Do I need DC-DC charger if I add Lithium batteries?

[phrench]

Will I damage my new TV electrical system or battery if I use "drop-in" lithium batteries in my trailer?

I understand the basics from reading other threads:
-The different chemistries of the lithiums and lead acid operate at different voltage ranges. Once the 7 pin connects the trailer and tv, batteries will try to equalize
-some people pull a tv fuse to protect it
-some disconnect immediately whenever the vehicle is not running
-while driving, some of your lithium charge will be depleted
-a DC-DC charger will allow your lithiums to charge while driving and will protect the tv (that's the limit of knowledge on DC-DC chargers)

Practically speaking what have people experienced?

[sdelam]

Your alternator will produce 14+ volts when the engine is running so you should not deplete any LiFePo charge while traveling. I found that my TV produces 14v+ @ ~13.8a with the TV at idle. Once the TV is turned off, the LiFePo start to discharge 2-3a for a short period until the voltage stabilizes between the TV and RV batteries. I don't see how this will have much impact on the TV battery since it is the same voltage produced by the alternator.

The concern is usually on alternator damage. Since the LiFePo will accept large volumes of current until full, if you had a large (mostly depleted) RV battery bank at the start of your trip and you had a large enough wire in your 7 pin connector, it could place a high demand on your alternator for an extended period of time. I suggest you measure the TV output when charging the LiFePo and see if you have a concern.

[phrench]

Thank you.
To clarify, the greatest risk is that I camp for 5 or 6 days and draw down the 200ah 100A bms lifepo to a low level. Then, jump in the vehicle and potentially damage the alternator in my brand new and very expensive (at least to me) vehicle due to the lifepo's ability to draw a lot of current to charge. The DC-DC controller would manage that current flow to protect the alternator. If the battery had a higher level of charge (?how much?) then the draw would be lessened and the tv battery protected. However, one of the major advantages of lithium in the first place is the ability to discharge it almost fully.
So if I am going to invest in lithium, and I move my trailer around alot, I will probably need a DC-DC charger to protect my alternator. Or add 120 lbs to the tongue of my single axle 3000lb trailer with a couple of 6V's.

[distracto]

From what I've read, the wire gauge and wire length from alternator to 7-pin creates enough voltage drop that the lithiums can only draw so much from the alternator.

There may be others here who know more than I, but we had no issues hauling our lithiums around all last season without a DC-DC charger.

[tomkatb]

Ditto distracto

The voltage I see at my batteries is always 13. Doodly squat. No matter what the tv is reading.

[distracto]

This typically applies more to motorhomes where the house batteries and engine batteries are so close together.

[Tabasco_Joe]

The other advantage of the DC-DC is that you can charge the lithiums to full charge while traveling. That way you are topped off when you stop.

[phrench]

...so my Hyundai Palisade which is rated for only 5000 lbs with factory towing package is likely built with light wiring to the 7 pin. The light wiring should serve to increase resistance and limit the charging demands being put on the alternator without the need to wire a charge controller into the system...

I don't get the vehicle for a week, so that bit about the wiring is purely an assumption. I would think that the pickup drivers would have more robust wiring, but who knows with these electrical gadget filled SUV's.

[distracto]

Originally Posted by Tabasco_Joe

The other advantage of the DC-DC is that you can charge the lithiums to full charge while traveling. That way you are topped off when you stop.

This is a good point, my rooftop solar charges while I drive so I've never felt i needed to add to the charge.

[KMP44]

Originally Posted by Tabasco_Joe

The other advantage of the DC-DC is that you can charge the lithiums to full charge while traveling. That way you are topped off when you stop.

Agreed, but I'm trying to find the same definitive answer as phwrench - IF charging while driving is not important to me, will towing with the 7 pin connected to a large LiFePo battery bank without a DC to DC controller hurt the alternator or melt the wires? My truck is listed as having an "extra heavy duty alternator" (not sure how many amps it can produce). The question that seems to be hard to find a clear answer to is will the BMS try to pull too much current for the rest of the system, especially in a situation where you leave camp with a low state of charge. I have enough projects to do, so dedicated wires and a controller aren't something I want to mess with. Just want to be able the tow the thing without damaging the electrical system on the truck. Seems like the BMS might try to pull more than the tv can handle - so real world experience is super helpful here.

[sdelam]

My real world experience is with a 2020 Duramax with a 220a alternator. At idle, the battery (2x 100AH battle born at 60% SOC) received 13.7a through the 7 pin connector per the Renogy battery monitor.

[KMP44]

Originally Posted by sdelam

My real world experience is with a 2020 Duramax with a 220a alternator. At idle, the battery (2x 100AH battle born at 60% SOC) received 13.7a through the 7 pin connector per the Renogy battery monitor.

Thanks - re-reading your earlier post, I see you had that in there. 13 amps seems like a pretty manageable load on the system.

[Tom48]

Originally Posted by tomkatb

Ditto distracto

The voltage I see at my batteries is always 13. Doodly squat. No matter what the tv is reading.

While I am a lithium iron phosphate proponent, and have great concern about my alternator in the motorhome. The factory wiring would connect my lithiums with a cable and that would be too much draw for my alternator in the long run.

I don't have the same concern for that little trickle charging wire that feeds through the seven pin connector. I know I built my setup with diodes when I designed the hookup for my towed vehicle but even at that, I unplug the towed vehicle from the motorhome if I'm going to be stopped for more than an hour to avoid any risk of drawing excessively from my truck battery or my house batteries.

