7-pin Rewire

[Marine359]

I’d like to hear from any TT owner who has rewired their trailer 7-pin plug/wire to provide 12vdc charging current for the trailer house battery.There seems to be some You-Tube info out there, but I don’t know if the conversion is safe, and what long term damage may be done to the tow vehicle alternator.

If you have done this conversion, how many amps do you draw through the 7-pin, and did you in-line fuse it directly to the battery? Some in the forum recommend using a dc/dc charger. But wiring through your tow vehicle is expensive, and so is the charger. So is a 7-pin conversion a cheap short cut?

[DavidM]

If your TV has the factory tow package, it almost certainly is ready for 7 pin charging, well almost. If it was added later maybe yes, maybe no.

The factory package sometimes leaves the fuse out so the pin is dead. It is pin #4, upper right as you look at the TV's connector. The TV's manual sometimes discusses where it is, so you can install a 30A fuse- typically.

If it was wired after purchase, then that fuse place holder still exists, you just need to find it and wire it to the connectors pin #4.

Virtually all TV's only supply power to the pin when the ignition is on.

If you have to wire it from scratch, no fuse, no protected circuit this is what I would do:

Install a relay good for 50A continuously, triggered by the ignition with its source from the battery or alternator terminal with a 50 fuse near that connection. Then using #8 wire, run a +/- circuit through the TV to the pin.

Why are you using #8 wire and a 50A fuse? So you can get more current to the trailer's batteries. The more typical #10 wire produces too much voltage drop to pass much current. I measured only 10A to my well discharged battery that slowly rose to about 5A when it became charged. #8 wire should double that.

There is no need for a DC to DC charger. The current is inherently limited by the voltage drop in the long wiring.

David

[RocknRoll]

Originally Posted by DavidM

If your TV has the factory tow package, it almost certainly is ready for 7 pin charging, well almost. If it was added later maybe yes, maybe no.

Thanks to Dave for clarity on this. I recently discovered that leaving the connector plugged in will continue to draw power from the TV even when off.

Including the connector diagram and how to test with a multimeter.
https://housetechlab.com/how-to-test...th-multimeter/

[Morich]

This is one of those things that come up so often and it seems to be so obvious that lots of folks have some strange ideas of what makes current flow as well as general everyday business.
First what makes current flow seems to be a big mystery, so perhaps some real low ball explanation will help as the concern about needing super big wires is really a non-starter if we think it through.

By definition, voltage is the thing that makes current flow, so we first need to think about what voltage means and what voltage we will have if we connect a trailer to a tow vehicle. Any dummy knows we use 12 volts but that is not the voltage we need to think about when we ask how much current will go from front to back!!!

It is common to use 12 volts as the voltage as we most often use ground as the reference and when measured to ground we DO get 12 volts. But if we are speaking of the voltage DIFFERENCE from front to back, we are not tying the battery to the trailer frame ground but to the trailer battery positive side.

If we tie a tow vehicle with a 12 volt battery to a trailer with a semi rundown battery reading like 10 volts to ground, the "difference in potential" is only 2 volts!!! So if we tie the charge voltage to the frame we might need a REALLY big fuse and wire to keep it running but if we tie it to the battery, we don't need very much to cover a 2 volt difference, do we?

So we have vehicles with 7 pin connections built from the factory for use to charge things like trailer batteries or the big sets of batteries used on big bass boats with all the new electronics. It may be just my trust in things but if I buy a new truck that is set up for towing, I assume it will work for me, much the same as it works for all my friends with other trailers they pull and I expect the builders to know what works!

So if I don't want to get down to figure out how and why it works, is it okay just to assume it will work for me because it works for everybody else who does it by just using what the factory provides?

[tinglett]

I tested my 7-pin connection a little last fall, but have more to check when it gets warmer this spring (now that I've had more time to think about it).

My SUV powers pin-4 only while the engine is running. I found it has a 40A fuse and I assume also a relay to switch it on/off (I haven't hunted down the relay). I haven't determined what size wire is used either within the SUV, or within my 2108DS's towing cable.

I have actually seen a 17.88A current draw as measured by my Victron shunt when idling and charging a Battle Born LiPO4 battery. I was surprised to see it that high, but the battery was quite low at the time (just under 12v), so I suspect this is about all I'm going to get. No doubt it is limited by the wiring. That's just as well as I don't want alternator issues. At that time, as the battery charged it got to about 13v, but the current was down to 6A.

It would be interesting to measure voltage at the engine side to see what it's putting out. I have a gauge in the dash, but it's a bit crude to read. I think it only gets up to 14.1v or so. With luck maybe higher, but I suspect voltage will always be down quite a bit at the hitch due to the wire. In the spring I'll try to measure both ends and see if that jibes up with a voltage drop calculation for a guesstimate of the length and gauge of wire.

I would also like to verify that the 2108DS doesn't draw any power at all from the SUV when connected with the SUV engine off. I think that'll be trivial to determine by just checking my shunt to see if the battery is charging or draining. I don't really intend to ever leave it connected that way, but would be a good thing to know anyway. Considering it only puts out voltage when running, I fully expect to see there is no power since it is probably cut off by a relay. But I'll check just in case.

But to the topic at hand, hopefully it's clear that my 7-pin connector was always good to go. While searching for some info on my SUV I did find at least one Toyota Tundra owner who was missing his fuse.