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Lithium in Trailer / AGM in Tow Vehicle

AnOldURAnOldUR Member Posts: 1,283
in Battery/Electrical & Solar
My old Wrangler does not have an isolation relay, so if I leave the 7 way plugged in over night the higher voltage lithium will back feed into the AGM to balance the batteries. Is this a correct statement? If so, is there a risk of damaging the AGM by over charging? What about while driving?
Stockton, New Jersey
2020 nuCamp T@B 320S * Jeep Wrangler

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    webers3webers3 Member Posts: 404
    Same problem here, I haven't figured out a solution so I just unplug the 7 way
    2017 T@B 320S   2019 Jeep Cherokee - Southern Connecticut
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    TabmanicTabmanic Member Posts: 36
    I just removed the Black wire (charge wire from TV) in the junction box and put some shrink tube on it, problem solved. My solar keeps the Tab topped off just fine.
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    HoriganHorigan Member Posts: 614
    Same here.  In my case, I pulled the fuse under the hood for the charge wire.  Need to leave the fuse for the brake controller though.
    Rich
    2019 T@b 400
    2013 Toyota Highlander 3.5L V6
    Bellingham WA
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    Yoshi_TABYoshi_TAB Member Posts: 378
    Hi,

    I don’t have any empirical data, but I’m 6 weeks out on a trip with two lithium batteries and a Jeep GC.  I have not observed any flow of voltage/current in either direction.  I have observed no voltage changes other than what is normally expected .  I’m sure there is something however small, but nothing to show via smart shunt or voltage readout on vehicle .
    2021 TAB 320 BD
    2021 Jeep Grand Cherokee
    Southern Maryland
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    AnOldURAnOldUR Member Posts: 1,283
    Tabmanic said:
    I just removed the Black wire (charge wire from TV) in the junction box ...
    Are you talking about the junction box behind the tub on the trailer or something in the tow vehicle?

    Stockton, New Jersey
    2020 nuCamp T@B 320S * Jeep Wrangler

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    gulfareagulfarea Member Posts: 506
    I put a switch in the tv dashboard that disconnects tv from T@B when I don't want them together. Art
    2019 TaB 320 S Boondock Edge
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    Grumpy_GGrumpy_G Member Posts: 450
    Don't overthink it. The voltage of a fully charged LiFePo battery is 13.6V which is below the typical charging voltage of an AGM battery of over 14V. Modern vehicles run in the 14V range and the AGM battery is fine with it. 
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    KK1LKK1L Member Posts: 95
    I put a Victron Orion in-line between the junction box and the battery. It gives me peace of mind that the LiFePo is treated correctly and the connection between the T@B and tow vehicle is one way and only active with the TV running. On my 400 it is mounted under the dinette. The battery wire runs through the floor there from the junction box, so very easy to put in-line by splicing into it.

    There should not be any great harm by having them connected, but the two battery types do not have the same resting voltages or characteristics. There will be some kind of current flowing most likely from the LiFePo, but unless one or the other is seriously discharged it won't be too much (not based on measurements, but just thinking about it).

    73 es God Bless de KK1L, Ron <><
    2023 T@B 400 Boondock "Running up a T@B"
    Jericho, VT
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    AnOldURAnOldUR Member Posts: 1,283
    Grumpy_G said:
    Don't overthink it ...
    Guilty! My overthinking was that if the TV's AGM was fully charged, there would be nothing controlling the LiFePO4's from over charge it. I put a lot of miles on with this configuration with no apparent damage, so there must be something that I'm not understanding.

    Silly question, but doesn't the TV have something like the Victron's float mode to maintain, but not over charge.

    Stockton, New Jersey
    2020 nuCamp T@B 320S * Jeep Wrangler

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    Grumpy_GGrumpy_G Member Posts: 450
    Modern vehicles have a battery current sensor and the PCM reduces the voltage when the current into the battery drops below a certain threshold. In other words it behaves like float mode. 
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