Charging lithium via 7 pin wiring harness

RVLiFe

Solar Enthusiast
My 2018 GMC 3500HD Duramax has the 7 pin wiring harness that provides a trickle charge to my 5th wheel battery while I’m driving. It works great for lead acid and AGM batteries. Should I disconnect this wire if I replace the RV battery with a lithium battery? I assume it’s providing about 13.6v which itself won’t hurt the lithium battery but I’m concerned with the higher voltage lithium hurting either the 220amp alternator or the two onboard lead acid batteries in the truck.
does anyone have any experience with this? I know this isn’t necessarily a solar issue but it is required information in order for me to upgrade to lithium for my future solar plans.
 

snoobler

Solar Honey Badger
Welcome to the forum.

Try this search:


Worth reading a few of them.

The voltage concern is that LFP actually has LOWER voltage than lead-acid during the majority of its charge and can force an alternator to output high current for much longer than lead-acid, which can wear out an alternator.

Another concern is the rating of the harness - typically 20-30A.

Many folks deal with this by inserting a DC-DC converter, Renogy or Victron Orion, to control the charging between the two systems.
 

RVLiFe

Solar Enthusiast
My truck has a 220 amp alternator because it is prepped for a snow plow so in theory, it is supposed to provide additional output for accessories that might get installed on a truck that would cause a battery to drain. I agree with the DC to DC charger but the wiring isn’t of super high gauge anyway so unless I run new wires from the alternator, I don’t see any benefit with installing one. I think it might be best for me to find the fuse and disconnect it to prevent any issues with the alternator. With a 30 amp fuse, I would think that this would blow before any alternator issues anyway. I will read the links and see what else I can find regarding this question. Thank you for the links and the help.
 

Mark44

Retired Traveler with a yen for solar and LIFEPO4
My truck has a 220 amp alternator because it is prepped for a snow plow so in theory, it is supposed to provide additional output for accessories that might get installed on a truck that would cause a battery to drain. I agree with the DC to DC charger but the wiring isn’t of super high gauge anyway so unless I run new wires from the alternator, I don’t see any benefit with installing one. I think it might be best for me to find the fuse and disconnect it to prevent any issues with the alternator. With a 30 amp fuse, I would think that this would blow before any alternator issues anyway. I will read the links and see what else I can find regarding this question. Thank you for the links and the help.
The 7 pin is only good for a trickle charge. The wire is so thin that amperage will be very low.
The DC-DC converter will allow for more direct charging, but if you are charging the batteries otherwise (generator etc.) I don't think you really need to count on the 7 pin connection for charging.
If you will be going Solar, you are going to be charging constantly with sunshine, so honestly I would just ignore it. You never know what other circuit is counting on that 7 pin connection so watch out with disconnecting it.
 

RVLiFe

Solar Enthusiast
The 7 pin is only good for a trickle charge. The wire is so thin that amperage will be very low.
The DC-DC converter will allow for more direct charging, but if you are charging the batteries otherwise (generator etc.) I don't think you really need to count on the 7 pin connection for charging.
If you will be going Solar, you are going to be charging constantly with sunshine, so honestly I would just ignore it. You never know what other circuit is counting on that 7 pin connection so watch out with disconnecting it.
I just want to make sure that having everything connected isn’t going to harm the lithium battery or the alternator. I’m not looking to charge any batteries from it by any means. I’ll keep it connected and watch the battery on the Victron SmartShunt and keep an eye on my batteries voltage in the truck. If anything is looking out of wack, I’ll pull the fuse. I’ve already located the fuse to the 7 pin power wire. I’ve pulled it and all the lights on the RV still work so it’s isolated and only runs directly to the battery so I’m not concerned with unplugging it if I have to. Again, I just want to make sure that leaving it plugged in isn’t going to have any adverse affects. I’ll soon find out I guess.
 

Mark44

Retired Traveler with a yen for solar and LIFEPO4
If you have additional concerns, I think you could always put a heavy weight diode in to prevent backfeeding form either side.

Your plan sounds solid to me, but hey I'm just a retired old white guy!
 

Tyler702

Desert dweller, quiet prepper
I just want to make sure that having everything connected isn’t going to harm the lithium battery or the alternator. I’m not looking to charge any batteries from it by any means. I’ll keep it connected and watch the battery on the Victron SmartShunt and keep an eye on my batteries voltage in the truck. If anything is looking out of wack, I’ll pull the fuse. I’ve already located the fuse to the 7 pin power wire. I’ve pulled it and all the lights on the RV still work so it’s isolated and only runs directly to the battery so I’m not concerned with unplugging it if I have to. Again, I just want to make sure that leaving it plugged in isn’t going to have any adverse affects. I’ll soon find out I guess.
Following your thread. Just curious if you've encountered any issues with your setup so far? Mine is similar, however, I'm just tapping into the 7pin 12V wires to charge my milkcrate LFP that sits in the bed of my truck, while driving to a boondocking campsite. The BMS shows only about 88W going to the milkcrate while on the highway, so it's a slow charge, but sufficient for my current needs. I, too, was concerned about overworking the alternator. (2018 Ram 1500) The milkcrate is a 12V 200Ah LFP.
 

RVLiFe

Solar Enthusiast
Following your thread. Just curious if you've encountered any issues with your setup so far? Mine is similar, however, I'm just tapping into the 7pin 12V wires to charge my milkcrate LFP that sits in the bed of my truck, while driving to a boondocking campsite. The BMS shows only about 88W going to the milkcrate while on the highway, so it's a slow charge, but sufficient for my current needs. I, too, was concerned about overworking the alternator. (2018 Ram 1500) The milkcrate is a 12V 200Ah LFP.
I’ve actually never tried charging while driving using the alternator. I’m sure that I wouldn’t have any issues since I have a SmartShunt installed and can easily monitor while driving. I just figured that I don’t have any real need to charge while driving at all. It’s too easy to charge via AC adapter or Solar that a DC/DC converter would just be a waste of money for me and isn’t a priority. I can easily travel for 5 days without having to plug into anything while still using my leveling jacks, slides and a few lights. Good enough for me. You may have convinced me to at least give it a shot some time and see what kind of charge I will get. It’s easy enough to pull the plug if I don’t like what I’m seeing.
 

