Trailer 12VDC

The 12VDC feed in the 7 pin connector seems to be switched so not supplying power when the ignition is off. Does anyone know if there is a battery isolator or voltage switched relay on the this DC source?

 

Yes - there is an isolation relay. Search thru the EWD's in this link for all needed info.
https://www.tundras.com/threads/ultimate-tundra-wiring-diagrams-collection.81448/

 

Devron11,

Thanks much for the link. That answers that question and several more.

 

It's in the link but I remembered that schematic from this thread I read the other day..

 

I was looking to disable the circuit so any charging was stopped. I have a lithium system in the trailer and didn’t want the alternator to be put under stress. I pulled relay “Sub batt” and that seemed to do it but I am not too sure if that relay supplies power to any other system in the truck.

On our second trip I just left the relay in and it didn’t seem to affect anything. I think the Lithium charge level was sitting at ~13.5V when we started driving and that might be too high to force the charging system to start working.

Eventually, I would like to disconnect the “+” circuit all together. I don’t want to run a DC to DC converter. I would rather just use the solar to charge the batteries in the trailer.

 

Most LFP batteries have a BMS (battery management system) either internal or external that will prevent overcharging. I have the same concern as you as I plan to switch to LFP batteries soon, but my concern is more along the line of managing the current in flow to the battery. An LFP battery bank can be charged at a much higher current level than the battery isolator can handle (>40A) and batteries will generally take all they can get. Unless the battery isolator acts as a current limiter also I am thinking a DC to DC charger with a lower rating with a charging profile for lithium batteries is the best answer.

 

I am thinking about getting another Lithium battery so I would have 500ah total. I am concerned about the load on the alternator if the truck tries to charge the batteries. Might not help the mileage either with the alternator running full out. My solar setup would be able to maintain the battery(s) without need for assistance from the truck. If a DC to DC charger can limit the current to the batteries, that would work to limit the load on alternator without having to cut a wire.

 

Has anyone found the charge line fuse? I pulled the SUB BATT relay but still saw 12v on the charge PIN. I haven't been able to locate the actual fuse.
I have the same concern with just recently switching to LifePo4's. I don't need the charging, just don't want to do aby damage.

 

On our last trip, I just left the relay in. The truck can’t really overcharge the trailer battery as the Lithium battery voltage is higher and the BMS on the battery will look after it.

The other option is to disconnect the 12v feed at the 7 pin connector.