrem commented ·

I think you're indicating that you personally disconnected your 7-pin + wire from your house battery and directed it to the input of your DC Charger. That way the TV power only goes into the DC Charger rather than into the house battery and the DC Charger.


I agree that that's the most ideal way to do it if one is willing to give up the traditional function of the 7-pin + wire. It eliminates the charging loop.

However, "The DC-DC charger input and output must not be connected together" is an incorrect statement of absolute. There are drawbacks to it (namely, that it causes a charging loop) but "must not" is not technically correct. That charging loop can be overcome by using the DC Charger's triggered switch function.

Chime in, @AirStreamPictures, to let us know how your project is coming along.

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Kevin Windrem avatar image Kevin Windrem Bob K commented ·
Respectfully BobK, there is no way the DC-DC charger will function with its input and output tied together. I'm an electronics engineer and know what I'm saying. In invite you to try it.


Look at the math: The TV generally produces around 13.5 volts at the battery with the alternator running. A modest 4 amp current from the trailer over 20 feet of #10 wire drops that to 13.3 at the trailer.

In order to fully charge a lithium battery, an absorption voltage of around 14.2 volts. With the DC-DC charger output connected to its input the house battery voltage will never exceed the voltage at the 7-pin (output voltage must equal input voltage since they are connected together). To achieve that, the DC-DC charger would need to produce enough current to raise the voltage to 14.2. This would include the TV voltage all the way back to the battery. But that current must come from the alternator which will reduces it's output current to zero when it sees a voltage above 13.5 volts.


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Bob K avatar image Bob K Kevin Windrem commented ·
Other than the charging loop problem, let's ask @AirStreamPictures if it works, since it seems that's how he currently has his wired. That'll give us a real-world datapoint.
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airstreampictures avatar image airstreampictures Bob K commented ·
This discussion has all been helpful. I'vere-wired the DC/DC charger, and it seems to be working, though everything is not perfect. What I've done is to use a pigtail (in fact a small buss bar) to connect the (-) side of the 7-pin to the DC/DC as well as the house (-), but I've wired the (+) lead from the 7-pin only to the input of the DC/DC charger. I've then wired the output of the DC/DC to the (+) and (-) bus bars that connect to the house batteries.


This works, and delivers a useful amount of charging current to the batteries. The circuit from my tow vehicle (a Mercedes GL) is 10AWG and fused at 20 amps, so it's a good match for the Orion 12/12-18. I'm using alternator sensing to control the Orion, and so far, this seems to work well enough, though in stop-and-go traffic it switches on and off, which throws up a message in the truck that rear blind-spot sensing is deactivated every time it does. I expect that using the running light circuit as a trigger would prevent this, but II wonder if that would cause the alternator to overheat in slow traffic?

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Kevin Windrem avatar image
Kevin Windrem answered ·

The negative wire from the 7-pin must be connected to the trailer chassis. It can also go to the negative input of the Orion. The Orion output connects to house DC electrical system as you have described and will at some point also connect to the trailer's chassis. That's all OK.

10 gauge in the truck is a bit small. I had 12 gauge in my truck and replaced it with #6 and used the truck chassis for the negative. I used the trailer chassis for the negative and ran #6 for the positive and get close to the 18 amps of charging current.

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