How do I determine if DC-DC charger is necessary?

[uncle_bob]

Originally Posted by kmrice

Thanks, as always, for the helpful replies. I’m just looking to get some charging on lithium batteries while towing. I’m thinking the DC/DC would fit in the compartment where my solar converter is stored on my 2022 GT 23FBT (under the front of the right bed).

Hi

Since the wire size does not change as you place the DC / DC here or there, the only issue is protection from the weather. You can "hit" the 12V bus (that goes to the batteries) from a lot of places in the trailer. Any of them will work ok for the sort of DC/DC you feed on a 7 pin connection.

Bob

[Steamy1]

Bob is correct; installing the DC-DC charger between the 7-way pigtail and the positive and negative buss in the trailer is the easiest method. Mine is installed under the bed in our 23FB, next to the invertor and busses, along with the smart shunt for the monitor.

Attached is a PDF of a wiring diagram I made up to help me keep everything straight.

[Projet2907]

Originally Posted by Rolind

I have been reading up on this for the last several weeks as I am considering swapping my AGMs for battleborn GC2s. As near as I can tell, there are three issues; alternator protection, current moving from the trailer to the TV battery and adequate charging from the tow vehicle. There is information here and on BBs and victrons website that seems to suggest that
1). alternator protection with no more than three lithium batteries is not necessary
2). if a battery isolator or dc to dc charger is not used, there may be a small drain of the trailer batteries to the TV that is unlikely to be significant
3). If you really want good charging from the tow vehicle, you need a dc to dc charger and a more robust wire run from the alternator.
I would call or email BB and victron and see what they recommend. Pls keep us updated on their recommendations, I am very interested in what they say.

Also I know there are folks here that are much smarter than I on this issue, so pls chime in.

What does BB means?

[Bill M.]

Battle Born

[masternate]

Originally Posted by Steamy1

Bob is correct; installing the DC-DC charger between the 7-way pigtail and the positive and negative buss in the trailer is the easiest method. Mine is installed under the bed in our 23FB, next to the invertor and busses, along with the smart shunt for the monitor.

Attached is a PDF of a wiring diagram I made up to help me keep everything straight.

Careful!

After researching this topic a little too much, I finally took the plunge and installed a DC charger. Running it off of the 7 pin is sketchy at best. After spending so much time to assemble the information, I decided to post a video about it. Hopefully it helps some here.

Plan / Diagrams: https://youtu.be/uONgZkrzkhY
Installation: https://youtu.be/OdZdg4KZ_5M

[pond]

Originally Posted by Steamy1

Bob is correct; installing the DC-DC charger between the 7-way pigtail and the positive and negative buss in the trailer is the easiest method. Mine is installed under the bed in our 23FB, next to the invertor and busses, along with the smart shunt for the monitor.

Attached is a PDF of a wiring diagram I made up to help me keep everything straight.

Ok...this drawing puts this thread together for me. Nice. I've read the thread half a dozen times plus YouTubing it and I think I've got it.

The 712 Vitron and 20amp Dc-Dc 20 amp Renogy arrived today. I only have a portable solar panels.

The question I still have is do I install manual breaker if I'm getting my power from the running lights, since it's always on running the rear-view camera. Does the Dc-Dc stop drawing when the lights go off and when the trailer lights connector is dis-connected?

Thanks if anyone is still monitoring this thread.

Mike

[Castaway]

Please correct me if I am wrong but this is what I think I understand about DC-DC chargers.
1. If I have my Li-Ion batteries fully charged before traveling the batteries will not be drawing power from the TV alternator

2. I should unplug the 7 pin connector from the TV when parked if I do not have a DC-DC charger.

3. I do not need a DC-DC charger if I only have 1 or 2 Li-Ion batteries.

Is this correct?

Also what size alternator is needed to charge my Li-Ion's with no DC-DC charger?

[richard5933]

Originally Posted by Castaway

Please correct me if I am wrong but this is what I think I understand about DC-DC chargers.
1. If I have my Li-Ion batteries fully charged before traveling the batteries will not be drawing power from the TV alternator

2. I should unplug the 7 pin connector from the TV when parked if I do not have a DC-DC charger.

3. I do not need a DC-DC charger if I only have 1 or 2 Li-Ion batteries.

Is this correct?

Also what size alternator is needed to charge my Li-Ion's with no DC-DC charger?

You could be correct, but it all depends on the particulars of your setup.

