Do I need a DC-DC charger?

[uncle_bob]

Hi

The typical wire used on a 7 pin cable is 10 AWG. The same is true of the wire used on the truck to get the power back to the socket. Depending on the truck, it's fused at 30 or 40A. If you try to pull 30A out of it, the voltage at the load ( in the trailer, at the end of the 7 pin cable ...) will be something like 6 to 8 volts. That's not very useful. ( There's a range because the engine RPM's matter ....).

If you drop back to under 20 A, you can get above 10V. That is useful. 10A of charging current will put 50AH into your batteries in a 5 hour driving day. If you are running solar, that comes in on top of this. At 20A, you would get 100AH of charge on the same 5 hour day. Drive 8 hours, you get more.

For us, without any fancy added wires, the DC-DC does a fine job keeping the trailer topped up while we are in motion. No need for generators or anything else.

If you *do* go with custom wiring, either put in a second alternaohtor or do a deep dive into the capabilities of the stock part. They tend to have great big bold print numbers like 375A and a bunch of fine print stuff that turns out to mean you don't get use more than about 30A for your DC-DC....

Bob

[nvestysly]

At the risk of adding to the confusion...

In the system I installed there are two parallel paths for voltage between the tow vehicle and trailer - 1) the new path I added using heavy gauge welding cable and 2) the existing 12V wire via the 7-way connector.

1) The DC to DC unit does not allow back feed to the tow vehicle so there's no issue with reversing the flow of electrons in that path.

2) In my installation (yours may be different) there was a possibility of a higher voltage at the trailer and a lower voltage at the tow vehicle. I decided to install the diode in order to prevent the trailer from charging the tow vehicle.

[foobar]

Originally Posted by nvestysly

At the risk of adding to the confusion...



In the system I installed there are two parallel paths for voltage between the tow vehicle and trailer - 1) the new path I added using heavy gauge welding cable and 2) the existing 12V wire via the 7-way connector.



1) The DC to DC unit does not allow back feed to the tow vehicle so there's no issue with reversing the flow of electrons in that path.



2) In my installation (yours may be different) there was a possibility of a higher voltage at the trailer and a lower voltage at the tow vehicle. I decided to install the diode in order to prevent the trailer from charging the tow vehicle.

If I understand your system description correctly, you have a DC-DC on your parallel path using the welding cable and a diode on your original wiring through the 7-pin connector.

If this is a silicon based diode, the diode will add a 0.7v voltage drop in normal path of current flow (current flowing from the tow vehicle to the trailer) and will block current in the opposite direction.

With the diode in place and the DC-DC active, I bet an ammeter will show zero current through the 7-pin connector due to the combination of diode voltage drop and resistive voltage losses in the wiring leaving the low resistance path through the welding cable plus dc-dc providing a higher voltage of 13.5-14+v inside the trailer.

Ugh, the above description is ugly and hard to follow. Basically what I am saying is that the dc-dc will result in a higher voltage on the trailer side of the diode and a lower voltage on the tow vehicle side of the diode, so the diode will not be conducting. This is what you intended, but you can achieve the same effect by pulling the fuse that feeds the 7-pin power cable out from the tow vehicles fuse block.

[Wazbro]

Originally Posted by nvestysly

At the risk of adding to the confusion...

In the system I installed there are two parallel paths for voltage between the tow vehicle and trailer - 1) the new path I added using heavy gauge welding cable and 2) the existing 12V wire via the 7-way connector.

1) The DC to DC unit does not allow back feed to the tow vehicle so there's no issue with reversing the flow of electrons in that path.

2) In my installation (yours may be different) there was a possibility of a higher voltage at the trailer and a lower voltage at the tow vehicle. I decided to install the diode in order to prevent the trailer from charging the tow vehicle.

It would have been cheaper and more reliable if you just left the 7-pin wire disconnected after you disconnected it to add the diode. Especially since it will be useless to have it connected with the diode for reasons Foobar stated.

[SallyForth]

Originally Posted by Bluetick

I’m replacing my lead-acid batteries with a pair of 100 Ah lithiums. Do I need a DC-DC charger? And how many amps? Can I mount it on the trailer A-frame near the battery box?

I found my solar and new lithium batteries in my Airstream trailer were doing a fine job trying to charge my tow vehicle via the trailer charge wire in the 7-pin trailer umbilical cord while underway. Not what had hoped.

I installed a Victron Orion Tr DC to DC Charger in the Airstream.

The setup criteria of the Victron Orion allowed me to defeat some of the less desirable properties of the smart BCM controlled alternator of my F150.

