Lance TC - lithium - DC-DC charger question

[orourkmw]

I’ve just purchased a 2020 Lance 975 that will sit in my 2022 F350. I have to install the Lance 6-pin at the front of my bed. I will also replace the two 12V lead acid house batteries with one 200 ah LiFePO4. The Lance has a battery separator, which appears to be installed just upstream of the house batteries. I think I need a DC-DC charger to protect the 270A alternator (as well as to get charging benefits), and am considering 30A-40A. Lance specifies at least 8 gauge hot and ground wires from the truck to the 6-pin. I am thinking that to protect the alternator from a large current draw, ideally I should run 2 or 4 AWG wire from the truck battery all the way to the DC-DC, and back, and not connect to the 6-pin plug (so skip the 8 ga). I think CamperJeff explained the potential complications of feeding the 6-pin. I would mount the DC-DC at the battery box, which is ~4 foot from the pin connector, so I’ll have to see how to feed the wire through. My questions are these: First, am I understanding this properly? If I can keep the battery separator in-line and ahead of the DC-DC, can I avoid the D+ signal wire that the DC-DC uses to isolate truck from house? I think the DC-DC only takes as large as 6 ga, so I’ll need to narrow down as close to the end as I can manage. Is there still a risk of overheating when running lighter gauge wire between the relatively short run from the battery isolator through the DC-DC to the LiFePO4? Any guidance would be appreciated.

[computermonkey]

https://www.youtube.com/watch?v=jgoIocPgOug

I've been using a Renogy 12V 40A DC to DC On-Board Battery Charger that I wired a power on off the tail lights on the camper. I use a 80 amp breaker at the truck to turn on/off and a 60 amp fuse between the battery and the dc to dc charger.

[Camper_Jeff___K]

Lithium changes the parameters of TC battery charging. An old lead acid or newer agm work just fine with simple truck and TC battery separation devices. I kept the battery types the same in everything. All lead acid 12.6 volts or agm 12.8 volts so one system wouldn't discharge into the other even if fully charged. I used to use a constant duty solenoid switch to connect and separate the two systems with a 4 gauge +/- wires to the TC. My old agm batteries could take a higher charge current than the lead acid but nothing compared to what the lithium batteries will accept. The lead and AGM internal resistance was high enough, no extra current limiting was needed. With lithium, a large battery bank can look like a near short drawing a lot of current causing a need for a current limiting device. I have seen the solar panels pouring 60 amps current into the life batteries and more with the engine running and the Victron dc dc 30 amp charger contributing to the mix. The lithium batteries look much much more like a dead short when they are significantly discharged. Your alternators could handle the load an agm could accept. The alternator needs to be spinning fast in hope of being able to handle the load demand. Victron has a couple videos explaining and demonstrating this and it becomes very clear very fast why you need a dc to dc charge controller if you want to charge your life batteries off your alternator. I'm a proponent of Victron products and they have served me well. When you do setup a dc to dc charge controller, be sure there is no connection to your TC system by the old 12 volt charge wire. It is of no use to you anymore excepting that you can use it for the key on trigger to activate the dc to dc charger turning it on and off automatically when you run the truck. You can also run a separate dedicated wire to do that. I did it to avoid possible conflict with the chance I might be towing something with electric brakes or other system. So remember, the more lithium batteries you have, the more you will need a dc dc controller. For large requirements, I believe you can wire multiple units in parallel to increase current if your electrical system can handle it. I currently have 675 watts solar on the roof and 404 amp hours lithium batteries. I am generally fully charged within 4 hours of sun up. After full charge, I switch my fridge from gas to 120 volt AC and run it off solar till low afternoon sun. The new system was pricey but it sure makes a difference camping. A favorite feature about lithium is it is always at 13.2 or more volts. The lights are always bright, the water pump always runs fast and hard, the FF fans work at full speed, everything electrical in the camper runs great and there is never a slowing down or brownout condition like AGM and lead acid have. After having made the jump to lithium and all it's particulars, I highly recommend it. It's not a necessity and if you're happy with the old tech, by all means save the money and keep doing what you're doing.

[Camper_Jeff___K]

JoeChiOhki wrote:
This stuff has gotten so complicated in the last ten years or so.

In the olden days, we just put a 90amp self-resetting marine breaker on the firewall on the line coming back to our 100amp rated Lift Gate Plugs for our charging umbilical between the truck and camper with 4 or 2 gauge wire all the way back and connected to our big banks of batteries.

Hey Matt, that's basically how I used to do it before getting 400 ah of life in the TC. Now it's DC TO DC and no 12 volt direct connection from truck to TC bat bank. You don't want the higher charger voltage feeding back through the 12 volt 7 pin connection creating a loop and throwing off your ecm voltage regulator.

[otrfun]

One of things rarely discussed in these types of discussions is the pre-existing alternator load prior to connecting the dc2dc charger.

