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Old 12-03-2023, 05:12 PM   #1
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2021 27' Globetrotter
Carlisle , Pennsylvania
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How do I know if I need a DC to DC Charger ?

After all the solar and battery upgrades I just finished, thought I was all ready to go, now i'm reading I might also need to add a DC to DC charger to my system.
I installed two GC2 100ah BB lithium batteries, set battery and solar charger correctly. My tow vehicle is a 2018 GMC Sierra 2500HD Duramax, has dual batteries and dual alternators and a 7 pin plug on the bumper I hook up to tow my 2021 GT 27FBQ. Do I really need to add a DC to DC charger or am I ok the way it is ? Seems like this never ends.
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Old 12-03-2023, 06:31 PM   #2
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The core of the issue is that it takes a higher voltage to charge Lithium batteries, than it takes to charge lead-acid batteries, and with the trailer side at a higher voltage than your tow vehicle, you will get unexpected consequences. Most folks will say you should do *something*, it is not mandatory to get a DC-DC charger, but you should do something to protect each side from the other. I went for about a year and a half with simply disconnecting the charge wire in my 7-pin harness, so that was my method to isolate the two from each other. I have since installed a DC-DC charger because I wanted the benefit of restoring batteries on long towing days when solar is sparse. Hope that helps you. There are other threads on this topic, with more details, lots of opinions on what is the best course, but in the end you will have to decide what works best for your needs.
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Old 12-03-2023, 06:37 PM   #3
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I'll do what you did for the time being and unhook it,
Do I disconnect the #4 wire on the 7 way trailer plug for the time being ?
Or just get it over with now and install the DC to DC charger, is there much involved in the install ? My GMC already has a #10 awg wire to the plug.
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Old 12-03-2023, 07:39 PM   #4
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Most everyone with one has an opinion on this. We have 2 ea 100 amp lithium and no solar, we have the all electric refrigerator. I ran 4ga cable from the truck battery through Anderson connectors to a 40amp DC-DC charger in the trailer which is hooked up to the DC distribution buss. The pull was 52amps from the truck battery when the trailer batteries were at 65-70% charge. Our batteries top off within 2 hrs of driving.

My current understanding of those who use the Victron Orion-Tr Smart 12/12 18 amp and their #10 cable in the 7-pin, they experience an on and off charging experience as the #10 cable can't continuously carry the current load from the DC-DC unit.


I'm sure they will chime in.

Steve
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Old 12-03-2023, 07:48 PM   #5
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I think most DC-DC chargers have multiple inputs (at least two). I don't currently have solar but one of the inputs on ours is for solar and we may want to add that later. You already have solar so that should be an easy installation. The other input is from the tow vehicle. 10 gage wire sounds large but it's not very large when considering voltage drop. It may work fine but I suggest you consider a completely separate supply line from your tow vehicle to the trailer.

As others have mentioned there are lots of threads on the subject. Here's the information from our installation.

https://www.airforums.com/forums/f37...le-202849.html

I originally did not include DC-DC converter, instead opting for a diode to solve the potential voltage disparity issue. I ultimately installed the DC-DC converter and glad I did. It was easy to install and leaves me the option of adding solar - rooftop or suitcase.
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Old 12-03-2023, 09:41 PM   #6
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Whether one needs a DC-to-DC charger is related to how the rig is used. If you have solar that can recharge while on the road, then likely DC-to-DC is unnecessary (assuming the sun is shining when you drive). Or, if you usually go from pedestal to pedestal then you also likely can do without DC-to-DC charging.

But, if you want the security of knowing that your trailer batteries will be charging while underway even if traveling under cloudy skies or at night, then a DC-to-DC charger will be your friend. Especially if you often leave a campsite after a night of boondocking and are heading to another boondocking site, being able to charge while underway is important.

