Battery charging wire

[J Martin]

I'd like some clarification on how to run a power wire from my tug battery back to my harness. I thought I didn't have to, as I was going to splice into the 12V outlet at the rear of the vehicle but I am told the wire gauge isn't big enough to handle the current. Is that true? If it is, then how do I connect it? Do I just run a wire from the + terminal to the power line on the plug, then a wire from the negative terminal to the ground on the plug?
I'm no electrical whizz...
Thanks

[dsheets70]

I'd like some clarification on how to run a power wire from my tug battery back to my harness. I thought I didn't have to, as I was going to splice into the 12V outlet at the rear of the vehicle but I am told the wire gauge isn't big enough to handle the current. Is that true? If it is, then how do I connect it? Do I just run a wire from the + terminal to the power line on the plug, then a wire from the negative terminal to the ground on the plug?
I'm no electrical whizz...
Thanks

More than one inquiring minds want to know. I too am interested. Good question. Thanks for asking.

[John Perry]

More than one inquiring minds want to know. I too am interested. Good question. Thanks for asking.

I have a 10 gauge wire running from the tug battery to the trailer plug. I also have a 30amp circuit breaker in the line near the battery. The battery is already grounded to the vehicle so if you have a good ground at the recepticle on the tug and a good ground on the trailer side that should be good enough.
Remember to unplug your trailer from the tug when you are camping or you might find your tug battery is dead.
Hope this helps,

John

[Con]

You have to install a battery isolation solenoid in addition to the 30 amp. circuit breaker.
That way you don't drain your tow vehicle battery. Most RV shops will have the solenoid.
Most of the newer vehicles in the last several years that come from with factory with the tow package are all ready pre-wired for the charging circuit and brake circuit.
They only thing you have to add is the right plug on the wires for your brake controller and plug it into the fuse box under the dash.
On the last 3 Chevy Z71 pickups I didn't have to wire up a thing, even had the 7 pin plug mounted.
On the older vehicles though you have to wire as up as I mentioned. And be sure to use the heavy gauge wire. While you are at, it run a wire for trailer brakes now even if you don't need it yet.

[Herb P]

Yup. I just did this. I used 12ga for the brake line (probably overkill) and 8ga for the charge wire. Whenever I get a new (to me) vehicle, I throw away the battery cables and make new ones from 1ga or 0/1 (depending on what I have lying around at the time). Including new grounds to the body, frame and engine... So far, I have no vehicles newer than '95 though so that may be unnecessary on newer vehicles. The fuse holder I had on hand wasn't water proof so I put it in a small waterproof container with 2 grommets for the wire. The brake wire and charge wire are sheathed in expandable cable shield and then the whole thing is attached to the truck...

We'll see how it holds up.

[Patrick M.]

I keep it simple and just do a 8-10 gauge wire and a 30amp fuse. Ground wire from hitch plug to suitable screw in rear bumper area (but weather proof.)

All you have to remember is to not keep your trailer plugged in while running your refrigerator on 12v! (Ask me how I know this! )

[Per Walthinsen]

It depends partly on what you want the battery/alternator to provide to the trailer. If you want it to be able to charge the trailer battery until it is fully charged, such as in half a day's driving, you may have to beef the wiring up.

Think of it as competing water pressures, one part coming from the tug battery/alternator and the other from the trailer battery. In order to charge, the pressure from the tug has to be greater than the existing "pressure" currently in the trailer battery. The greater the difference, the better the charging.

This is where the wiring comes in: inadequate wiring produces voltage drop, so by the time the tug voltage reaches the trailer battery it may be not much higher than the existing battery charge.

I found a section of my charge wire to be 12 gauge, it was not doing much. Changed it to 6-gauge with a 40-Amp Maxifuse near the alternator (hard to work with, requiring heavy lugs and different connectors to be practical), and the results exceeded my expectations. The alternator will now charge the batteries, run my frig and a towel warmer at the same time with ease.

If you really want it to work well, there is probably no substitute for hefty wiring.

[John Perry]

You have to install a battery isolation solenoid in addition to the 30 amp. circuit breaker.
That way you don't drain your tow vehicle battery.

I am installing an isolator today. I recently purchased a slide in camper that makes it almost impossible to disconnect from the truck electrical system once it the camper is installed. If I was only using the trailer I would not have bought an isolator and saved the $45.00.

[Brian B-P]

Just a note on the "ground" connection...

That wire (normally white) and connection will carry all of the current for the trailer lights, battery charging (or running the refrigerator, etc), and the trailer brakes. That could be a lot of current, and (due to the lights and brakes) it is critical. I would not settle for the typical self-tapping screw in a random bit of steel - ran a 10 gauge wire (in hindsight it should be heavier) all the way from the 7-pin connector to the battery negative terminal. That wire also has a terminal bolted to the hitch receiver frame as a supplementary "ground" connection.

The comments about resistance to current flow apply equally to the "ground" side: you can have the heaviest cable sold for the positive wire, and the flow of current will still be restricted by the resistance of the negative (ground) wire and its connections.

[Brian B-P]

...Most of the newer vehicles in the last several years that come from with factory with the tow package are all ready pre-wired for the charging circuit and brake circuit.
They only thing you have to add is the right plug on the wires for your brake controller and plug it into the fuse box under the dash...

