DC/DC Renogy Trailer Install

[alfaeric]

I thought we were talking 12g. Wasn't the question around 10g or 12g? Not sure how 16g got into the question....

If you need a .1ohm resistance to get 2.5v drop at 25A, and the resistance is 1.6ohms/1000ft, I get 62.5 ft to get 0.1ohms.

Again, it's just simple math for the wire. Perhaps you have other issues. Connectors add resistance, as do poor crimps.

 

[Short_Shot]

I thought we were talking 12g. Wasn't the question around 10g or 12g? Not sure how 16g got into the question....

If you need a .1ohm resistance to get 2.5v drop at 25A, and the resistance is 1.6ohms/1000ft, I get 62.5 ft to get 0.1ohms.

Again, it's just simple math for the wire. Perhaps you have other issues. Connectors add resistance, as do poor crimps.

Because I posted actual voltage drop with 16 awg wire using an actual dc to dc charger.

There's photos of all my measurements and the setup if you'd prefer to critique it.

No connectors involved. Tinned, stranded oxygen free copper wire. A bit over 18% voltage drop with 10 amps over ~25 feet.

And it's close enough for practical application to what all those calculators return. Calc says just under 17%, but as stated you need to add in the terminals at the devices and whatnot, so I'm calling it good enough.

So is the calculator wrong or are you?

 

[Short_Shot]

So I'm legitimately trying to figure out if you're wrong or if I'm wrong and my little experiment just so happened to also be wrong in exactly the same way.

So here's a bunch of calculators returning values on both sides of my result, within 2% or so.

I've been wrong before, and I'd like to correct my error if that's the case here, but it seems pretty much everything lines up with my results within a margin of error that's mostly irrelevant for this application.

 

[alfaeric]

FWIW, I'm not using one of the calculators.

I'm using V= IR.

We know V is 2.5V
We know I is 25A

So then R = 0.1 Ohms.

Then I am looking up the standard resistance for wire gage, and I'm seeing 1.6ohms/1000ft for copper 12g. Another chart has it a 0.00521 ohms/m, which is 19m to get 0.1ohms.

16g is 0.0132 ohm/m, so you need 7.6m to get 0.1 ohms.

When I looked on one of the calculators, they include a factor of safety- I'm just using the raw numbers.

Maybe measure the resistance with your meter.

 

[Short_Shot]

I know what your doing.

I've had, and aced, plenty of tests on exactly that.

And yet my results in practice often match closely with the calculators which all seem to disagree with you.

That said, I never did understand the discrepancy, and since it's been brought up again now I'm on the subject and it'll annoy me until I'm passed out drunk in my camper later.

Impedance is a thing but it's dc, so that's out. Temperature coefficient perhaps, as the wire was about 20c above ambient after a while.

_______?

 

[alfaeric]

I know what your doing.

I've had, and aced, plenty of tests on exactly that.

And yet my results in practice often match closely with the calculators which all seem to disagree with you.

Cool beans.

Good luck with that.

 

[Short_Shot]

So you can't explain why my documented results match(ish) all the calculators, but disagree with you, so... that's it? Just give up?

 

[AndyRonLI]

On the subject of fuses, a clarification.

Note that my F350 has a 25 amp fuse labeled battery charge that has been confirmed to be inline with the battery charge pin of the 7 pin connector.

The 40 amp fuse has yet to be investigated. There seemed to be some confusion about this.

I too find it difficult to envision Ford using a 25 amp fuse on wiring that cant handle 25 amps. SAE does have standards. Ford does not want hundreds of nuisance complaints because that fuse keeps popping and they dont want things catching on fire. A 25 amp fuse would handle most trailer loads using SLA batteries., but would likely pop before wire damage if you had a somewhat discharged Lithium battery. Thus the DC/DC converter requirement

So the question is if the 25amp fuse is likely to be oversized for the wiring. Not is the 40 amp fuse oversized (clearly it would be!)
Not the 30 amp fuses on the lights!

 

[Short_Shot]

I would find it difficult if I hadn't seen it in several vehicles lol

 

[AndyRonLI]

I would find it difficult if I hadn't seen it in several vehicles lol

Could you be specific. Have you actually seem fires/ fuses blow/ real issues with undersized wiring etc? I am not a mechanic, but have worked on a few vehicles and certainly have seen some crispy wires, but generally in old cars and in the engine compartment. And never actually seen a failure. Would think if this were a habit with Detroit (or anyone else for that matter) I would have heard of some recalls for wiring harnesses. Have heard of a few for wire harness chafe but never actual wire harness wires.
Just curious.

 

[Short_Shot]

That's not at all what I said. At what point did I say any of that??

I said I factually have many times seen factory installed trailer wiring undersized relative to the fuse they install.

As I've said, oh idk, 500 times, that's not a problem with a lead acid battery at the other end, and as the other guy pointed out its usually not a big issue with a lithium either because the voltage drop causes a drop in current draw at the battery. Kind of hard to pull 20 amps if the incoming power is only 0.1v over the battery and whatnot.