1. If they are fully charged they might not be pulling a charge, but depending on your TV's wiring the TV might be pulling a charge from your trailer's lithium batteries and depleting them. The lithium batteries at full charge will typically have a higher voltage than a tow vehicle battery, which could trigger your tow vehicle's alternator to not do anything and just let the trailer's batteries do the charging.

Also, depending on what you have running in the trailer you could deplete your batteries somewhat on an extended day of driving. If this was followed by a night off-grid you could wind up without enough reserve to make it through the night.

2. This only applies if your tow vehicle keeps the connection open when the key is turned off.

3. The number of lithium batteries is not the issue. The issue is whether or not the batteries you have will be charged adequately when you arrive at your destination to make it through the night (or however long til you can charge again.) This is a simple math equation - weigh the batteries' reserve against the predicted need and see where you stand. Add to the equation whatever charge you get from your 7-pin connection, if any.


Your last question - about the size of the alternator - looks like a trick question. The answer is that likely the limiting factor is not the alternator. The limiting factor in most tow vehicles is the wiring/circuit that supplies power to the 7-pin connector. It night only have a 20-amp fuse. Even if it has a 30-amp fuse, unless the wiring is adequately sized it will never be able to actually deliver that much at the 7-pin. To know how much charge you can expect from your 7-pin you'll need to explore the specs on your TV.

[uncle_bob]

Originally Posted by pond

Ok...this drawing puts this thread together for me. Nice. I've read the thread half a dozen times plus YouTubing it and I think I've got it.

The 712 Vitron and 20amp Dc-Dc 20 amp Renogy arrived today. I only have a portable solar panels.

The question I still have is do I install manual breaker if I'm getting my power from the running lights, since it's always on running the rear-view camera. Does the Dc-Dc stop drawing when the lights go off and when the trailer lights connector is dis-connected?

Thanks if anyone is still monitoring this thread.

Mike

Hi

The running lights are a completely different wire on the 7 pin than the charge wire. It is not impacted by the DC/DC. The wire to the DC/DC normally only goes to the batteries via the main DC bus bar arrangement.

Bob

[uncle_bob]

Originally Posted by Castaway

Please correct me if I am wrong but this is what I think I understand about DC-DC chargers.
1. If I have my Li-Ion batteries fully charged before traveling the batteries will not be drawing power from the TV alternator

2. I should unplug the 7 pin connector from the TV when parked if I do not have a DC-DC charger.

3. I do not need a DC-DC charger if I only have 1 or 2 Li-Ion batteries.

Is this correct?

Also what size alternator is needed to charge my Li-Ion's with no DC-DC charger?

Hi

1) => typically this is true. As noted above, you need to have zero loads pulling current in the trailer for this to be correct.

2) => Unplug is different than disconnect the charge wire. Pulling the charge wire off of the DC bus in the trailer is the alternative to having a DC/DC.

3) => You don't *need* much of anything. If your decision is to charge batteries while in motion *and* you have lithiums, you need a DC/DC. Since a smaller battery bank will run down faster than a big one, you probably will find a DC/DC more useful on a small bank than on a large one.

Alternator: If you are just pulling 10A into a DC/DC the stock alternator likely is fine. If you go for something fancy, a 400A alternator may not be big enough. It very much depends on how crazy you get.

Bob

[Castaway]

Originally Posted by Bill M.

A dc-dc converter would be a great thing to have. Seems like a small additional cost. But I doubt if necessary in that you can disconnect the charge wire on the 7 pin if you would rather. That will isolate the TV and the trailer.

Which wire would that be, and would disconnect from the busbar in trailer?

Gary

[kscherzi]

As someone earlier noted, we don't know what the electronics within the truck would do if confronted with the higher voltage of charged lithium going backwards through the 7 pin connector. Some may stop it, some may let it through until the truck battery is cooked, in some the truck electronics may burn out, and sometimes it just may do nothing.

A DC to DC converter eliminates those unknowns, as does permanently disconnecting the power terminal from the 7 pin.

[jeffb831]

Originally Posted by Castaway

Which wire would that be, and would disconnect from the busbar in trailer?

Gary

You don't need to physically disconnect any wires. All you need to do is identify the fuse in the TV that is on the circuit for the trailer power and remove that.

[uncle_bob]

Originally Posted by Castaway

Which wire would that be, and would disconnect from the busbar in trailer?

