I just got a Prodigy P3 controller and am about to install it. I'm pulling a 2 axle trailer with electric brakes. The trailer has the required break away switch and an on-board battery. When I bought the trailer (used) I had to take the plug off because it's different than the connector on my truck. Now I have to wire the new plug. The lights are self explanatory, but what does the trailer's battery hook to? As for the trailer brakes, I'm guessing they have ground and hot wires... the hot wires all connect and go to the wire from the brake controller?
Thank you!

Usually the middle post of the plug.

7 Pole Plughttp://www.wallaceracing.com/wiring_7_pole.jpg

Johnta1 has the right plug pattern, but wonder if it applies? Is your new/used trailer an enclosed or a travel trailer? If it's an enclosed with a smaller maybe motorcycle battery? than from what I can remember, you don't need to run a charge line to it as the small battery can't handle the constant charge without a constan't load. I seem to remember that it could get overcharged and you'll need to replace the batter constantly. If it's a travel trailer, then of course the charge line is needed as the travel trailer will have a car/truck or RV battery that can take a constant charge without burning out. I need to replace my RV batteries every two years anyways. Next set of batteries will be (2) 6-volts. Last longer.

Just my $.02

Anybody know for sure?

The trailer battery will go to the 12V,#4 terminal on the plug. If its the small battery I wouldnt go directly without a resister. All this battery needs is a trickle charge. Trailer places will sell them. The center terminal on your truck would be back up lights if factory.

The center plug is 12V supply; if someone wired backup lights to that, they'd be on all the time.

Anyway, if you use the center plug to charge trailer batteries, how do you prevent the batteries from drawing so much current that it keeps burning the fuse? I'm using a 30amp fuse, and I won't allow any more than that for that size wire.

I started thinking about my trailer hookup.
I think the middle one is for the reverse lights on my trailer.
The connection across from the ground is the 12 volt feed I use for the trailer battery and inside lights.

The center plug is 12V supply; if someone wired backup lights to that, they'd be on all the time.

Anyway, if you use the center plug to charge trailer batteries, how do you prevent the batteries from drawing so much current that it keeps burning the fuse? I'm using a 30amp fuse, and I won't allow any more than that for that size wire.


Center pin is back-up lights, not 12v supply. We usually wire a 20amp thermal reset, (circuit breaker) at the trailer battery.

Oh, you're right, 12V is the black wire on pin #4, sorry.

Did you ever check what the self-resetting breaker does? Like if it keeps engaging and breaking all the time or if it stays engaged most of the time?

Oh, you're right, 12V is the black wire on pin #4, sorry.

Did you ever check what the self-resetting breaker does? Like if it keeps engaging and breaking all the time or if it stays engaged most of the time?


It protects the system, however if the trailer battery is not fully charged, the breaker will remain open and never charge it up. b
Basically it is cheap protection. works well as long as both batteries are good.

It protects the system, however if the trailer battery is not fully charged, the breaker will remain open and never charge it up. b
Basically it is cheap protection. works well as long as both batteries are good.

That seems to be a big disadvantage of electrical brakes. If you don't use the trailer frequently, the battery will be somewhat discharged after some time and the brakes don't work in case the trailer comes off the hitch ...

Wouldn't it make more sense not charge the battery from the vehicle but to use a battery charger instead? You'd have to actually charge it every now and then, that might be more reliable.