Or, if you're like me and kind of OCD about being able to tick off all the boxes and be prepared no matter what, it's not that expensive or difficult to install just in case.
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Old 12-04-2023, 12:37 AM   #7
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Quote:
Originally Posted by bzizzi View Post
I'll do what you did for the time being and unhook it,
Do I disconnect the #4 wire on the 7 way trailer plug for the time being ?
Or just get it over with now and install the DC to DC charger, is there much involved in the install ? My GMC already has a #10 awg wire to the plug.
If you are comfortable in general doing 12vdc electrical work, I think both are straightforward to implement. I think the DC-DC device is best located in the electrical bay of the trailer, somewhere near the 7-way harness, but there are other install options. The hardest part is working in that confined space. I removed one of my dinette bench seats for the work, to make that easier and less risky. I don’t remember for sure the PIN number in the 7-way harness but it’s fairly easy to confirm via some internet research. When I first uncoupled the +12v charge wire, I simply removed that wire from the +12vdc terminal strip where it was connected, wrapped it with electrical tape, and left a label on it. Nothing more required. About 18 months later I installed the DC-DC device as a better long term solution for me. Hope that helps you.
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Old 12-04-2023, 10:47 AM   #8
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Hi

Even with lead acid batteries, charging via the 7 pin connector is iffy at best. The voltage drop in the wiring results in very modest amounts of charge being delivered. This is not something new, it's always been "part of the problem".

Lithiums do make things a little worse because of their higher voltage needs. However it's not a night / day sort of thing. You didn't get much with lead acid and now you get a bit less. With either battery type a DC/DC improves the amount of charging that gets done when towing.

How much charge do you get via 7 pin / lead acid? Typical reports are in the "couple of amps" range. Is that 3A or 5A on average? It's going to depend on a bunch of things. Also there aren't a lot of folks running shunt based monitors on lead acid systems. Thus the amount of accurate data is a bit lacking.

With both lead acid and lithium, there is a chance that your trailer battery will *discharge* into your TV as you run down the road. This is a pretty remote chance with lead acid. With a fully charged lithium setup, it can and does happen. For years and years folks put diodes inline with the charge lead to stop this. The gotcha there is that the voltage drop in the diode reduces the amount of charge you get even further.

As noted above, simply disconnecting the charge wire "solves" the problem. For many, putting in a good DC/DC (.... there often are issues ...) is the answer.

Bob
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Old 12-04-2023, 05:42 PM   #9
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I talked to a EE at Battle Born today to set the record straight, he said while I'm towing very little current will flow from TV to batteries from 7 pin connector, however once vehicle is shut off the current will flow from batteries to the TV. To avoid any issues he told me to simply unplug 7 pin connector if I'm going to leave trailer hooked up to TV for a extended period of time like hour or more without vehicle running.
I guess I could hook charge wire into one of my upfitter switches.to make it simple. Said current will always flow towards the lower voltage. This was right from BB.
He also told me to get rid of the fuse I installed on positive battery terminal. Said BB lithium batteries are internally protected and fuse just creates a bottle neck of resistance.He also
e-mailed me all the correct settings for my Victron MPPT solar charger and IP65 Smart shunt.
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Old 12-05-2023, 07:56 AM   #10
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Quote:
Originally Posted by bzizzi View Post
I talked to a EE at Battle Born today to set the record straight, he said while I'm towing very little current will flow from TV to batteries from 7 pin connector, however once vehicle is shut off the current will flow from batteries to the TV. To avoid any issues he told me to simply unplug 7 pin connector if I'm going to leave trailer hooked up to TV for a extended period of time like hour or more without vehicle running.
I guess I could hook charge wire into one of my upfitter switches.to make it simple. Said current will always flow towards the lower voltage. This was right from BB.
He also told me to get rid of the fuse I installed on positive battery terminal. Said BB lithium batteries are internally protected and fuse just creates a bottle neck of resistance.He also
e-mailed me all the correct settings for my Victron MPPT solar charger and IP65 Smart shunt.
I am considering adding a DC-DC converter/charger. If I do, is it absolutely necessary to break the charging path in the 7 pin? I always disconnect the 7 pin anyway, once parked.

I'm putting together my questions for my call to Battle Born.
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Old 12-05-2023, 08:07 AM   #11
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Quote:
Originally Posted by BGClassic View Post
I am considering adding a DC-DC converter/charger. If I do, is it absolutely necessary to break the charging path in the 7 pin? I always disconnect the 7 pin anyway, once parked.

I'm putting together my questions for my call to Battle Born.
If you're going to run a DC-to-DC with it's own power source (recommended) then you should definitely disable the charging path form the 7-wire harness.

