Couple of questions about DC-Dc charging

BFL13 wrote:
Imagining some numbers:

Output Watts = 40a x 14.6v (just before Bulk ends) = 584w

Efficiency 85% (WAG--same as a typical converter) so
Input Watts = X amps x 14v (alternator output) = 584 x 100/85 = 687w

So at 14v input (no voltage drop) amps = 687/14 = 49 amps
With 1v drop, at 13v input, amps = 687/13 = 53 amps

So with some better info you could get closer for the amps.

so for the 40amp model chage your efficiency to 90% and max charging output is 500 watts so depending on the battery chemistry you could be only putting out a max of 34ish amps. (efficency says up to 90% so I am assuming that and using 90%) victron is 87% but they are powering bluetooth with theres also.

they state an input voltage of 8 to 16V but looking at normal peramiters I would expect it to be 13.2 to 14.8 in a vehicle, but we would have to account for voltage loss for the wiring you use.

if you are ouputting 500 watts with a 90% efficiency that means the unit is takeing in 556 watts from the altanator, which we can figure out amps out as it is dependent on the voltage now.

so at 14.8 (which is where mine will run untill the ecm tells it to go lower I would be using 38 amps roughly to output the 34 amps assuming no line loss.

now if that drops to 13.2 at the charger due to line loss then I would be using 42 amps to provide the same charge level, and if we had a failing battery and we were only getting 11 volts we would use 50 amps and at the bottomish of the input range 9.3V we would draw 59.7 amps from the battery, which we can't so I am assuming these numbers would be for a regulated DC power supply input if you are tapping into a weird output voltage instead of an altanator.

this topic was put out at the right time as I am looking into dc to dc chargers and hadnt realy dove into how they work and this forced me to do some reading into the specs and ratings, now I guess I have to compare different brands and see what the difference is vs price.

Steve