DC-DC Charge install Reflection 29RS

[Guest]

There is nothing wrong with considering the trailer frame as an extremely large AWG wire.

Good point Scott. The Renogy is made to be isolated but in fact both input and output negative terminals can be chassis grounded for the smaller 20amp model.

Jim

[howson]

There is nothing wrong with considering the trailer frame as an extremely large AWG wire.

I would think so, too. My input comes from the experience I had with the Hydrastar hydraulic brake controller. MORryde attached the negative to the camper frame while the instructions specifically said to attach to the vehicle's negative. When I swapped the wiring (long thread somewhere here on the subject) it did make a difference in the lag between truck and camper brake application.

From the Hydrastar manual:


In email conversations with Hydrastar regarding the lag I was experiencing (and the wiring issue I found courtesy of MORryde) the Hydrastar engineer followed up with this hand-drawn schematic:




Some may say this is an apples-to-oranges comparison (DC to DC Charger vs the Hydrastar). My input is based on the assumption that it's a good practice to eliminate as many connections (within reason) between the + and - electrical path in a low voltage application.

[Scott'n'Wendy]

attaching the negative to the camper's frame means the electrons have to "find" their way back to the battery versus a direct path.

There is nothing wrong with considering the trailer frame as an extremely large AWG wire.

[howson]

netazure -- I can't give you an engineering-level rationale reason for this input, but I'd attach the 12vDC negative directly to the 7-pin's negative. Probably not writing this 100% correct, I'll give it a go anyway: attaching the negative to the camper's frame means the electrons have to "find" their way back to the battery versus a direct path.

If you have a Mictuning box (which many of us have installed in lieu of the OEM-provided junction box) it's very easy to do.

[netazure]

Thanks Alex & Brian!

Interesting about the long trip and it not charging the entire time. I do have solar as well (that can manually be shut on / off) but have read maybe the renogy thinks the battery is full if the solar is operating. If only manufacturers sent all RV's ready for boondocking from the factory =) hah.

I'll update signature! its a 22 chevy 2500.

[AlexPeterson]

Country Campers Hey Brian,

What did you run (ground) the negative on the INPUT side of your renogy dc to dc charger to? Best I can tell is the 7-pin ground runs to the same chassis ground as everything else in my system so I assume that is fine as well.

Just want to make sure I'm not doing something dumb by running the INPUT negative right to the chassis ground in my camper... Since (almost) everyone that wires these directly to TV run heavy gauge wire to the positive and negative TV terminals.

Thanks,

-Mike

Mike, your signature does not indicate what tow vehicle you have. I connected mine as you have shown, using the 7-pin charge line. I have a 2022 F-150, which has some sort of "smarts" upstream of the 7-pin circuit. Anyway, when I connected it up for the first time, it worked, with the Renogy 20a DC-DC unit putting out about 17 amps (or, more accurately, my TT's batteries were drawing that). I recently took the first long trip with it, and noticed that the batteries were not notably charged more after 8 hours' driving. My conclusion is that the circuit did not like ~25 amps going in, and the truck shut it down. No fuses are blown, when I connect it now and start it, it will charge. What I do not know is how long it runs before the truck says no more. I'll be installing a solenoid, dedicated wires, etc. next.

To answer your question, I tied the input and output negatives together at the batteries, which is only a foot away from the DC-DC.

[Country Campers]

netazure

I believe I used the ground from the 7 pin wire for the input, the output ground went to a buss bar that is also grounded to the chassis and the battery.

Brian

[netazure]

Country Campers Hey Brian,

What did you run (ground) the negative on the INPUT side of your renogy dc to dc charger to? Best I can tell is the 7-pin ground runs to the same chassis ground as everything else in my system so I assume that is fine as well.

Just want to make sure I'm not doing something dumb by running the INPUT negative right to the chassis ground in my camper... Since (almost) everyone that wires these directly to TV run heavy gauge wire to the positive and negative TV terminals.

Thanks,

-Mike

[Country Campers]

First time out with the new Renogy DC-DC charger and I am very happy with the install. Started out with about 70% batteries and had full charge when we stopped after the first 2 hours of our trip, solar was on as well but this was very early in the morning. I did have it switch to only 1/2 output and seen 10 amps at times on the Victron app. I did not test at full capacity but will try in the future. This is a great way to get charge to the batteries while traveling, especially in non sunny conditions if you have solar. The 12 v fridge was running the whole trip and the batteries were at 100% apon arrival.

Brian

[Scott'n'Wendy]

No, not a victron video. It's been a while, lower budget flic.

[Guest]

It is common on rv forums. Not specifically here. There is a youtube video of an unregulated alternator trying to charge a lithium battery...direct connection...guess what happens. They don't mention that truck electrical systems are regulated.

Scott,

Was it the Victron video? If so the alternator they used was regulated but it overheated from the lack of cooling due to low RPM. They moved to a balmar marine alternator with temperature control that pulled back on the charge when it began to overheat.

The point is to not let engines idle with high loads for prolonged periods while charging a lithium battery via the auto alternator. Auto manufacturers do not use temperature compensation and need RPM and vehicle movement to cool. The 7 way will not cause overheating at idle in its original configuration while connected to a lithium pack.

Jim

[NickinCO]

It is common on rv forums. Not specifically here. There is a youtube video of an unregulated alternator trying to charge a lithium battery...direct connection...guess what happens. They don't mention that truck electrical systems are regulated.

This. Not technically here, just the net in general. I may disconnect the 7 pin power wire but I kind of like having it there for running the power jack, etc. Without it plugged in the power jack is pulling 11 amps. When my 7 pin is plugged in it pulls about 0.5a. Both SOK and Connected Current said I'm doing no harm by keeping it plugged in. It's been a while since I reached out to them but I believe battleborne said the same thing.

[Scott'n'Wendy]

I haven't followed every post closely--who is stating using the 7-pin with the truck's OEM-provided charge will damage a lithium? Not my intent to "call someone out" but it is important, especially on this forum, to provide accurate technical information.

It is common on rv forums. Not specifically here. There is a youtube video of an unregulated alternator trying to charge a lithium battery...direct connection...guess what happens. They don't mention that truck electrical systems are regulated.

[ncitro]

howson That's right, hey I did remember the discussion lol. I also agree that risk is minimal and for the utility I get leaving it connected I'm okay with it. Obviously like a lot of things in the rigs everyone has to make their own decision.

[howson]

... linked a article from AM solar stating the always disable the factory wiring when they do an install. I will see if I can find that discussion and link it.

AMSolar's concern was for the truck's wiring not the lithium battery. What they said in that post was it 's possible for more current to travel on the 7-pin's return ("ground") wire than what it was rated to carry. That's why they will not install systems like what we've got in our trailers. At least that's what my gray matter remembers...

Since we're ... if I understand what Garret is saying, by disconnecting the power wire I have not mitigated the post's concern. Current associated with the battery charger could still find it's way back to the battery via the 7-pin's return path. I judge this risk (for me) to be negligible since there is a robust and direct return path to the battery to and from the charger in my configuration. But I'm just a shadetree backyard owner/technician so each person should weigh and evaluate this topic/risk for themselves.