If you are going to use it to power the DC-to-DC charger, then you can set the DC-to-DC to automatically disconnect when the tow vehicle is not running, meaning that you can leave things connected.
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Old 12-05-2023, 08:41 AM   #12
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Quote:
Originally Posted by bzizzi View Post
I talked to a EE at Battle Born today to set the record straight, he said while I'm towing very little current will flow from TV to batteries from 7 pin connector, however once vehicle is shut off the current will flow from batteries to the TV. To avoid any issues he told me to simply unplug 7 pin connector if I'm going to leave trailer hooked up to TV for a extended period of time like hour or more without vehicle running.
I guess I could hook charge wire into one of my upfitter switches.to make it simple. Said current will always flow towards the lower voltage. This was right from BB.
He also told me to get rid of the fuse I installed on positive battery terminal. Said BB lithium batteries are internally protected and fuse just creates a bottle neck of resistance.He also
e-mailed me all the correct settings for my Victron MPPT solar charger and IP65 Smart shunt.
Hi

There are a lot of folks at a lot of companies. You never *really* know who you are talking to.

On a modern truck, the 7 pin power goes through a control module. The function of that module varies with brand, year, and model. Without the details of that module, you really have no idea if you will have discharge back to the truck or not. Does that apply to the next paragraph? Maybe it does.

If your lithium is fully charged at 14.4V and you are running down the road with your alternator putting out 13.4V: What's the higher voltage? The lithium is 1V higher than the alternator. Current flows from the trailer to the truck *while* you are in motion (in this case).

In terms of charging, is getting 5AH into the batteries after driving all day "some charge" ? Sure it is. Is 5AH going to make any difference at all to how long you can do this or that? Probably not.

Leaving things hooked up is not a good idea.

Bob
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Old 12-05-2023, 09:01 AM   #13
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Quote:
Originally Posted by bzizzi View Post
I'll do what you did for the time being and unhook it,
Do I disconnect the #4 wire on the 7 way trailer plug for the time being ?
Or just get it over with now and install the DC to DC charger, is there much involved in the install ? My GMC already has a #10 awg wire to the plug.
I just pulled the fuse for the charging wire on my 2018 Duramax.
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Old 12-05-2023, 09:01 AM   #14
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Quote:
Originally Posted by bzizzi View Post
He also told me to get rid of the fuse I installed on positive battery terminal. Said BB lithium batteries are internally protected and fuse just creates a bottle neck of resistance.
I have never heard this before. A fuse does not restrict the current flow, it only prevents it from exceeding it's rating when it melts. There may be a tiny bit off added resistance due to the additional connection point. I am very skeptical of this advice.
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Old 12-05-2023, 11:46 AM   #15
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Originally Posted by richard5933 View Post
If you're going to run a DC-to-DC with it's own power source (recommended) then you should definitely disable the charging path form the 7-wire harness.

If you are going to use it to power the DC-to-DC charger, then you can set the DC-to-DC to automatically disconnect when the tow vehicle is not running, meaning that you can leave things connected.
Thanks. I don't want to have another problem crop up, like a poster here claimed when he disabled that wire. He said the tail lights stopped working, or something to that effect. That wire should be the "7-way charge", and black if the AS drawings are correct.
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Old 12-05-2023, 03:30 PM   #16
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Here's the easy answer - in all cases, definitely install a DC-DC charger.

https://www.amazon.com/Renogy-Batter.../dp/B07Q5VYPCF

This Renogy 20A unit is able to be set to operate at 10A. That is the perfect rate that should be compatible with most every car and within the 7-pin power limits.

It doesn't really matter if you have lead-acid, lithium, AGM, a DC-DC charger will make a big difference. It can be set to charge any chemistry. It'll isolate and protect the car electrical system, and keep the trailer batteries from discharging because of voltage differences. More importantly, it will consistently deliver a solid 10 amps to the trailer, regardless of long wires or resistance to keep the batteries topped off.

Should have installed one long ago even with lead-acids, but with lithium's, there are more reasons to install one.
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Old 12-05-2023, 03:34 PM   #17
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Originally Posted by thewarden View Post
I just pulled the fuse for the charging wire on my 2018 Duramax.
That's even better
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Old 12-05-2023, 04:35 PM   #18
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Originally Posted by uncle_bob View Post
Hi

There are a lot of folks at a lot of companies. You never *really* know who you are talking to.

