Couple of questions about DC-Dc charging

NRALIFR wrote:
. . . If your friend’s wires are heating to the point where they’re burning off ~15 amps of current, he should probably be looking for materials and/or workmanship problems with his install. I don’t believe the wire gauge difference from 4 to 2 or your shorter circuit length can account for all of that.

I've done a bit of informal testing of our Renogy 40a dc to dc charger with an adjustable power supply. I think you'd be surprised with the conversion losses when you start dropping input voltage. The whole point of my last post was to show that voltage drop resulting from undersized cables, combined with conversions losses as the dc to dc charger tries to compensate for this voltage drop, uses (wastes) a lot of current.

The current losses my friend experienced with his 40a Renogy dc to dc charger is par for the course---nothing unusual or wrong with his installation. The 30 ft. of 4 gauge cable he used was the gauge and maximum run length Renogy recommended for the 40a Renogy. Also interesting to note that Renogy recommends the use of a 60a fuse for this particular length/gauge---which just so happens to be very close to the current my friend's Renogy consumes. If you calculate the standard voltage drop for 30 ft. of 4 gauge cable with a 60a load, you'll find it's almost 1.0v. The use of terminals and fuses add even more losses, let's say 1.3v. Even if you have good alternator voltage, say 14.0v., right out of the gate your dc to dc charger is only working with 12.7v. It ramps up current draw significantly to compensate. If you feel the case of a 40a Renogy while it's converting such a low voltage you'll be firm believer it is using 10-20a of current to pull off this conversion. The case gets *very* hot while both fans run full-on. Also, let's not forget the current consumed by the 1.3v voltage drop created by the cable, terminals, and fuses. It all adds up.

Folks have no choice but to use the proper gauge cables with their inverters. Use too small of cable with your inverter it simply shuts down. Not the case with a dc to dc chargers. They will silently ramp up current draw until either your fuse or alternator gives up the ghost.

Even though I have a 220a alternator in my truck, I still played it safe. I spent an extra $30 for 2 gauge cable and reduced the load on my alternator by at least 10-15a. $30 very well spent in my book.