If you're concerned about your towing vehicle then the simplest battery isolator can be bought in many auto parts stores or you can order a higher quality solid state battery isolator online. I don't think that it would hurt to have an isolator such that when the truck shuts down the towed vehicle is disconnected from the 12 volt source line.

Or just unplug the rig every time you stop

[Tom48]

Originally Posted by phrench

Thank you.
To clarify, the greatest risk is that I camp for 5 or 6 days and draw down the 200ah 100A bms lifepo to a low level. Then, jump in the vehicle and potentially damage the alternator in my brand new and very expensive (at least to me) vehicle due to the lifepo's ability to draw a lot of current to charge. The DC-DC controller would manage that current flow to protect the alternator. If the battery had a higher level of charge (?how much?) then the draw would be lessened and the tv battery protected. However, one of the major advantages of lithium in the first place is the ability to discharge it almost fully.
So if I am going to invest in lithium, and I move my trailer around alot, I will probably need a DC-DC charger to protect my alternator. Or add 120 lbs to the tongue of my single axle 3000lb trailer with a couple of 6V's.

In your scenario, a DC to DC charge controller wired from your tow vehicle battery then to a connector at the back and then wired over to your trailer batteries from that connector with about 6 gauge wire should put plenty of power in with the DC to DC charger and still protect the alternator. Have a 600 amp lithium battery Bank that seldom runs down below 70%, so topping off can mean 180 amps. With my 40 amp DC to DC I'm relatively well topped off by the end of a day's drive even if my solar is not producing due to overcast. When the solar is producing the DC to DC drops out by the time the voltage gets decent in the batteries

[phrench]

Am I allowed to rant on this forum?!?
Everyone has been super helpful, so I hate to rude….

But, I just wanted to buy a small trailer. Then…
-my 6 week order turned into 10 months
-the dealer made a “mistake on the price” and told me I needed to pony up another $2k or I could walk away. Prices went up so much that it was still a good deal, but c’mon
-turns out the fridge is 12v only. No going back now
-stock solar panels and battery won’t run the fridge more than a day or so
- seems I need more solar, so…
- get some portable panels
- get a mppt charge controller to replace stock pwm
- get lithium batteries or overload my tongue
- move batteries inside
- get connectors and proper cable gauges and a shunt with a battery power meter
- get a dc-dc charger to protect my expensive new (under warranty) tow vehicle
-do some major rewiring of my new (warranty) trailer with insulated bottom and limited access in order to move the battery connection inside, connect the new mppt charge controller, state of charge meter, dc to dc charger while not screwing up the existing pwm charge controller for the solar on the roof
-maybe use the “solar on the side” for my portable panels which turns out is often not even connected or uses too small wires, and then have to rewire it to the new battery location
-try really hard not to catch my trailer or new vehicle on fire, electrocute myself or others

(I know, I know, throw money at a problem and it will be fixed. i.e. take it to the dealer or another professional, replace with a new 2-way fridge, or buy a generator. I have some good reasons for those solutions not being appropriate in this case)

So maybe I’m just inflexible and spoiled. But I paid a lot of money for a new trailer with solar. With the setup I got from the dealership with a cheap, weeny little lead acid I can run my fridge for about a day. Is it too much to ask for my fridge to run without having to become a pseudo-electrician and spend an extra $2000!!!!!

[distracto]

I feel your pain... My dealer says he is not ordering any more 12v fridges, too many customers come back complaining. We knew what we were getting ourselves into and had a plan before we ordered it.

We had ordered it with a few things that the dealership wouldn't have ordered as a standard like the radiant barrier pack and the fridge. He didn't have any 257BHS models in the lot, so we put a deposit down and hoped we'd like the layout and such. His manager told him I'd lose my deposit if I didn't like the trailer when it arrived because of the fridge...

It's not a good design idea for boondockers, but cheaper for the manufacturer. Imagine the savings on the cost of the fridge never mind every gas line and connections they don't have to make!

[phrench]

Thanks Distracto, a little commiseration has calmed me down. I’ve just been trying to cram 12 pounds of solar and electrical knowledge into a 10 pound brain before my first camping trip.

[KMP44]

Originally Posted by phrench

Am I allowed to rant on this forum?!?

Sure! Hopefully when your new rig arrives all of this fades into enjoyment.

Yeah, the built in solar might be pwm with small wire, but it still functions. Test it out at home or close to home with a couple portable panels. You might get enough power to do what you need. Then all these other things can just be future mods if/when you decide you want to do them. And if it doesn't cut it, come back and vent some more!

[distracto]

Originally Posted by KMP44

Test it out at home or close to home with a couple portable panels.:

This.

Once you get it, you can put it through its paces on the driveway and see how long the fridge really runs on the batteries, or at least how fast you can recharge the batteries.

[Comanchecreek]

All I can provide is my experience, and not professing to be any sort of expert, or even knowledgeable. My older 2003 Duramax only had a 105 amp alternator. I upgraded my 4-6 volt golf carts to 300 ah of Battleborn Lifepo4's. First problem was the truck batteries, and the Trailer batteries were never disconnected. The other is, I could drive for hundreds of miles and little increase in battery charge. Wire was only 10 gauge that ran from alternator to RV batteries. To solve the problem I installed a "key hot only" relay in the original 10 gauge charging wire. I used that as the on/off switch for a Renogy 20 amp DC to DC charger. I ran a separate 8 gauge marine bonded cable from the truck batteries to the DC to DC charger. I now am fully charged when I arrive, and the batteries between the truck, and the trailer disconnect when I shut off the truck. I also upgraded to a 145 amp alternator which was an option for that truck. YouTube is your friend when setting up a system such as this, along with the powers that be on these pages.