Dc to Dc load on truck.

[Drooling Piston]

I know there are different amp ratings for the DC to DC chargers. I believe those ratings are what power the unit can/will put into your battery pack.
I am surprised that their adds usually do not list what amp draw this will put on the alternator that is powering it. This is a very important spec to know so u know what gauge wire and size of fuse and plug to install in the tow vehicle.
My question to all of you is, Do u think 30% more than the B2B rating is a fair number?
So the Victron 18 amp will probably draw around 25amps. The Renogy 40 amp unit will draw in the neighborhood of 52 amps.

 

[400bird]

It's just math

You multiply output volts x output amps to get output watts.

Divide output watts by efficiency of the DCDC to get input watts.

Divide input watts by your charging system voltage to get input amperage.

I'd recommend estimating your alternator voltage low (12 volts not 14 volts), so the wire gauge is sized for the largest amp draw the DCDC may use. Because as the input voltage goes down the input amperage goes up.


The reason most don't list input amperage because these things typically have a large input voltage range which leads to an equally large input amperage range.

 

[Drooling Piston]

It's just math

You multiply output volts x output amps to get output watts.

Divide output watts by efficiency of the DCDC to get input watts.

Divide input watts by your charging system voltage to get input amperage.

I'd recommend estimating your alternator voltage low (12 volts not 14 volts), so the wire gauge is sized for the largest amp draw the DCDC may use. Because as the input voltage goes down the input amperage goes up.


The reason most don't list input amperage because these things typically have a large input voltage range which leads to an equally large input amperage range.

I’m looking at the Victron 18 amp unit. It is rated at 220 watts.
the Victron website says it has an 87% efficiency rating. Once I divide 220 (output watts) by 87%, I got 253, divide that by 12 (volts) gives me 21 amps input load. Does that sound right?
So I could use a 30amp breaker at the starter battery, run 6 gauge copper wire to the rear of the tow vehicle (about 20 feet), 50 amp Anderson plug, continue with 6 gauge wire along the frame of the camp trailer to the battery pack in the rear of the trailer (about 34 feet) to the B2B.