On our Cummins (220a alternator; OEM configured, with no optional or additional electrical equipment connected) alternator load can vary from 40a to 70a. Load varies depending on the starter battery SOC and which accessories are in use (lights, a/c, etc.).

It's the remaining load capacity that determines whether a given dc2dc charger can be safely powered by a given alternator. Depending on the pre-existing load, a 160a alternator can potentially handle a given dc2dc charger with more headroom than a 220a alternator.

[S_Davis]

I have pulled 50 amps at idle with my 220 amp alternator every day now for almost a year with no issues.

[BFL13]

Can't answer about the camper isolator for how it works.

Note IMO you will have trouble keeping the input voltage to the DC-DC up and might want to do the neg input path to truck frame trick later if that happens.

Another factor is that with the Chev, I could plug the camper in at home and that would also maintain the truck battery. Now the converter can't get at the truck battery, so it needs another battery maintainer if parked for very long. Same as a MH in that regard now with the DC-DC in the camper.

[orourkmw]

One more question, please. I’ll be wiring up in the next few days. I plan to use the Lance truck plug with 8 ga wires from the truck battery to the camper plug at the front of the camper, which runs to the 30A DC-DC just upstream from the house battery. The other smaller (lights) wires will go to the 4-way pin at the rear of the truck. Since the DC-DC is itself an isolator, and the Lance camper has its own isolator (Blue Seas Battery Link Automatic Charging Relay), will there be any conflict if I leave the wiring through the Blue Seas ACR?

[otrfun]

srschang wrote:
. . . The alternator really doesn't produce anything at idle . . .

Most alternators run at ~3 times engine speed. Our Cummins idles at 700 rpm or 1000-1500 set to high idle. 700-1500 engine rpm would equate to roughly 2100-4500 alternator rpm or 150-200a according to this chart.

[srschang]

Ah, but I missed this part. The chart above is alternator RPM, not truck RPM.
Apparently my alternator is spinning 3.24 times faster than the engine RPM.

[ticki2]

Considering it’s a diesel it will be running in the lower rpm range further limiting its output

[srschang]

I found this interesting. This is for a 2019 or newer Ram 2500 & 3500 220A alternator which is my truck. The alternator really doesn't produce anything at idle. And it derates the alternator if the air temperature flowing through the alternator gets to 200 deg F.

[otrfun]

BFL13 wrote:
. . . With the truck engine off, if I forget to turn off the DC-DC it still does about 14 amps from the truck battery. Goes back to 20 when the truck is turned on. Oops, it is a Chev, no isolator, I must remember to go back into the camper and turn the DC-DC off. Easy to forget if you stop to go into a store, or worse go off for a longer time.

The DC-DC isolates the truck batt from the camper batt, but it does not isolate the camper from the truck unless you have a Ford. I could put an isolator in the camper somehow as others have--might save me someday.

Can't imagine using our 40a dc2dc charger without a battery isolator or equivalent to automatically take it off-line when the alternator is off-line. For us, just too easy to forget.

As for isolating the 7-pin pos (+), we simply disconnected it inside the 7-pin truck to TC pigtail connector. For us, it serves no useful purpose and is just a liability with the dc2dc charger online. If we did need the pos (+) for some reason, all we have to do is reconnect the pos (+) wire inside the 7-pin pigtail connector.

[time2roll]

JoeChiOhki wrote:
This stuff has gotten so complicated in the last ten years or so.

In the olden days, we just put a 90amp self-resetting marine breaker on the firewall on the line coming back to our 100amp rated Lift Gate Plugs for our charging umbilical between the truck and camper with 4 or 2 gauge wire all the way back and connected to our big banks of batteries.

I use the existing system. Works fine. No complications.

[BFL13]

orourkmw wrote:
BFL13: what is the size of your “bypass wire” that carries the current to the DC-DC?

I got into the wires for the 7-pin for the camper that comes up between the cab and the box and dug out the 12v wire before it gets to the truck's 7-pin connector. That wire is maybe #10 or even #12 not sure.

I then added on a length about 1 foot of #8 to come out of the wrapped cord and have another maybe 3 feet of # 8 wire-nutted to that and going up into the camper and onto the DC-DC input. I have a fatter wire for the neg on the DC-DC input that comes down and its end is jammed under the aluminum rail cap which makes it truck frame grounded.

I will try for a photo here of the ugly mess. If I ever get the ambition, I can do it properly someday.



I drilled a new hole beside the camper's 7-pin up under there for the pos and neg (to frame) wires. The battery box is in the front driver's side of the camper so it is all together in that front corner.

The 7-pins have no 12v pin 4 at all now. No brake line either--so it is sort of 5 pins 🙂

Note the other 7-pin at the back bumper is not affected by all this

Now I am thinking that wire nut on the red wires there could act as my isolator instead of going back to the camper to turn it off when we stop somewhere. The little B+ on/off wire goes to that red input wire.