I think this is important: don't try to reproduce what may already be done for you.

Unfortunately many tow vehicles which are very capable of handling Class 1 (up to 2000 lb) and Class 2 (up to 3500 lb) trailers have no such wiring, including minivans and SUVs not based on truck chassis. Although kits to handle the lights are readily available, you're on your own for the brake controller and battery charging.

[Herb P]

Just a note on the "ground" connection...

That wire (normally white) and connection will carry all of the current for the trailer lights, battery charging (or running the refrigerator, etc), and the trailer brakes. That could be a lot of current, and (due to the lights and brakes) it is critical. I would not settle for the typical self-tapping screw in a random bit of steel - ran a 10 gauge wire (in hindsight it should be heavier) all the way from the 7-pin connector to the battery negative terminal. That wire also has a terminal bolted to the hitch receiver frame as a supplementary "ground" connection.

The comments about resistance to current flow apply equally to the "ground" side: you can have the heaviest cable sold for the positive wire, and the flow of current will still be restricted by the resistance of the negative (ground) wire and its connections.

I guess I have to disagree with you. I think a vehicle frame can probably carry more current than a 10ga wire. I think an 18" run of 1ga from battery to frame, and then an 18" run of 10ga from frame to 7 pin connector is going to have far greater current carrying capacity than a 20 foot run of 10ga... (As long as you address corrosion issues with the right kind of paste and preventative maintenance).

I will agree that a sheet metal screw into a piece of random steel is not going to do much but one can always find at least one way to do something incorrectly.

[Brian B-P]

Herb's right, of course, there's lots of metal in the vehicle structure. Who knows, maybe my return current is mostly flowing through the hitch then body then ground cable, rather than my wire... but there must be a good path, by whatever conductor.

I think we do agree that the "ground" connection -whatever the method - carries all of the current and is thus important.

Keep in mind that the body grounds which we see in vehicles when looking at the lighting and accessory circuits - which may indeed use a self-tapping screw - are designed for only a few amps, and you want to carry ten times more current.

[Patrick M.]

And remember the ground wire (and the hot wire) in the 7-wire trailer connection carries all that current too, and God knows what size of wire that is!

Here's where I grounded my hitch connection. About 4' from the hitch, 10 gauge wire, high and dry.

[J Martin]

Thank you all for the info and especially the pictures. I ran out to Princess at lunch and bought some 8ga wire (on sale!) and loom to run it through. I am glad I only have to run one wire back from the battery. I'll try to ground the harness either on hitch or frame. What a wealth of knowledge here!
Jeff

[Pete Dumbleton]

In *addition* to the frame ground, I run a hefty ground wire under the truck along with the battery and brake lines, so I *know* that ground is a good one. Although the ground and battery wires in the trailer harness are relatively small, so are the fuses! The shorter I can make the small runs, the less total resistance I will have in them.

You guys who are using fuses may want to consider using auto-reset circuit breakers instead -- Unless you obsessively check the fuses or voltages, you will not know one is blown until your egg battery is kinda depleted (which is certainly not good for the battery) -- ARCBs don't cost much more than a replacement fuse in those sizes...

[Patrick M.]

You're absolutely right Pete (as usual). I thought I was being clever to use a fuse, so that if there were a short the breaker wouldn't be constantly resetting and possibly causing some problem, but the bigger issue is I NEVER check that fuse, so for all I know it IS blown!

Will change that out before the next trip!

[Brian B-P]

After earlier discussions (including Pete's explanations), I chose auto-reset circuit breakers for both the brake controller power supply and the battery charging line. For the brake controller it is essential (you can't plug a new fuse instantly when you need to apply the brakes); for the charge line I would be happy with a manual reset if I had an indication on the dash that the breaker had tripped.

[Brian B-P]

And remember the ground wire (and the hot wire) in the 7-wire trailer connection carries all that current too, and God knows what size of wire that is!

That's a good point: there are different types of 7-wire cable available, and only some of them have heaver-gauge power (black) and neutral (white) conductors. If you want to run electric brakes or charge the battery, it matters. Mine was what the nearest RV store stocks in bulk, and I think it has 10 ga for these conductors (and smaller for the rest).

[Patrick M.]

That's a good point: there are different types of 7-wire cable available, and only some of them have heaver-gauge power (black) and neutral (white) conductors. If you want to run electric brakes or charge the battery, it matters. Mine was what the nearest RV store stocks in bulk, and I think it has 10 ga for these conductors (and smaller for the rest).

Actually I meant the one on the trailer, which has the long run. On the Casita the battery is way back in the boondocks!

I've always had a auto-reset breaker on my brake controller, for the safety issue.

Seems like what we need is some type of wireless device showing current flow to the trailer!

[Herb P]

Actually I meant the one on the trailer, which has the long run. On the Casita the battery is way back in the boondocks!
Seems like what we need is some type of wireless device showing current flow to the trailer!

You want a hall effect current meter. It can detect current flowing through a line. The cheapies are AC only. The more expensive ones will tell you DC.

I just bought some 7 conductor cable from the local hitch place. The white/black were 10ga, the blue was 12ga, and the rest were 14.