7 Pin for charging + 4 pin for driving?


Well-known member
I currently have 4 pin for driving but I want to see if I can charge while in motion. Would it be possible to hook up charging only through the 7 pin so I don't have to rewire the entire trailer?


Expedition Leader
Verkstad, you're getting mellow in your dotage, you didn't tell him he'd burn his vehicle down.

The 7-pin hot is usually fused at 10- or 15-amps. You aren't going to do a lot of charging thru that. It would however be useful for backcharging your vehicle battery via a trailer-mounted solar setup, since most single 100W-class panel setups are only putting out ~5A


if you want to charge dead trailer batteries while driving you will need alot more than the 7-pin connector since like rayra said its low amps (and narrow gauge). You could run a seperate 4 or at least 8 awg off a relay to an anderson connector and wire the other end to the trailer battery for charging only. then keep the stock 4 pin for lights and everything. Or if you just want to top up the trailer battery and keep stock wiring on the truck, buy a 7 to 4 adaptor, plug into the 7 pin on your truck, adaptor to trailer wiring, and then just run the +12V off the 7 pin pigtail to the trailer battery.

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You don't have to rewire the whole trailer to convert from 4 to seven pin. Just get a 7 pin trailer side plug, cut the light wires off of the 4 pin and connect them to the 7, run a charge wire from the the aux pin directly to your DC-DC charger, or the positive battery post if you don't have the charger. You will probably want to upgrade the battery negative wire and the wiring ground wire on your truck to 8 AWG or bigger.

Or you could wire your trailer side 7 pin to a TV side 4 flat, just plug in your current 4 flat to that and add the charge wire to the aux pin, but why make so many different connections.


Depending on your set-up, and how your truck is already wired, you may want to run the charge circuit through a relay on the truck side so you don't drain your truck battery when the ignition isn't on.



Well-known member
@TantoTrailers put together a nice system for this purpose to supplement solar charging. Details are scattered about the thread, but this is a good starting point.

If sticking with existing wiring, I think using an inexpensive, dumb DC-DC converter/power supply is a better solution than trying to gain a useful charge by connecting directly to the 7-pin plug.


Well-known member
What kind of battery do you have in the trailer?

You can convert your 4pin trailer to a 7pin with a new main harness such as this: https://www.amazon.com/BougeRV-Weatherproof-Connector-Junction-Trailers/dp/B078N2T32T/
You'd just not hookup the brakes up to anything obviously and wire the aux power to your battery.. a small DC-DC power supply like @shade mentioned would allow you to charge it at a constant voltage despite any voltage drop so the battery can still get a full charge on a long trip.

Have you considered a fixed solar panel on the trailer? Thats what I ended up going with.. As long as its not super cloudy or night time I can get a good 20A of charge while driving, no extra wiring on vehicle.. Ive been able to cook food in the trailer while driving, and still charge my battery up.


So what is the intent of the 7 pin setup if not to provide a charging current to the trailer? Genuine question: historically, why did 7 become the standard?


Well-known member
So what is the intent of the 7 pin setup if not to provide a charging current to the trailer? Genuine question: historically, why did 7 become the standard?
It can deliver a modest charge for a house battery in a camper, as well as charge a small battery for the breakaway feature of electric brakes. Trailer manufacturers aren't concerned with anything past that, and the 7-pole system is a cheap way to accomplish the task.

The problem is that charging a battery (especially a lead battery) via the 7-pin plug may not get the job done, leading to a chronically undercharged lead battery. That's bad for the battery, and may mean that the house battery won't have enough power to meet demand. If the trailer is routinely plugged into mains power for long periods of time, it's not such a big deal. Otoh, it may not take long for a 100Ah battery to be discharged while primitive camping if the only charge source is through the 7-pin plug.


Well-known member
So what is the intent of the 7 pin setup if not to provide a charging current to the trailer? Genuine question: historically, why did 7 become the standard?
Electronic Brakes.. the battery on the trailer is just for emergency break away on the vast majority of commercial/cargo trailers on the road.. often all you need to do to know a trailer has brakes is look to see if it has a 4pin vs 7pin plug.. you putting a 7pin on a trailer w/out brakes would be rather abnormal and you might put a sticker or something indicating its not equipped w/electronic brakes.

Its designed for a maintenance current at best for the battery, not intended in the least for bulking up a deep cycle off the tow vehicle.. it comes with 3 extra circuits, Brake, +12V, and Backup Lights.. all things that tend to be useful on larger trailers.. tho backup lights are rarely implemented.
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I plan to use a 12v to 36v booster mounted near my trucks auxiliary battery to send 36v back through 10 gauge wire to the trailer where is will feed into my solar charger in parallel with solar panels. The higher voltage will eliminate having to run heavy gauge(1/0) wires 25’ back to the trailer batteries while still getting 400+ watts back there, and the solar control will step it back down to a proper voltage.
I haven’t seen examples where people actually have down this. I’ve amassed most of the parts but haven’t gotten it together yet.

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Well-known member
You can find em at your local hardware store in electrical section, I tend to favor the spring loaded push in ones but those are for solid copper.. these are much cheaper, but compared to 3 wire nuts in a junction box its much more compact.. try something like "3 wire coupler screw terminal".. I only said AC because its 3 wire, and DC is 2 wire so you might not find it in low voltage connectors.

Here's the whole kit-n-kaboodle: https://www.amazon.com/gp/product/B01IQUQF3Q/
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