Couple of questions about DC-Dc charging

S Davis wrote:
So I just took some readings on my Redarc 50 amp the system has 25’ of 1/0 DLO from the engine compartment to the bed of the truck where the charger and batteries are located. Charger ties into a 600 amp buss bar with about 6’ of 3/0 DLO to two sets of Trojan T105 batteries.

Truck idling on cold start.


Batteries starting at 12.4 volts no load.

Truck charging voltage 13.85 volts at 37 amps to the Redarc.

Redarc voltage 14.25 and 18.4 amps charging.

So that is 13.85 x 37 = 512.45w in vs 14.25 x 18.4= 262 out, and 262/512 = 51% system efficiency. Can that be right?

That made me go out and try some measurements with my 20 amp Renogy set-up where the input is via OEM (mostly) 7-pin to the camper.

I used the inverter to get some load on the full batteries so the Trimetic could show whatever the Renogy did. (However the Renogy load started at 19.7 and fell to 17ish later. Values kept changing with the inverter involved. EDIT--forgot the fridge 120v element draws less when it gets hot?--not a good choice for this test! Hard to take a snapshot running back and forth, can't be two places at once! I jury rigged an analogue 60 amp ammeter to the 12v 7-pin wire close to the camper end.

Truck started in idle at 14.5 volts but warmed up to more like 14, so I will use voltages from when warm. Had only 18.x amps of load I could find without the MW (fridge on electric and some fans) so hoped to see some pos on the Tri to get near 20 for output amps. Battery voltage from Trimetric.

Voltage drop from engine batt to Renogy. Renogy off, saw 14.0 at engine batt and 13.75 back at jury ammeter no amps. With Renogy on, it was 13.8 and 10.0 volts, so quite a drop so will use 10v as input volts--yipes! The engine batt voltage measured lower too, notice.

Ammeter was at roughly 30 amps on that jury wiring and at 10v, so call it 300w input. ( not very exact!)

Output was about (kept changing, hard to capture) 12.5v and 17.3 amps or 216w. System efficiency was 216/300 = 72%

but most of that must be my 7-pin wiring and the jury- rigged ammeter's R. The conversion voltage was 10 boosted to 12.5 for whatever Renogy efficiency that is at.

Bottom line is that it did draw more like 30 amps and output was under 20 in that test.

So it seems reasonable that with high R wiring to engine batt you could well see 60 amps with the 40 amper drawing near 40.

( If I want my 20 amper doing all it could, I should indeed improve that input wiring set-up. )