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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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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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