Renogy 20A dc to DC report

pianotuna wrote:
. . . the Renogy 20A DC to DC Battery Charger 12V, the unit puts out 20 amps, but draws 30 amps when doing so. This is the result of bench testing.

pianotuna wrote:
. . . I don't have an easy way to measure input amps to the unit . . .

I'm confused. Explanation?

Current (along with voltage) measurements at the alternator/battery (power source for the dc to dc charger) and the input terminals of the dc to dc charger are, for us, 2 of the 4 critical data points necessary to determine the overall quality and efficiency of any given dc to dc charger installation. The other 2 are the output terminals and battery terminals on the charge side.

pianotuna wrote:
. . . I have seen 16 amps going to the house bank at idle. The highest reading noted was 19.6 amps, when driving. I don't have an easy way to measure input amps to the unit . . .

The 3-4a drop in output current at idle vs. driving would concern me. This potentially indicates the alternator is unable to sustain the proper current/voltage at idle. In other words, the dc to dc charger may be overloading the alternator at idle. Every dc to dc charger I've installed has exhibited negligible to no drop/rise in output charge current when the engine is idling at, say, 750rpm vs. 2000 rpm.