On a modern truck, the 7 pin power goes through a control module. The function of that module varies with brand, year, and model. Without the details of that module, you really have no idea if you will have discharge back to the truck or not. Does that apply to the next paragraph? Maybe it does.

If your lithium is fully charged at 14.4V and you are running down the road with your alternator putting out 13.4V: What's the higher voltage? The lithium is 1V higher than the alternator. Current flows from the trailer to the truck *while* you are in motion (in this case).

In terms of charging, is getting 5AH into the batteries after driving all day "some charge" ? Sure it is. Is 5AH going to make any difference at all to how long you can do this or that? Probably not.

Leaving things hooked up is not a good idea.

Bob
I ended up pulling the 30A trailer battery fuse on my 2018 GMC Sierra 2500 Duramax, now hopefully I don't have to think about it anymore
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Old 12-05-2023, 10:33 PM   #19
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Quote:
Originally Posted by BGClassic View Post
Thanks. I don't want to have another problem crop up, like a poster here claimed when he disabled that wire. He said the tail lights stopped working, or something to that effect. That wire should be the "7-way charge", and black if the AS drawings are correct.
Properly installed, the DC-to-DC charger will be replacing the function of the charge wire in the 7-wire harness. It should have no impact on the function of the tail lights or anything else related to the 7-wire harness.

Quote:
Originally Posted by pteck View Post
Here's the easy answer - in all cases, definitely install a DC-DC charger.

https://www.amazon.com/Renogy-Batter.../dp/B07Q5VYPCF

This Renogy 20A unit is able to be set to operate at 10A. That is the perfect rate that should be compatible with most every car and within the 7-pin power limits.

It doesn't really matter if you have lead-acid, lithium, AGM, a DC-DC charger will make a big difference. It can be set to charge any chemistry. It'll isolate and protect the car electrical system, and keep the trailer batteries from discharging because of voltage differences. More importantly, it will consistently deliver a solid 10 amps to the trailer, regardless of long wires or resistance to keep the batteries topped off.

Should have installed one long ago even with lead-acids, but with lithium's, there are more reasons to install one.
Agree. There is no downside to having a properly-installed DC-to-DC charger in the setup, and lots of potential gain. It doesn't have to be that difficult or expensive to get one installed if you're able to do simple wiring yourself.
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Old 12-06-2023, 06:17 AM   #20
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So, I get that, with my TV having a lead-acid battery and an alternator, and with my camper having a pair of lead-acid batteries, I didn't worry about it, I just hooked up, drove, unhooked at camp, etc. My batteries would have gotten some charge, maybe not the most efficiently, but, not a risk.

Swapping the camper over to a lithium setup, I understand that the 'radically different' charging profile for a lithium battery means that I really wouldn't want to leave the 7-pin plugged in over a long period of time while sitting parked somewhere. I understand that a DC-DC converter will allow for a better charging profile for the lithium battery bank, and that to maximize the effectiveness, it's best to run a dedicated line from the vehicle battery to the rear bumper next to the 7-pin connector, then use an Anderson connector to have the line from there on in to the DC-DC to 'do it right'.

I suppose my question is, what if, you've reasons to not modify the TV? Maybe you rent a vehicle to tow with, or, are constantly trading vehicles, or maybe you have access to a fleet of vehicles to use, whatever... But, you're willing to install a DC-DC in the camper to help out anyway, even knowing you'll be at 10a or so instead of 50a or whatever: should you wire the live wire in the 7pin of the camper to go to the DC-DC? Or is that a bad idea?

But if you do install a dedicated line: do you 'need' to then disable the live wire in the 7pin, so it's not trying to also send to the camper batteries via a different route than the dedicated line? Or, is it okay to leave the 7pin stock, and simply make sure you don't leave it plugged in when parked for more than a gas-stop?

I'm interested in having a DC-DC to do 'better' than not having one, but I'm not as interested in running that dedicated line. I know it'd be 'best' to do such, but, if a DC-DC would still be an improvement without, I might be okay w/ that. (But if the DC-DC gain w/o a dedicated line is 'almost not worth it' without a dedicated line, maybe I'll need to more seriously consider such.)

Thoughts? Commentary, further discussion, ideas, etc., are welcomed...
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