Typically when connecting my trailer with a low *lead acid* battery the current rarely exceeds ~8a and very rapidly tapers off to almost nothing as the battery charges because it takes very little charge to surpass the low voltage it's supplying.

I said it's an issue when a device like a dc to dc charger is installed which will force current regardless of voltage drop.

This is NOT what the circuit is designed for and of course its easy to damage things when operating outside of design intent.

 

[AndyRonLI]

So, intended load is not the only threat to wiring. Chafe, fender bender damage, can all set up a scenario where there is current through a wire that exceeds its capacity. So if I have a short that exceeds the wire but not the fuse, I can have a fire. As there is no "specified allowable load" on that battery charger circuit., according to the manual, a 25amp fuse should indicate a 20 amp load is acceptable. You seem to be implying that the engineers knowingly used an undersized wire. I do think you are using an NEC mindset rather than an SAE or AIAA mindset. SAE in particular allows for more voltage drop on DC circuits than NEC does. The Saftey requirements for a house I sleep in at night are reasonably higher than the safety requirements for a car I drive in when I am awake. Aircraft voltages are much higher and even a the same percentages the drop is high as well. Weight and Space trade on voltage drop.

 

[Short_Shot]

I don't think they did so.

I know they did so. I own an example of it, and owned two prior examples of it.

It's still fused. A short even on a 16 awg will pop a 40a fuse before it burns the wire up, but a 35 amp load won't.

You know. Because it's not intended for that.

That wire is attached to an auxiliary stud intended for more than one application, at least in my GMs.

Stupid? For our application here, yes.

For the average user towing a cargo trailer? Not an issue.

 

[AndyRonLI]

So what was the design load is really the question. Any modern travel trailer will have a slide out or two, and some sort of Jacks.
On my rig, and I suspect on most, the last gasp gotta get out of dodge power source is the tow vehicle alternator. Mine slides pull in the mid teens, not sure of the jacks. So designing for a 20 ish load, should be a target. Would be nice if along with all the weight ratings, there was a power rating for tow vehicles as well.

 

[AndyRonLI]

So just a status.
When starting with a fully charged battery, the 20Amp DC/DC with current limiting (effectively making it a 10 Amp unit) can almost keep up with the refrigerator load, plus losses. A 12 hour drive sees me down about 10 Ah on a pair of 200 Ah Ampere Time batteries. The voltage drop is significant. I am showing about 10.8 at the charger. No sign of overheating. But definitely will be upgrading the tow vehicle wiring. to handle a full 40 amps. I could stay even with 20, but if I am off power overnight, that silly residential fridge sucks about 40 Ah. I would like to see the system have enough to keep the fridge running, and have some current to spare such that the batteries increase their charge, rather than decrease it. This new target is so that the Air Conditioning is not competing with the AC/DC charger when I am on generator, My Honda 3000 will run at least the 8000 btu unit but not when the the charger is pushing 40 Amps into the battery. I think thats only 6ish amps AC but it breaks the bank on the generator. The 3000 is good for about 23 - 25 amps, Going to see if a softstart kit helps with the AC problem,

 

[Tyler702]

Wow. What a thread this turned out to be! Lots of info to chew on. I took the liberty of unhooking the housing from behind my '18 Ram 7pin and discovered that the main 12V + and Neg wires are 10g, (black and white wires) the rest appear to be 12 and 14. I purchased this 7pin connector w box, and tapped into the 12v leads only, with 8g wire to an anderson connector, and that feeds to my milkcrate 200Ah 4S LFP bank. No trailer, just the milkcrate in the bed of the truck with the cable draped over the tailgate to charge while I drive, for my 1 to 2 day boondock camping purposes. What I found, while monitoring the bms via Bluetooth from in the cab, while on the highway, I'm getting anywhere from 7 to 10A of charging going into the milkcrate from the 7pin. No dc/dc unit involved, just straight 7pin to LFP bank. I figure my bms will offer protection so I don't overcharge, plus, I can still monitor it from in the cab, and turn off the charging switch once I feel its reaching full capacity. I also noticed that when I turn off the truck, while still connected, there seems to be about a 55-60w draw from the LFP. Not sure where that power is going?, back to the truck battery? so I simply turn off the discharge switch while driving, so there's no backfeed of any sort when I arrive at my destination and turn off the truck. Then I unhook the 7pin, enable the discharge switch in the bms, and my milkcrate power bank is ready for use. Am I rolling the dice with this setup? Whats the downside?

