Charging Lithium Batteries From Your Tow Vehicle

I’m not an electrical engineer, nor do I play one on TV. So for those of us trying to follow along at home, is there an issue charging a single Lithium battery (say 100 amp hr) from an average TV without some modification between the two? If I’m following this thread correctly the answer seems to be somewhere between “don’t know” and “ probably not”. You guys clearly understand the topic much better than most of us. When contemplating making a change from lead acid to Lithium, the average bloke only contemplates what the compatibility issues might be on the RV. Should we be paying more attention to the TV end of the equation?

Thanks all.

There is a couple of issues. The first which is being discussed here is the charge current from the tow vehicle is not large enough to recharge a battery very much....charge current between 5 and 10 amps. It wont fully recharge a fully depleted battery during a normal day of travel. I address this by running a heavier gage wire and a dc-dc charger to control the charge current. you do not want to use a heavy gage wire without a charge controller as lithium batterys will use how ever much charge current you want to send at them.... Some will accept 200 amps with no issue, the problem is most alternators are 175 amps max. You will burn out your trucks alternator with out some regulation, be it a charge controller or a long small gage wire.

The second issue on trailers with large solar arrays and lithiums batterys is the charge voltage is signifiantly higher that what a standard tow vehicles max voltage is. The solar array feeds 14.8 volts or so back into the truck causing issues with brake controllers. I know one of the big three has a recall to address the issue of high voltage feeding back into the truck from the trailer.
 
Last edited:
There is a couple of issues. The first which is being discussed here is the charge current from the tow vehicle is not large enough to recharge a battery very much....charge current between 5 and 10 amps. It wont fully recharge a fully depleted battery during a normal day of travel. I address this by running a heavier gage wire and a dc-dc charger to control the charge current. you do not want to use a heavy gage wire without a charge controller as lithium batterys will use how ever much charge current you want to send at them.... Some will accept 200 amps with no issue, the problem is most alternators are 175 amps max. You will burn out your trucks alternator with out some regulation, be it a charge controller or a long small gage wire.

The second issue on trailers with large solar arrays and lithiums batterys is the charge voltage is signifiantly higher that what a standard tow vehicles max voltage is. The solar array feeds 14.8 volts or so back into the truck causing issues with brake controllers. I know one of the big three has a recall to address the issue of high voltage feeding back into the truck from the trailer.

Agreed. This is one of the other factors I was alluding to. I don't allow my solar to charge when towing (accidentally a few times probably).
 
I’m not an electrical engineer, nor do I play one on TV.

The issue with communicating is that we all need to speak the same language and when that can't happen, mistakes happen. I used to live in Japan as a kid and you learn real quick about how to ask 'Where is the toilet?'. As for researching for information, the internet is a great place to learn, but there are a lot of clowns out there that like to share their opinion based on just that. Their opinion.

I would suggest to learn the basics so you can understand some of the language that is spoken. If not, you might just crap in your pants. If you rely on what person A said or person B, how do you really know that both of those are clowns and person C is the expert?

People have a real issue with electronics and I guess I did too when I first started turning wrenches. I had to break it down for a pea brained person like me to get. Maybe take a few classes with YouTube and try to 'get' what they are saying. Once you totally get the basics, then it's easier for you to understand what is being said or at least figure out who are the clowns in this circus we are living in.

Basically the item with the most power [voltage] will supply the rest of the system that has less. So if you are charging, the higher voltage will charge the lower voltage. How fast it will charge is limited by several things, but wiring is one of them. The larger the wire, the more power can flow. The shorter the wire can flow more. If you had a 300A alternator, it will not charge the battery at 300A if the wiring will not allow that much current to go through. It might try and it might just blow something, like a fuse, circuit breaker, or even the wiring, if that is the weakest link.

Accepting someones words as fact, can get you into real trouble. Sublet the problem to a professional or learn some of the basics so you get what language that is being spoken. I have found that if I am in a hurry, sublet is the answer. I had a roof leaking in my house a few years ago. Nope, I did not do that task.

Not all of us are wired to understand everything in life [did you get that?]. I am fortunate that I don't have any interest in brain surgery or want to even experiment with that. I am unfortunate that I don't trust a lot of professionals, so I tend to do things myself. My parents told me I was not stupid and to 'look it up' if I asked a question. If I asked question and backed it up with information about me looking up the facts, he would guide me to the answer, never did he give me the answer, but he guided me on the right path. The path on finding facts can be fun, time consuming, but in the end, you will learn something.
 
This is true, Research for yourself, Sadly many professionals have a financial interest in what they are saying. I dont trust them either
 
There is a couple of issues. The first which is being discussed here is the charge current from the tow vehicle is not large enough to recharge a battery very much....charge current between 5 and 10 amps. It wont fully recharge a fully depleted battery during a normal day of travel. I address this by running a heavier gage wire and a dc-dc charger to control the charge current. you do not want to use a heavy gage wire without a charge controller as lithium batterys will use how ever much charge current you want to send at them.... Some will accept 200 amps with no issue, the problem is most alternators are 175 amps max. You will burn out your trucks alternator with out some regulation, be it a charge controller or a long small gage wire.

The second issue on trailers with large solar arrays and lithiums batterys is the charge voltage is signifiantly higher that what a standard tow vehicles max voltage is. The solar array feeds 14.8 volts or so back into the truck causing issues with brake controllers. I know one of the big three has a recall to address the issue of high voltage feeding back into the truck from the trailer.