Gary

Hi

The best and safest approach is to trace out the wires from the in trailer end of the 7 pin cable as they go to the DC bus bar assembly in the front of the trailer. There is only one wire that goes from the 7 pin to this assembly. Since color codes seem to be a bit random, identifying it by color is never 100%.

Pulling this or that fuse *might* work or it might not. Some trucks fuse the input to a magic module and let that fuse deal with a number of things all at the same time. Other trucks put in relays or fuses here or there. Without a full wiring diagram for the truck, there is no way to be sure of what this or that brand did on this or that model in this or that year and with this or that option package installed.

Lots of fun

Bob

[pond]

Originally Posted by uncle_bob

Hi

The running lights are a completely different wire on the 7 pin than the charge wire. It is not impacted by the DC/DC. The wire to the DC/DC normally only goes to the batteries via the main DC bus bar arrangement.

Bob

Sorry I need such detailed answers.

To the input side of the DC/DC charger the wires can come from the negative and positive busses instead of splicing into the charge wire and ground wire from the TV 7-way pin?

Mike

[richard5933]

Originally Posted by pond

Sorry I need such detailed answers.

To the input side of the DC/DC charger the wires can come from the negative and positive busses instead of splicing into the charge wire and ground wire from the TV 7-way pin?

Mike

Not really.

The DC-to-DC charger takes power from the tow vehicle and converts it to the proper charging voltage for the trailer's batteries. The input side of the DC-to-DC charger must connect to the tow vehicle - both the negative and positive connections.

Many people use the 12v+ from the 7-wire cable to get the power from the tow vehicle to power the input side of the DC-to-DC charger. To use this wire to power the DC-to-DC charger it's necessary to pull it from it's usual connection in trailer and connect it directly to the input side of the DC-to-DC. You're not really 'splicing into it', but rather you're relocating it from its connection point in the trailer to the input of the DC-to-DC charger.

The output side of the DC-to-DC charger provides the proper voltage to the trailer so the batteries can receive a proper charge. It is often connected directly to the 12v+ bus bar in the trailer's wiring.

The 12v negative wire from the 7-wire harness needs to connect to both the negative input on the DC-to-DC charger and to the trailer's negative bus bar.

[uncle_bob]

Originally Posted by pond

Sorry I need such detailed answers.

To the input side of the DC/DC charger the wires can come from the negative and positive busses instead of splicing into the charge wire and ground wire from the TV 7-way pin?

Mike

Hi

The DC / DC stuff is in no way AS specific. If it's freaking you out, having a local RV tech drop it in is perfectly legit. As long as he doesn't have to dig to get to the DC bus or go crazy mounting things ( you pay by the hour ....), it should not be a crazy expensive thing to hav them do.

Bob

[pond]

Originally Posted by uncle_bob

Hi

The DC / DC stuff is in no way AS specific. If it's freaking you out, having a local RV tech drop it in is perfectly legit. As long as he doesn't have to dig to get to the DC bus or go crazy mounting things ( you pay by the hour ....), it should not be a crazy expensive thing to hav them do.

Bob

HaHaHa...freaking-out.

Richard5933's explanation nailed it, thanks Richard5933. I'm into the weeds and things are going fine.

Great thread, thanks,


Mike

[kmrice]

Steamy1, thanks so much for the outstanding wiring diagram. It does an excellent job of helping those with limited experience in electronics (like me) understand things.

If you are still following this thread, I have a few questions:

How does the Jack connect to the DC negative? I only see a cable to the positive terminal of the battery.

Why does the Jack not run through the Shunt?

What is the B+ Shunt Wire running from the shunt to the positive DC Buss Bar? Is that how the Shunt gets power to run itself?

Thanks again for your great contribution to this very helpful thread.

Karl

[uncle_bob]

Originally Posted by kmrice

Steamy1, thanks so much for the outstanding wiring diagram. It does an excellent job of helping those with limited experience in electronics (like me) understand things.

If you are still following this thread, I have a few questions:

How does the Jack connect to the DC negative? I only see a cable to the positive terminal of the battery.

Why does the Jack not run through the Shunt?

What is the B+ Shunt Wire running from the shunt to the positive DC Buss Bar? Is that how the Shunt gets power to run itself?

Thanks again for your great contribution to this very helpful thread.

Karl

Hi

The jack typically grounds to the frame of the trailer. The frame is always connected to DC negative. There are a number of "jumper points" in the trailer that do this.

The shunt needs a 12V power source. You typically run it straight to the battery so it can read out exact battery voltage.

Bob