 

[Short_Shot]

Wow. What a thread this turned out to be! Lots of info to chew on. I took the liberty of unhooking the housing from behind my '18 Ram 7pin and discovered that the main 12V + and Neg wires are 10g, (black and white wires) the rest appear to be 12 and 14. I purchased this 7pin connector w box, and tapped into the 12v leads only, with 8g wire to an anderson connector, and that feeds to my milkcrate 200Ah 4S LFP bank. No trailer, just the milkcrate in the bed of the truck with the cable draped over the tailgate to charge while I drive, for my 1 to 2 day boondock camping purposes. What I found, while monitoring the bms via Bluetooth from in the cab, while on the highway, I'm getting anywhere from 7 to 10A of charging going into the milkcrate from the 7pin. No dc/dc unit involved, just straight 7pin to LFP bank. I figure my bms will offer protection so I don't overcharge, plus, I can still monitor it from in the cab, and turn off the charging switch once I feel its reaching full capacity. I also noticed that when I turn off the truck, while still connected, there seems to be about a 55-60w draw from the LFP. Not sure where that power is going?, back to the truck battery? so I simply turn off the discharge switch while driving, so there's no backfeed of any sort when I arrive at my destination and turn off the truck. Then I unhook the 7pin, enable the discharge switch in the bms, and my milkcrate power bank is ready for use. Am I rolling the dice with this setup? Whats the downside?

I can confirm the same results.

The low current to the lithium battery is a result of a combination of voltage drop across the trailer wiring and the high voltage of the lithium battery causing a decreased difference in potential.

Since you have larger wiring you might want to test this with your battery discharged rather low and see if it stays within reason. If it does took be fine.

However if you take the battery down to 10v cutoff you might see massive current pulled until the battery gets back up to its 13 ish volts.

You can avoid discharging it that much and avoid the issue of course.

 

[Tyler702]

I can confirm the same results.

The low current to the lithium battery is a result of a combination of voltage drop across the trailer wiring and the high voltage of the lithium battery causing a decreased difference in potential.

Since you have larger wiring you might want to test this with your battery discharged rather low and see if it stays within reason. If it does took be fine.

However if you take the battery down to 10v cutoff you might see massive current pulled until the battery gets back up to its 13 ish volts.

You can avoid discharging it that much and avoid the issue of course.

I've not ran the bank below about 40%, but it would be a good test to get it down to say, 5 or 10%, hook it up, start the truck, and see how much power it draws at such a low state of charge

 

[Zwy]

Wow. What a thread this turned out to be! Lots of info to chew on.

Well, I tried since the beginning to tell some members what I would have done. Which later they realized what should be done, been there, done that.

I took the liberty of unhooking the housing from behind my '18 Ram 7pin and discovered that the main 12V + and Neg wires are 10g, (black and white wires) the rest appear to be 12 and 14. I purchased this 7pin connector w box, and tapped into the 12v leads only, with 8g wire to an anderson connector, and that feeds to my milkcrate 200Ah 4S LFP bank. No trailer, just the milkcrate in the bed of the truck with the cable draped over the tailgate to charge while I drive, for my 1 to 2 day boondock camping purposes. What I found, while monitoring the bms via Bluetooth from in the cab, while on the highway, I'm getting anywhere from 7 to 10A of charging going into the milkcrate from the 7pin. No dc/dc unit involved, just straight 7pin to LFP bank. I figure my bms will offer protection so I don't overcharge, plus, I can still monitor it from in the cab, and turn off the charging switch once I feel its reaching full capacity. I also noticed that when I turn off the truck, while still connected, there seems to be about a 55-60w draw from the LFP. Not sure where that power is going?, back to the truck battery?

That's about a 5 amp load. You should wait some period of time as modules on the truck go to sleep and see what the draw is but usually you see amps drop from a few amps to 50ma as the modules go to sleep. However you have a 5 amp draw which might actually be a failed alternator diode. But the starting battery would be dead overnight most likely. I would first establish what the parasitic draw of the truck is with the TT unhooked. Then determine if the TT is causing some type of parasitic draw when hooked up.

As long alternator isn't charging over 14.5v or voltage drop on the wire is enough to keep power banks volts below 14.5v, you could be OK, however if the LFP bank has a runner cell, you could find the runner cell will have shortened lifespan as it runs in it's upper voltage range all the time. With a B2B, you could have a charging voltage set to limit the pack voltage where the runner cell runs closer to 3.4v to 3.5v. This would extend lifespan on that cell.

so I simply turn off the discharge switch while driving, so there's no backfeed of any sort when I arrive at my destination and turn off the truck. Then I unhook the 7pin, enable the discharge switch in the bms, and my milkcrate power bank is ready for use. Am I rolling the dice with this setup? Whats the downside?

Nothing really, I advocated just using such a method instead of a B2B for low amp charging. One could install a diode to prevent backfeed to the truck from the power bank but I'd first determine if the parasitic draw the vehicle has while sitting unhooked. 5 amps is not a small draw. Are you certain something isn't on in the vehicle?

 

[Short_Shot]

Well, I tried since the beginning to tell some members what I would have done. Which later they realized what should be done, been there, done that.

But if you want more charge current than that you'll still need a dc to dc charger, just for the opposite reason, and assuming the wire can deal with it.

I'd still be very concerned about the low state of charge where the lithium is under 12v and much more apt to draw a lot more current.


As for the draw back into the tow vehicle, I see the same thing when my smart alternator cuts out. The trucks voltage drops lower than the lithium battery and current flows back out of the trailer.

Having recently discovered the availability of a 9a victron dc to dc I'm very tempted to buy one. That will ensure constant charge and no discharge, with a maximum current of around 11-12a or so through the trailer wiring and absolutely no manual intervention.