DC-DC Charger Set-up

[mmnzs]

I am confident someone here has the answer to this but I couldn't find a thread.
I have installed a new electrical system in my travel trailer. Largely because of the 12V Fridge which made the factory set-up unusable but that's another thread. Renogy 3000W inverter, 400Ws from Solar panels, Rover Elite 40A charge controller, 30A Auto-Transfer switch and a set of 200aH Lithium batteries. All is working well. Both the Sun and shore power will charge battery bank. Last consideration for charging is the tow vehicle.
I've looked online but can't find a definitive solution for the recommended set-up.
I now have a big lithium battery bank and I'm concerned about the workload on my alternator if I were to simply connect the 7-pin cable to the running tow vehicle when the batteries are low. Add a DC to DC charger you say?? Exactly. So my question is what is the ideal setup? The Renogy manual suggests the unit should be close to the house battery which means mounting in the trailer. To accomplish this and avoid any smart alternator issues you seem to be required to add a new fused cable directly from the starter battery to the trailer hitch where you will use something like Anderson quick connectors to connect to a new cable on the trailer side directly to the DC-DC charger. This also means disconnecting the 12V aux power from the 7-pin harness which comes from the tow vehicle fuse panel. Can anyone confirm this is the necessary setup or have any experience installing it this way?

[Scott'n'Wendy]

First step. Decide how much amperage you want to supply to the battery bank. Lots like 40-60amps or just moderate like 10-20amps
That will determine all your following steps.

[Guest]

For Lithium batteries many can be charged up to 1C which for example would be 100 amps on a 100ah battery. But for the best in battery longevity, a charge rate of 0.5C would be ideal between longevity and charge time. So for the 100ah battery example, a 50 amp charge would be best all around. Always check with the battery manufacturer first before buying an expensive system.

Jim

[cdn.shaun]

For Lithium batteries many can be charged up to 1C which for example would be 100 amps on a 100ah battery. But for the best in battery longevity, a charge rate of 0.5C would be ideal between longevity and charge time. So for the 100ah battery example, a 50 amp charge would be best all around. Always check with the battery manufacturer first before buying an expensive system.

Jim

I agree with the 0.5C calculation for long life of the Lithium batteries.

I believe mmnzs has a 'set' (pair?) of 200ah batteries, therefore 400ah total capacity. Based on this, technically 0.5C would 200amps, correct? And perhaps why he's looking to maximize tow vehicle charging the bank while he drives but not overwork/stress the alternator?

I'm planning for my own larger (200-500ah) bank in the spring and appreciate the chance to learn here of the best options to consider.

[Country Campers]

There have been plenty on here that have installed the 40 amp Renogy DC-DC charger. They also had to run a dedicated wire package from the engine compartment to the rear of the truck and then from there to the front of the RV where the DC-DC is mounted, just as you described. If you have dual alternators or one very large one this may be the way to go, also if you have large usage while traveling and need the 40 amps to keep up with the usage. I, and a few others, have installed the Renogy 20 amp version and use the stock wiring on the truck to power it. I do not have much usage while traveling so the 20 amps or even the 10 amps will do fine for me, plus I did not have to do all of the other work. The stock 7 pin positive, with out the DC-DC charger, will only put out about 5-7 amps, not very much when you are trying to charge a lithium battery that is looking for 14.4 v. With that the stock wiring should not hurt the lithium battery at all.

Brian

[Scott'n'Wendy]

I think the charging calculations need to be done considering the tow vehicle electrical system...not the optimal charge rate for the battery bank.
You wanna punch out over 50-60A to the trailer batts..then....yes...you will severely shorten your alternator's life.

[mmnzs]

There have been plenty on here that have installed the 40 amp Renogy DC-DC charger. They also had to run a dedicated wire package from the engine compartment to the rear of the truck and then from there to the front of the RV where the DC-DC is mounted, just as you described. If you have dual alternators or one very large one this may be the way to go, also if you have large usage while traveling and need the 40 amps to keep up with the usage. I, and a few others, have installed the Renogy 20 amp version and use the stock wiring on the truck to power it. I do not have much usage while traveling so the 20 amps or even the 10 amps will do fine for me, plus I did not have to do all of the other work. The stock 7 pin positive, with out the DC-DC charger, will only put out about 5-7 amps, not very much when you are trying to charge a lithium battery that is looking for 14.4 v. With that the stock wiring should not hurt the lithium battery at all.

Brian

I actually own a 20A Renogy DC-DC charger installed in my work trailer. Maybe I'll borrow that and see what I can get out of the stock setup before I jump to the new wiring and the 50A 12V dcc50s DC-DC On-Board Battery Charger with MPPT. I live in the interior of British Columbia and it's quite sunny here. Maybe I'll stay charged without much from the tow vehicle. I do like the idea of a backup though. I can always add it later if the 20A doesn't work well enough.
Thanks
Michael

.

