Lost wiring Victron DC-DC with 7 wire

[tunnelboy]

Ok, so I have a 16' Bambi. I dropped a LifePO4 battery in the tongue. I also changed out the charging unit in the circuit breaker panel assembly. So I can charge the battery when Bambi is plugged into 120V.

But I was told that if I connect my tow vehicle to the Bambi, and connect the 7 wire connector, I would need a DC-DC charger so I don't blow out my alternator. I bought an isolated DC-DC Victron 18A. I took the two wires from the 7 wire (BK & WH) and connected those to the Victron in. Then ran two cables on the out side and to where the 7 wire used to be. I connected my tow vehicle and did a brake check and all the lights were very dim. Barely visible. I removed the charger and disconnected the battery.

I'm confused on what went wrong here. I'm thinking I should have gotten a NON-Isolated. But I bought this so long ago and can't return it. Can I still use it? If so, how to wire?

Thanks

[BigAl]

Not positive but I think you should have left the white wire intact and just tap into it for the dc-dc input. Then connect white output to ground of trailer. Check on Victron site for wiring diagrams.
Al

[uncle_bob]

Hi

The color codes on the cables going to the 7 pin adapters are not always "standard".

The ground wire from the 7 pin already goes to the trailer ground. It needs to stay there / no benefit to moving it. Just wire the ground for the input and output. of the converter to the trailer chassis ground.

The "hot" wire from the 7 pin goes to an auto-reset breaker. That breaker attaches to the main 12V bus bar. Typically this "stuff" is up in the front of the trailer somewhere. All you need to do is move that wire from the breaker to the DC/DC input.

At that point you will have all 18A trying to run from your TV to the battery.

The gotcha is that a lot of vehicles can't supply that much current to the 7 pin connector. Without knowing what the TV is in this case, there's no way to guess at that part.

Just for reference: No, you won't fry your alternator without the DC/DC. That information is talking about a van installation with giant cables running to the alternator. This is *very* different than what you are doing.

Fun !!!

Bob

[tunnelboy]

So the TV is a 2006 4Runner Ltd v8. If this isn’t needed, is there any point in using it at all? It’s a LiTime 100ah fwiw.

[Bill M.]

I have not done a Li conversion. I read and am thinking about it though. My under standing is that you need the DC-DC converter if you actually want to charge the Li battery while towing. If not care about charging while towing you can just disconnect the charge wire from the 7 pin. That may not be needed if the truck cuts off the charge line to the trailer when the truck is not running.

[Forester2]

Lots of great videos on correct way to hook up Victron dc/dc chargers including correct fusing and breakers. You have the correct charger.

[uncle_bob]

Originally Posted by tunnelboy

So the TV is a 2006 4Runner Ltd v8. If this isn’t needed, is there any point in using it at all? It’s a LiTime 100ah fwiw.

Hi

It's a good bet that the power wire on the 4Runner is a < 15A sort of circuit. An 18A converter will overload the circuit. A lot of these modern vehicles do fun stuff when that happens.

You need the DC/DC in order to charge the lithium battery. Without the DC/DC the lithium will power the 4Runner. The voltage on the Lithium is higher than the voltage the typical auto power system is targeted at.

Fun !!!

Bob

[tunnelboy]

Thanks for all the info. I can't find it now, but when I searched a couple of days ago, it said the factory 7-wire on my TV has a 30 amp relay and 30 amp fuse.

I think maybe the safest thing to do is to install the DC-DC to cover all the bases. So I have the hot and ground (negative) coming in off the 7 wire in the Airstream. I will LEAVE the ground connected to the bus bar and run another ground from the busbar to the input on the Victron, and another wire from the output of the Victron back to the bus bar. Then the hot from the 7 wire will go directly to the Victron in and an out to the location where the 7 wire was.

Does that all sound correct? Since the 7 wire will only be powered when the TV is on, the Victron should not drain any power from the TV when not being used?

So effectively what the Victron is doing is just limiting the amount of amperage being draw from the TV and also not allowing the Lifepo4 to power backwards to the TV?

[uncle_bob]

Originally Posted by tunnelboy

Thanks for all the info. I can't find it now, but when I searched a couple of days ago, it said the factory 7-wire on my TV has a 30 amp relay and 30 amp fuse.

I think maybe the safest thing to do is to install the DC-DC to cover all the bases. So I have the hot and ground (negative) coming in off the 7 wire in the Airstream. I will LEAVE the ground connected to the bus bar and run another ground from the busbar to the input on the Victron, and another wire from the output of the Victron back to the bus bar. Then the hot from the 7 wire will go directly to the Victron in and an out to the location where the 7 wire was.

Does that all sound correct? Since the 7 wire will only be powered when the TV is on, the Victron should not drain any power from the TV when not being used?

So effectively what the Victron is doing is just limiting the amount of amperage being draw from the TV and also not allowing the Lifepo4 to power backwards to the TV?

Hi

If you put 30A through the typical 7 pin wiring you will get about 0 volts at the trailer end of the connection. (ok, maybe it'll be 3 volts). There are a lot of examples of that sort of wiring getting below 8V at 20 amps. (with the alternator at 14.5V). Your DC/DC really would like to see something above 9V on the input.

One interesting factoid:

The resistance of all that wire goes up as it gets hot. We all talk about resistance at 70 degrees. If you are out on a "sunny day" the wiring will be a *lot* hotter than 70 degrees. It also will heat up as you push current through it. Your magic 8V at 20A may well become 5 or 6V at 20 amps in the heat.

Bob