-- A B2B still requires the same size wiring. Amps is amps. A B2B will help with voltage drop, but you still need properly sized wires.
I thought it was common practice to use a larger gauge wire to reduce voltage drop when charging straight from the alternator. The Redarc states a 4AWG wire is required for the 1240D to run 30ft. If your alternator is at 14.7 volts, 40 amps over 30ft. reduces this to 14.1V at the battery. In order to have a charging voltage of 14.4 you would need to size up to a 1/0 AWG wire. So I thought the DC-DC charger would let you get away with a smaller, more manageable, wire.
The alternator outputs 165amps
It does, but not many peoples battery banks can take that kind of charge. If you have a 200Ah battery bank a 40A charger is right at the charging sweet spot assuming .2C.
I understand you are under the impression that dc-dc charge will charge battery faster
And this is all marketing ****************...
Not faster, but fast enough. I thought that after the battery is done the bulk stage the current to finish charging is reduced. Most of the time spent charging a battery is in the slower absorption stage which a 40A charger on a 200Ah bank would be more than adequate.
As for marketing, what about battery life? Is there truth to a 3-stage charger being better for your batteries? If so, considering the price of an ACR and MPPT is about the same as a DC-DC charger wouldn't the DC-DC charger be a better choice for your batteries longevity?