[Guest]

Yes 0.5C on a 400ah battery bank is 200 amps. You would need to calculate the load on the rig to feed such a charger if one is available that big. For DCDC a dedicated line with the larger DCDC would work. 60 amps from an alternator found on most HD trucks is no issue.....as long as your moving to move air over the alternator.

Jim

[mmnzs]

Thank you for all the responses. Sounds like I will need to upgrade the wiring to maximize charging speed with this new lithium battery bank. I'll just add it to the ever growing list of projects.
In the meantime since I have a 20A DC-DC charger and I am going camping soon I think I'll try it out before upgrading. I am seeing reference to issues with smart alternators (Variable Voltage). When the start battery is full the smart alternator limits the current and therefore won't charge the 2nd battery. I don't think I have a smart alternator on my 2015 Tundra but that would certainly limit effective charging of the Lithiums.
Anyone have any experience with this?

[Country Campers]

It is important to note that unless the 12 v dc system in the RV is "calling" for a large amount of amps the DC-DC will not put out a large amount of amps. If the battery bank is depleted somewhat then the DC-DC will provide up to the amperage it is rated.

Brian

[Yoda]

I am confident someone here has the answer to this but I couldn't find a thread.
I have installed a new electrical system in my travel trailer. Largely because of the 12V Fridge which made the factory set-up unusable but that's another thread. Renogy 3000W inverter, 400Ws from Solar panels, Rover Elite 40A charge controller, 30A Auto-Transfer switch and a set of 200aH Lithium batteries. All is working well. Both the Sun and shore power will charge battery bank. Last consideration for charging is the tow vehicle.
I've looked online but can't find a definitive solution for the recommended set-up.
I now have a big lithium battery bank and I'm concerned about the workload on my alternator if I were to simply connect the 7-pin cable to the running tow vehicle when the batteries are low. Add a DC to DC charger you say?? Exactly. So my question is what is the ideal setup? The Renogy manual suggests the unit should be close to the house battery which means mounting in the trailer. To accomplish this and avoid any smart alternator issues you seem to be required to add a new fused cable directly from the starter battery to the trailer hitch where you will use something like Anderson quick connectors to connect to a new cable on the trailer side directly to the DC-DC charger. This also means disconnecting the 12V aux power from the 7-pin harness which comes from the tow vehicle fuse panel. Can anyone confirm this is the necessary setup or have any experience installing it this way?

First off I would recommend you find out what you TV alternator is capable of and the current loading. A 60A Renogy DC-DC at full load can draw in the neighborhood of 90A. Renogy has recommended fusing sizes for thier other units. My new truck has the max alternator package of around 475 amps I believe the 2023 with the optional 3K in bed inverter will probably have more, but then again what does you rug have, and what is actually available for use? Your dealership or a good battery shop can test you system to verify the capability. No many install instructions say to connect to the alternator. I did not do that - instead I tied to one of my two batterys as the charges lines from the alternator to the batterys was big enough to handle the load.

As for an install you can find mine here https://gdrvowners.com/forum/operati...harger-project There are several other installs documented out there too so search. Several have used the stock 7 pin wiring with the 20A unit with good results. These are stand alone units and not combined with an MPPT solar controller. My DC-DC is wired directly to the battery through a circuit breaker.

Hope this helps
Keith

[Scott'n'Wendy]

My new truck has the max alternator package of around 475 amps

Holy cow! Are there cables the size of a baby's forearm under the hood? Wonder how much horsepower an alternator like that would demand when outputting 400+ amps?
Thats double the size of the alternator in my 2018 Dodge. (I don't like the name "Ram')

To the op, if you have an alternator like Keith's, I'd install a 60A DCDC. No point in having all that capacity just power up the radio. lol

[Yoda]

LOL They are only 2awg welding cables. I wanted to make sure no voltage drop issues and as the run length (65" total both directions; give or take) is so long for the 90A draw.

I used the chart found here https://www.bluesea.com/support/arti...r_a_DC_Circuit

The dual high output alternators come stock with the instant cab heat option on the 6.7 - speaking of which I need to find that switch. I imagine if I have that on, I cant have the DC-DC running. BTW for my flooded lead acid I have the DC-DC cut back to 30A.

[Guest]

DCDC will put a load on the TV charging system where the alternator will accommodate this load since the DCDC is a draw on the system. Its not like the lead acid battery with high internal resistance that never becomes charged from the 7 way without a DCDC.

In setting this up you would not want to directly connect to the alternator but to the TV battery which is monitored in many cases for load, SOH and SOC. The other is to not let a DCDC become active with the TV sitting idle for long durations.

Jim