AndyRonLI

Solar Enthusiast
So I swapped out my FLAs for Lithium, there is a thread on here somewhere. I put a 20amp Renogy DC/DC charger inline with that trickle charge. And then set it to the Lithium profile, and enabled the charge limiter. That limits me to 10 amps charging. The real issue is the truck side wiring. Lots of threads on this site. With the existing truck wiring, 7 pin connector, and existing trailier wiring all at 10gauge, I do get a voltage drop. Even limited to 10 amps the voltage on the input side of the is 10.8 Volts, though the charger pulls it back up to 14 to keep the Lithiums happy.
I can just stay even with my residential fridge on a long drive. So as long as you dont need too much power the 7 pin will suffice. But I recommend the DC/DC or other isolator approach to keep the amperage down. Not for the batteries, but for the wiring and possibly the alternator.
My 2 cents

 

rickst29

Solar Addict
My truck has a 220 amp alternator because it is prepped for a snow plow so in theory, it is supposed to provide additional output for accessories that might get installed on a truck that would cause a battery to drain. I agree with the DC to DC charger but the wiring isn’t of super high gauge anyway so unless I run new wires from the alternator, I don’t see any benefit with installing one. I think it might be best for me to find the fuse and disconnect it to prevent any issues with the alternator. With a 30 amp fuse, I would think that this would blow before any alternator issues anyway. I will read the links and see what else I can find regarding this question. Thank you for the links and the help.
You could use my scheme instead - boost the "trailer battery charge" Voltage under the truck hood. (I boost to a well-regulated 36.0V, on the basis of a dashboard switch). Then within the RV, I detect the high Voltage and auto-switch that "TBC" wire to become input to an MMPT Solar Controller. The MPPT controller is tuned for Lithium batteries and has them at very short distance.

This allows up to 450 Watts along the "7 pin" Bargman cable and connectors, while keeping amperage below 15A on that wiring path. My SUV appears a single "450 Watt" Panel, with V(mp) just a bit below 36 volts. Most of the single-box DC->DC charger controllers (Renogy type) must be installed in the RV, rather than the Truck, and pull much higher current on a pair which runs only around 13.5 Volts (and loses Voltage from there, with increasing current and wire losses).
 

shadowsteve

Solar Enthusiast
You could use my scheme instead - boost the "trailer battery charge" Voltage under the truck hood. (I boost to a well-regulated 36.0V, on the basis of a dashboard switch). Then within the RV, I detect the high Voltage and auto-switch that "TBC" wire to become input to an MMPT Solar Controller. The MPPT controller is tuned for Lithium batteries and has them at very short distance.

This allows up to 450 Watts along the "7 pin" Bargman cable and connectors, while keeping amperage below 15A on that wiring path. My SUV appears a single "450 Watt" Panel, with V(mp) just a bit below 36 volts. Most of the single-box DC->DC charger controllers (Renogy type) must be installed in the RV, rather than the Truck, and pull much higher current on a pair which runs only around 13.5 Volts (and loses Voltage from there, with increasing current and wire losses).
What are you using to boost to 36v? Is it up to the user to ensure they don't plug in a trailer that can't handle 36v on the 7pin connector charge line?
 

rickst29

Solar Addict
What are you using to boost to 36v? Is it up to the user to ensure they don't plug in a trailer that can't handle 36v on the 7pin connector charge line?
I am using one of these for the "Boost", under the hood. It is important that the maximum output from the MMPT into the batteries be the limiting factor, I am allowing only 28A maximum there (on a "30A" Solar Controller): https://www.ebay.com/itm/274406991976

It's been running for years with no issues. It's up to the user of the truck (which is me), and my dashboard switch is pretty blatant about controlling "something unusual". I simply leave "Zombie Lights Switch" in the "Off" position, except when towing the compatible Trailer in non-daytime hours. Here's the dashboard image:

https://diysolarforum.com/attachments/new-tv-wiring-zombie-switch-jpg.43547/

For towing regular trailers, "TBC" simply defaults to normal under-the-hood Voltage (about 13.4 Volts in my Toyota SUV.)The compatible Trailer has pretty good Solar on the roof, which is connected into the MPPT controller when the Dashboard switch has NOT been turned "on". A Relay in the Trailer detects high Voltage, and its switched lead controls another Relay - that second does the actual switch between "real solar panels" versus switched Bargman TBC "one big fake panel" on the MPPT Solar controller input. I could use a 3rd Relay to switch TBC back into the 12V system, but my Trailer is never towed by another vehicle - and would always be running from Solar anyway, except at night. As you have already figured out, a regular "TBC" running at 13.5 volts (less after wiring losses) is nearly useless for charging batteries trailer batteries anyway.

My main Thread (with wiring diagrams and parts list) is here, if you haven't found it via "search": https://diysolarforum.com/threads/r...rs-of-mppt-charge-solar-in-the-trailer.20730/
 
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rickst29

Solar Addict
I feel that my scheme only makes good financial sense when you already have a Trailer-installed MPPT supporting "regular" panels for camping. When you already have that MPPT, the whole thing costs barely $100 (plus beers for all the installation work). However, if you add another $170 or so for a decent MPPT, the finances become much less attractive in comparison to other schemes.
 
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