[wanatrail]

Good question: I too am in the process of replacing AGM with LiFePO4. Adding two (2) 100Ah Ampere Time. I started to think i would 'need' a DC-DC charger, ordered one and now with delivery in a day, I believe I don't really need it at all. I am not going to run a 6AWG wire to the TV battery etc etc nor connect to the 7pin plug either (small wire on that one). Yes it will take longer to charge via a unchanged 7pin. At any rate you need to change out the Converter to a LiFePO4 profile style.$$$.

[Steamy1]

I installed a Renogy 20A DC-DC charger to have additional charging capacity to the LION Energy Safari UT 1300 105Ah batteries (2). I used the existing 10 AWG positive and ground wires in the 7-pin connector.

We spent four days boondocking at Lake Powell, using the pump for two showers every day, lights, Sirius radio, and a little heat in the mornings, cooking, etc. Drew down the LION lithium batteries to 67%.

Drove to Capital Reef, and the batteries were back up to 100%, by charging using the Renogy dc-dc 20a charger while traveling and enabling us to camp again without hookups for several days.

I think a dc-dc charger is handy and just another tool in managing power.

[Bluetick]

One thing I still don't get. Once you run a wire the dc-dc charger, what do you do about the trailer lights and brakes? Do you continue to hook up the 7-pin when you tow?

[richard5933]

Originally Posted by Bluetick

One thing I still don't get. Once you run a wire the dc-dc charger, what do you do about the trailer lights and brakes? Do you continue to hook up the 7-pin when you tow?

Absolutely... You will still need the 7-pin to power everything else and run the brakes. You will be using 6 out of the 7 pins. The new pair of wires are dedicated to charging the batteries in the trailer.

My charging wires end at the bumper with an Anderson connector, and you can see the 7-pin connector just to the right.

On the trailer side, I've wrapped the charging cable alongside the 7-pin so that they both move together, and only the last foot or so they're separated to plug into their receptacles.

[Bluetick]

Originally Posted by richard5933

Absolutely... You will still need the 7-pin to power everything else and run the brakes. You will be using 6 out of the 7 pins. The new pair of wires are dedicated to charging the batteries in the trailer.

My charging wires end at the bumper with an Anderson connector, and you can see the 7-pin connector just to the right.

On the trailer side, I've wrapped the charging cable alongside the 7-pin so that they both move together, and only the last foot or so they're separated to plug into their receptacles. Attachment 426782

Thanks, I think I'll just use zip ties to keep the 7-pin and new dc-dc charger wire together as they enter the trailer. But how do I disconnect only the battery charging circuit on the 7-pin? Is it just a matter of pulling the fuse where the 7-pin connects to the positive terminal of the battery?

[richard5933]

Originally Posted by Bluetick

Thanks, I think I'll just use zip ties to keep the 7-pin and new dc-dc charger wire together as they enter the trailer. But how do I disconnect only the battery charging circuit on the 7-pin? Is it just a matter of pulling the fuse where the 7-pin connects to the positive terminal of the battery?

Not sure how much accessibility you have to the wiring inside front of the trailer, but on mine I disconnected the power feed from the 7-pin cable where it joined the 12v panel, taped the end thoroughly, and called it done.

[FloydCoVa]

Richard, can you tell which wire from this pic I need to disconnect?

[jeffb831]

The trailer connector has 7 wires and there are only 5 connected in your box. The power wire is *usually* the black one but that may not always be the case.

[FloydCoVa]

Exactly

[FloydCoVa]

So maybe the black bypassed the black box and went directly to the always on bus?

[richard5933]

Originally Posted by FloydCoVa

Richard, can you tell which wire from this pic I need to disconnect?

Attachment 426785

I also had a couple of wires which went directly to their connection, bypassing the box.

The wire you want to disconnect is the one which goes from the 7-pin cable to the 12v power system. It might connect in the 12v fuse box, a bus bar, or another similar place. If you have the tow vehicle and the trailer nearby each other, you can plug in the trailer and see which wire carries 12v on it even without the lights, brakes, etc. being on. That's the line you want to disconnect.

[jeffb831]

Originally Posted by FloydCoVa

So maybe the black bypassed the black box and went directly to the always on bus?

That's possible. Trace the 5 wires back towards where they enter the trailer and you should be able to determine where the black (and white) wires are split off.

[Silvr_Bullet]

Is there a dedicated fuse in the tow vehicle for the 12v power supply to the trailer ? Thanks

[Tyler2you]

Originally Posted by FloydCoVa

So maybe the black bypassed the black box and went directly to the always on bus?

Pic below is from the 2015 FC manual. At least it says the wire is black :



https://www.airstream.com/wp-content...manual-147.pdf

[Tyler2you]

Originally Posted by Silvr_Bullet

Is there a dedicated fuse in the tow vehicle for the 12v power supply to the trailer ? Thanks

Yes, that wire will be protected with a fuse in the tow vehicle.