“Hybrid” DC-DC charger wiring

[GarryandKristi]

Goal: Eliminate house battery drain due to DC fridge while traveling to arrive at dry camping sites with a fully charged battery.  Hybrid to refers to keeping existing charge wire when adding another charge line of heavier gage. 

After upgrading to a lithium battery (300 AH Epoch) I wanted to be able to negate the amp draw from our 3-way fridge (dometic RM2454) and other small loads that total about 16-17 amps. That fridge used a 100% or nearly duty cycle on DC.  I purchased a Renogy 20 amp DC-DC charger.  Initially I replaced the 14 awg gage wire Oliver spliced from the 12 awg pigtail power wire terminus to the charger with 10 awg.  Why Oliver  went to a smaller gage than the pigtail  is beyond me although this was back in 2018. I also spliced a 10 gage wire to the pigtail common 10 gage that Oliver in its wisdom had also spliced to a smaller gage wire.   This worked fine if I triggered the current limiting feature that reduce the charging current to half (10 amps) of the rated. When I tried to charge at 20 amps it only produced about 14 amps and resulted in a very warm pigtail plug as you might suspect. 

So I relented and decided to run 8awg wire through my pickup (2015 RAM 1500) and Oliver E2 - but with the intent of also using the pigtail charge wire to carry some amperage. Doing some calculations using the wire resistance of 10 and 8 awg and knowing that the voltage would be the same at the junction I planned near the charger input I figured about 2/3 of the amperage would be carried by the new 8 awg line but after installation found that it actually carried 3/4 of the amperage probably in part by neglecting the loss through the tiny 12 v wire  pin in the 7-pin socket on the pickup. The ground pin has about 3x the surface  area of all the other wire pins in the socket   

Fiberglass rods that I had  were helpful in running wire and I only used a 2 or 3 foot ground leg to the chassis frame rail near the bumper so essential just ran the positive wire under the truck.  I had to remove the front of the bathroom sink to pull site into the trailer. I have read that some have run wire without messing with the sink but they must be magicians!  I tap into the trailer running light wire (green) to activate the charger.  First results after an initial camping trip were good.  When testing at idle I measure 25 amps in to get 20 out and a voltage drop of 1.1 volt from about 14.0 at the battery (I had calculated a 1 volt drop and I guess got lucky. So I wasn’t going after the big amps as my objective and figured I could run smaller gage wire in tandem with the existing pigtail to meet my needs.  

30 amp fuse off positive battery terminal

Andersen-type plug with t-handle above bumper

entrance gland/strain relief (1/2 inch) behind propane tanks on left 

input on right (8awg after junction of 8 and 10 awg) and output on left (10 awg). I rummaged an old DPDT switch and used one side of it to activate the current limiting feature that should not be needed after running the 8 gage wire. 

[Ronbrink]

Your “hybrid” approach is interesting to say the least! I initially installed a Renogy 20A DC-DC, which specified a minimum 4 AWG cable size. That said, seems you have a grasp on your limitations with expectations met.