xcntrk

I'm looking for a 12v power source to run my air-bags compressor. The 7-pin trailer harness connection has a switched ignition 12v 30amp source that would be perfect. The compressor requires switched ignition and 15-amps.

But then I got to thinking, what is using this 12V power source when I have my TT hooked up and might that amp draw combined with the compressor amp draw, exceed the 30-amp fuse in the fuse box? Apparently it's supposed to "charge the battery" but what exactly does that mean? Is there a built-in trickle charger somewhere? Obviously my TT has a battery and converter with a built in 12v charger that when plugged into 120v shore power charges the 12v system. But is all that actually running off this 12v source when the trailer is hooked up going down the road? Anybody know if this is a good power source for an airbag compressor?

brulaz

Shouldn't be a problem. If there's no 110V, the TT converter won't be doing anything anyway. But you'll probably want to unplug your trailer anyway to get at the 7 pin.

I've been thinking the same thing for my air compressor, but haven't got as far as you. Is the 12V trailer circuit really 30A? Right now I'm clipping the air compressor directly to the truck's battery, but building an adapter for the 7-pin might make things easier.

xcntrk

Yeah it's definitely 30amp because I had to install that fuse & relay that comes loose in the glovebox (part #AL3T-15A416-AA).

My hope is to splice into this power source at the trailer harness pigtail, so I would actually have both the compressor and the 7-pin trailer connector attached at the same time. That's why I'm questioning how the TT uses the tow vehicle 12v source and for what purpose.

brulaz

When the truck is connected to the trailer and the trailer is not connected to 110V, the TT battery is charged via the truck's alternator.

Not sure what happens when you've got the trailer and truck connected, the truck running and the trailer connected to 110V. Guess both the TT converter and truck's alternator will be putting V to the trailer's battery. Probably not a problem.

I know that the 110V converter and solar panels are often connected to the trailer's battery at the same time without major issues ... but having two competing V sources can screw up battery charging patterns, like one might try to "equalize" at 15.5V while the other is tries to maintain "float" mode at 13.2V.

seaeagle2

If you have electric trailer brakes, the battery charge also charges the backup battery for the breakway switch battery (at least on boat trailers with electric over hydraulic brakes, maybe the breakaway on a travel trailer is powered by the "house" battery)

xcntrk

On my TT the breakaway is definitely on the house battery. On my car-hauler, it's as you describe on your boat trailer with a separate small battery.

brulaz

Hmmmm, I'll have to check if the 12V on the charging line is only there when the trailer brakes are detected. That would complicate things for me, as I wanted to use it without the trailer connected. I have Ford's built in brake controller.

Oh well, maybe extra long jumper cables would be easier.

xcntrk

There is definitely 12v power on that circuit whether it's in use or not. It's ignition switched power. If your pin is not hot, then you're missing the fuse/relay pack I posted above.

smokeywren

The 7-pin RV plug has two wires that provide 12-volt power. One is usually used to charge the battery in the RV. The other is usually not used for anything unless your trailer has backup lights. But the backup light wire is "hot" only when the tranny is in reverse gear.

I don't claim to know much about automotive 12-volt wiring, but I would NOT use the battery charge wire to also power an air compressor. Run a dedicated hot wire from the battery to the compressor, and add a switch so you can switch it on only when you want the compressor to run. You may also want to add a relay to the circuit.

brulaz

I'll have to double-check my fuse, but xcntrk says the 12V charging circuit on the 7-pin outlet has a 30A fuse. My 12V air compressor has a 15A fuse. Sounds like it should work.