battery charging question

[Jon Vermilye]

Originally Posted by Raz

I guess I'm missing something here. If there is no continuity between the coil wires on each brake magnet and the magnet housing, how will the return wire become part of the circuit.

Because most tow vehicles use the frame as a ground, or at the least, tie the negative wiring to the frame. Your hitch "connects" the trailer frame to the tow vehicle frame. The brake negative from the tow vehicle is tied to the frame, if not at the brake connections, at the trailer hitch through the tow vehicle's frame.

For the breakaway switch to work, the converter ground must be tied to the trailer brake ground, which, if no where else, is tied to the trailer frame through the tow vehicle frame (again, at the hitch).

If the only connection between the trailer brake negative is through a wire you ran from the converter, it should be heavy enough to carry any battery ground fault current. This is why Escape, along with most trailer manufacturers add a heavy (#6 or #4 wire between the battery negative & the frame of the trailer). It protects the rest of the negative wiring wiring from carrying a large fault current.

Escape, like most fiberglass trailer manufacturers does not use the frame as a current carrying wire except under fault conditions. They run wire pairs to fixtures & appliances.

[Raz]

First, I didn't wire it, the manufacturer did. Next, you said

"If the only connection between the trailer brake negative is through a wire you ran from the converter, it should be heavy enough to carry any battery ground fault current". The brake magnets are wired no different than a light bulb. A light bulb wouldn't carry a fault. There is no connection between the brake wires and the frame at the brakes.

[Jon Vermilye]

Originally Posted by Raz

First, I didn't wire it, the manufacturer did. Next, you said

"If the only connection between the trailer brake negative is through a wire you ran from the converter, it should be heavy enough to carry any battery ground fault current". The brake magnets are wired no different than a light bulb. A light bulb wouldn't carry a fault. There is no connection between the brake wires and the frame at the brakes.

Unless a pair of diodes are used to isolate the two grounds, can you explain how both the trailer & the converter can supply a negative connection to the brake magnet without being tied together?

If the tow vehicle braking system is tied to the tow vehicle frame (and most are) then through the trailer hitch, the trailer frame is tied to the tow vehicle frame, and the negative wire.

Try using a multimeter on the ohm scale & measure between the trailer's negative brake connection & the trailer frame with the tow vehicle connected.

[Raz]

Originally Posted by Jon Vermilye

Unless a pair of diodes are used to isolate the two grounds, can you explain how both the trailer & the converter can supply a negative connection to the brake magnet without being tied together?

They are tied together.

If the tow vehicle braking system is tied to the tow vehicle frame (and most are) then through the trailer hitch, the trailer frame is tied to the tow vehicle frame, and the negative wire.

Again I agree. Trillium wired a common point where all return wires are connected. The negative battery connection and the charge line go there aswell. But if the brake magnet is not connected to the frame at the brakes, why would it carry a fault current?

[Raspy]

While it's true that the hitch can be what grounds the trailer to the TV, and that it can carry some current, it's a very bad connection that should never be counted on to do any work, or be reliable. With any grounding problem searches, look to this point first, by disconnecting the hitch and seeing if the continuity disappears. At best, it's a poor and variable connection.

Oliver had a series of their trailers with missing ground straps, where charging and the brakes then defaulted to the hitch as the ground, or negative conductor. This led to charging problems and TV warnings that something was wrong.

The easiest first way to check for a proper continuous ground, back to the TV, is to disconnect the hitch and see if the batteries are charging. Do this with the 7 pin plugged in to the TV, and with the TV running.

The negative wire from the TV, through the 7 pin, must be grounded somewhere on the trailer and be continuous to the trailer battery. Or go to a negative buss with a connector to the battery and the trailer frame. The trailer brakes must be grounded to this same point, or to a solid frame lug, to prevent braking current from having to conduct through the suspension system.

This assures that brake current from the TV can complete the circuit back to the TV, that the battery can charge, and that the TV will not throw a code because of a bad connection to the trailer.

[Raspy]

Originally Posted by John in Santa Cruz

both my Casita and my Escape have the chassis grounded to both the 120VAC ground wire, and the battery negative.

Jon, I've wondered about this for some time. Would it be better to have the 120 Volt ground be continuous back the the pedestal and beyond, but not continuous with the 12V negative? If there was a discontinuous ground back the the pedestal, and a shorted hot, it would heat up the whole trailer and TV.

What is the best grounding method for the 120 volt side?

[Jon Vermilye]

What I was trying to point out (apparently not too well) is that whether or you want it or not, the trailer brake negative is connected to the frame of the trailer. During a fault such as a wrench between the + battery connection and the frame (through the propane tanks, tongue jack or other trailer part connected to the frame) while removing the battery will cause considerable current to flow through the negative brake conductor. If the battery negative is properly connected to the frame with a large enough wire, that will limit the current through the brake connection, but with hundreds of amps flowing, a typically sized brake negative could be damaged.

As to grounding or bonding, NEC requires the 120V ground be bonded to the trailer frame, as well as the case of a converter. This is to protect the trailer user from shocks between the trailer hardware & the earth.

If you do not bond the negative 12 volt side of the battery & converter to the frame, you run the chance of the only connection between the battery & frame is the trailer brake negative, typically a #14 wire. This would not survive long carrying the fault current of the slipped wrench.

As I mentioned earlier, after a slipped wrench fault, it would be wise to check the condition of the trailer brake negative.

[Raspy]

Jon,

That is a good point I had not thought of before. The trailer brake grounding wire being overloaded by a short. But I still don't see how that could happen. 12V + comes from the TV brake controller, to the magnet, and then on to the trailer frame for a ground, or 12 -. Then, the frame is grounded to the TV negative through the 7 pin. But the trailer frame cannot ground through the brake ground in any way that I can see. A shorted wrench from the battery + to the trailer frame would complete the circuit through the frame to battery Negative conductor, not the brake negative. Isn't that right?

[Jon Vermilye]

The path is from the wrench to the frame to the - brake wire (through the connection at either pin 7 or at the converter ground tie) to the - battery post. Again, there should be parallel connections between the battery & frame that will share the fault current, but some don't believe in grounding the battery to the frame, in which case the brake negative might be the only path between the frame & battery.

[Raspy]

Jon,

Isn't the brake ground really just the magnet coil negative? It provides no path from the frame to the trailer battery negative that I can see. The brakes use the frame as a conductor to ground, but the frame can't use the magnet coil as a ground conductor, as that circuit only goes to the Brake Controller +. A short from the + terminal to the trailer frame would complete through frame grounding wires, not backward through the magnet. If the house battery is grounded to the trailer frame, and it must be to operate, that would be the path of a shorted wrench. What am I missing?

[Jon Vermilye]

Originally Posted by Raspy

Jon,

Isn't the brake ground really just the magnet coil negative? It provides no path from the frame to the trailer battery negative that I can see. The brakes use the frame as a conductor to ground, but the frame can't use the magnet coil as a ground conductor, as that circuit only goes to the Brake Controller +. A short from the + terminal to the trailer frame would complete through frame grounding wires, not backward through the magnet. If the house battery is grounded to the trailer frame, and it must be to operate, that would be the path of a shorted wrench. What am I missing?

Yes, you are correct. I sketched it out, and with the way fiberglass trailers interconnect the tow & trailer grounds, it is unlikely that the brake negative from the converter or 7 pin charge line would be damaged.

I was using the typical frame based brake connection typical of some stick built trailers & utility trailers where the negative brake magnet wiring is connected to the frame at the wheel.

Sorry for the confusion...

[Raspy]

Jon,

Thank you very much for following through on this and clearing it up.