Lithium battery charging in FW of truck

Slownsy wrote:
Also Renogy recommendations for power to there dc to dc chargers are rather large for a long run

Reading the installation instructions for the Renogy 60A DC-DC charger they recommend 4 AWG and a 90A fuse for lengths up to 30'.

This seems like over kill me !

Many trucks wired with 7 pin trailer harness use a 50A fuse a 10 AWG wire for B+ in the factory wire harness. I would be comfortable leaving that as is for a 40A DC-DC charger.

Slownsy wrote:
And would really like to understand how if possible for batteries to draw that much A true a small wire. Also Renogy recommendations for power to there dc to dc chargers are rather large for a long run. I do actually have 00 cable from batteries to rear drawbar but that is for my winch.
Thanks Frank.

It is more about getting a constant voltage to the Li Batteries.

My OEM charging was fused at 60 amps. I did blow the fuse 3 times before I replaced it with an auto reset 50 amp breaker. I did double up on the charging path and have seen over 70 amps (How much over I do not know as my ammeter stops at 70 and starts flashing).

Slownsy wrote:
Also people are installing dc to dc chargers because of voltage drop in wire and smart alternators not supplying sufficiently. So how is all this happening,or is it. Please be gentle as I am no electric genus.

A DC-DC charger is the BEST solution for charging ANY auxillary battery ! A good DC-DC charger will properly charge the battery through all of the stages of charging, regardless of the voltage coming in (minimum about 10V) and prevent excessive draw drove the vehicle charging system.

May not be what you are looking for, but I have the Renogy 20 amp DC-DC in the truck camper and use that instead of 7-pin charging. The 7-pin 12v wire in the truck has a 40 amp fuse, so that indicates what the wire can stand.

I made the 7-pin 12v wire in the camper the pos input to the Renogy instead of having it on the battery as usual. It does a steady 20 amps no matter what the alternator voltage is, and pulls maybe 30 amps from the truck, so that is within that wire's ampacity and 40 amp fuse. Much better amps than using the 7-pin itself.

You can add the 20 amps to whatever solar amps you get if you set the Renogy DC-DC to the same voltage (it has some choices) as your solar controller's set Vabs.

You have a bigger alternator than in my truck so in theory, you could get the bigger amp DC-DC than the 20, but then you run into wiring ampacity issues where the 40 amper (output) will pull 60 amps input from the truck.

You could do something clever with the winch wiring so it is powered without turning on the winch.

I used a 12v connection in the camper instead of using the truck ignition method to turn the DC-DC on/off. Be same in the 5er. You want the DC-DC near the house batts, so it would not go in the truck like they would have it in a MH.

Thanks so far, I do have 600w on the roof, but am a kind off belt and braises person. And would really like to understand how if possible for batteries to draw that much A true a small wire. Also Renogy recommendations for power to there dc to dc chargers are rather large for a long run. I do actually have 00 cable from batteries to rear drawbar but that is for my winch.
Thanks Frank.

If you get an adequate solar system you’ll never look back. That’s all you need.

If you don't really need alternator charging via 7-pin, which is low amps and so not much use, you can avoid the whole question.

With solar on the roof of the trailer, it can do a better job than the 7-pin

Just "turn off" the 12v on pin #4 of the 7-pin (yank its fuse is one way). The signal lights etc still work from the rest of the pins.

If you do want alternator charging, then as you note, it gets complicated! :(