How To Install A Roadmaster Hy-Powered Diode On Trailer 7-way

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Question:

I have the RM-690 diode. I have a trailer and installed lithium batteries and a new power converter. Ive read conflicting installation procedures for the RM-690 diode. Could you please be explicit with instructions? So far: 1 connect the positive wire from the trailer to the anode side of the diode and the other side to the positive wire of tow vehicle 2 connect the diode in the positive of the trailers 7-pin connector with the anode side facing towards the tow vehicle and, 3 severe the 12 volt hot lead from the deep cycle battery between the battery and 7-way connector. No reference to direction. One says: allows power to flow from tow vehicle to trailer batteries preventing backfeed to tow vehicle Second says: ensures that power can only flow from the trailer to the tow vehicle preventing reverse current Where should I place the diode and which direction? I would like to place it between the batteries and input to trailer if possible. Please help

asked by: Randy

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Expert Reply:

Hi Randy,

Are you just trying to charge the trailer batteries from the tow vehicle's 7-way? If so you're using the 12v aux pin on the 7-way and could place the diode on the trailer side just before the junction box to prevent any current from backfeeding to the tow vehicle. The face of the diode has an "in" and "out" labeled on it. The "in" will be power coming in from the tow vehicle. The "out" would be the power going to the batteries. With these diodes current can only flow from in to out, if it ever went both ways then you'd know that the diode has failed and needs to be replaced.

That being said, you're not going to get a good charge for your batteries using just the 7-way. The 12v pin on the 7-way is typically low amperage so if you're needing more than just a trickle charge you'd want to look into a DC to DC battery charger like the Redarc BCDC. These come in a few different amperage ratings so you'd really want to determine your power draw/demand to size the charger. The smallest option they offer is 12 amps with the # RED96FR. If this is for a camper that you'd have plugged into shore power the trickle charge from your 7-way would likely be enough to get you by though.

If you have any other questions just let me know and I'd be happy to help.

expert reply by: Kevin C

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Randy

11/13/2024

Kevin, I don't necessarily need to charge the lithium batteries from tow vehicle. My issue is that I seem to have received differing connection issues via research on the internet: a) It is generally recommended to install a diode between a trailer with lithium batteries and the tow vehicle to prevent reverse current flow and protect the to vehicle's electrical system. b) Install a diode to ensure that power can only flow from the trailer to the tow vehicle, preventing reverse current from the tow vehicle battery potentially damaging the trailer's lithium battery system. This seems to show a current flow in different directions. Am I correct? Also, a previous response to another customer question was: In your case, you will want the Roadmaster Single Park Light Diode part #RM-690 (which is what I have) to eliminate back feed. To install the diode, you will need to severe the 12 volt hot lead from the deep cycle battery between the battery and 7-way connector. The diode will need to be placed on the trailer. (Direction not stated.) This would be easiest for me if it protects the system. Would the "IN" be from the batteries and "OUT" towards the trailer 7-way connector? Sorry to be so confused on this one. ;)

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Kevin C.

11/14/2024

@Randy Installation of the diode is going to differ depending on the intended purpose so it's going to be a case by case thing. For "B" I'm not quite sure why they would want the power to flow from the trailer to the tow vehicle but they could be needed if they were using the trailer's battery to power some kind of accessory on the tow vehicle. The more likely case is that answer is listing it backwards and would need to be corrected, but without seeing what the whole situation is it's hard to say. You just need to cut into the 12v power wire coming from the trailer 7-way cable that runs to the junction box. The "in" will be the side facing the tow vehicle and the "out" will be the side facing the trailer. Battery power "in" from the tow vehicle and "out" to the trailer. If you reverse this the trailer will not receive 12v auxiliary power from the 7-way/tow vehicle.

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Robert

8/7/2025

Hello Kevin, in reference to the question above concerning the install of the RM-690, I understand that I am going to use the diode to keep my lithium battery bank which is located in my slide in truck camper from drawing more than 12v from my trucks charging system. I understand that the IN side of the diode will connect to the truck charging system and the OUT side of the diode is towards the camper battery bank. Installed in this manner it will keep the lithium battery bank from drawing any power except what the diode is letting through from the trucks charging system ie. 12v, there by not causing harm to the trucks alternator. My question is, where to physically locate the diode? On the vehicle prior to the 7 pin connector or on the camper itself near the battery bank or battery compartment? Also the hot wire is normally which connection on the pin connector?

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Robin H.

8/8/2025

@Robert you can put it on either side but I would stick with the trailer side wiring for the diode installation. Don't want to risk any issues with truck wiring or triggering some kind of weird error code if you cut into it. The 12V on the vehicle is going to be in the 1 o'clock position. I attached a help article which shows 7-way connectors and the typical functions based on position.

Helpful Links

faq-wiring-7-way.aspx

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