Lithium in Trailer / AGM in Tow Vehicle

AnOldUR

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?

webers3

Same problem here, I haven't figured out a solution so I just unplug the 7 way

Tabmanic

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.

Horigan

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.

Yoshi_TAB

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 .

AnOldUR

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?

gulfarea

I put a switch in the tv dashboard that disconnects tv from T@B when I don't want them together. Art

Grumpy_G

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. 

KK1L

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).

AnOldUR

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.

Grumpy_G

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.