Thank you for the response
 
This is true, Research for yourself, Sadly many professionals have a financial interest in what they are saying. I dont trust them either

Actually I did research. As was mentioned earlier a possible issue with the TV has never been discussed in material I have reviewed. That’s why this thread caught my interest.

But I appreciate the suggestion. Thank you.
 
Last edited:
Great mod. We also decided on a DC to DC charger. It is installed in the camper but the power for it comes from the 7 pin running lights. I took the power of my battery through a circuit breaker to an electronic switch powered by the Ford upfitter switch circuitry. I don't actually have the upfitter switches installed, but the wiring is there in all F-350 SuperDuty trucks so you can tap in and use. I simply put an on off switch by the truck ignition switch so I can control when battery current is going to the charger. Used Anderson cables and connectors and made an extension cord with Anderson cables on both ends to connect truck and camper when towing. Works well and can give battery a full charge even on those days when solar can't keep up with the refrigerator. We have the Renogy 40 amp model and it works well with our set up
 
Hey thank to all that responded to my questions with explanations as to they "why's" of the 7 pin with LFP. I really appreciate it. Now I am going to try to conduct a couple of tests to see for myself what happens.

a) I want to see if having my solar panels charging puts any charge to the 7 pin on the trailer. I've never thought about that before and again never saw anything in the truck that would raise a concern. I put 10,000 miles on last year running 965W of solar the entire time without any problem. I can't think of a way that I would be able to test for back feeding on the truck side. I guess if there were enough current I would be able to see a difference at the battery when the harness is connected vs not connected.

b) I want to watch my battery monitor shunt when the 7 pin is connected and disconnected to see how many amps actually make it to the battery bank. Just really curious now about all this.

Thanks again!
 
Last edited:
There have been several questions about just using the 7 pin connector to charge your Lithium battery bank, along with a few questions about damaging your Alternator if you do decide to simply use the 7 pin connector to charge your Lithium batteries.

In regard to damaging your Alternator, the problem is two-fold: first, charging Lithium batteries could draw too much power all at once, and secondly, charging a larger Lithium battery bank could require your alternator to run at a high output (amperage) rate for an extended period of time. If you simply ran a couple of 1/0 cables from your truck battery to the camper there is a pretty good chance you would burn out your alternator if your coach batteries started with a low SOC.

Victron produced a video explaining why you want to have a DC-DC Converter between the truck's electrical system and the Coach's electrical system.

The video is pretty scary: How Not to Blow Up Your Alternator When Charging Lithium Batteries

When it comes to explaining exactly why you should install a DC-DC Converter in your truck and why it matters; Tom Morton from "Mortons on the Move" YouTube channel, gives a very detailed explanation of how the Alternator operates and what are the risks associated with not installing a DC-DC Converter: RV Alternator Charging Overview

Note: The video will start at the point when Tom begins talking about the Alternator charging. If you want to view the video from the beginning simply back up your video player.

I hope these two videos will help everyone understand the Alternator/Lithium Charging issue better.
 
I read all of the pros and cons, and decided to pull the charging fuse and just let the solar handle it. Its been working well like that, and I even run the fridge on electric while under tow
 
I just installed a 100 Ah Lithium battery. Reading this I would like to disable the 12 volt aux charging function from the truck. Pulled the owners manual and went to the fuse section - I assume I pull Fuse F30 for my truck (see snip below from the manual). This should kill the 12V aux charging pin but nothing else, right? Thanks. fuses.png
 
I read all of the pros and cons, and decided to pull the charging fuse and just let the solar handle it. Its been working well like that, and I even run the fridge on electric while under tow

This is how I operated for the first few years (I disconnected the 7-pin wire at the top of the pinbox on the camper).
 
I just installed a 100 Ah Lithium battery. Reading this I would like to disable the 12 volt aux charging function from the truck. Pulled the owners manual and went to the fuse section - I assume I pull Fuse F30 for my truck (see snip below from the manual). This should kill the 12V aux charging pin but nothing else, right? Thanks.View attachment 50579

That looks right to me, however, I would check it with a volt meter at the battery terminals on the coach.

Note: the following steps might be best performed with a slightly discharged battery.

First, I would turn off the Solar (if I had Solar) and check the battery voltage without the 7-pin plugged into the truck.

Then I would plug in the 7-pin and check the voltage with the truck running (Note: on Ford trucks, you need to step on the brake with the 7-pin plugged in to allow the computer to sense the trailer brakes before the 7-pin Aux charging will activate). Your voltage should be higher with the truck running.


I would then remove the fuse and check to ensure the battery voltage is lower than before I removed the fuse.
 
I install my DC to DC charger in my trailer. I wired it so it turns on when the running lights are on which also turns on the trailers backup. Camera. I ran 6 gauge solid copper wire to back of my truck with an Anderson Connector. I used 6 gauge solid copper jumper cables with the ends cut off to go from the Anderson Connector to the DC to DC changer. The jumper cable wires are attached to each other giving it a cleaner look.
 

Try RV LIFE Pro Free for 7 Days

  • New Ad-Free experience on this RV LIFE Community.
  • Plan the best RV Safe travel with RV LIFE Trip Wizard.
  • Navigate with our RV Safe GPS mobile app.
  • and much more...
Try RV LIFE Pro Today
Back
Top Bottom