I would like the best way to charge 48V batteries from 12V car and using just Victron equipment. 48V is just easier and more cost effective for our project.
Victron Orion 12 - 48 V
But then you only have a very low charging current on the 48 V side …
At this time there is no 12 to 48 DC DC charger from Victron. There are the DC-DC Converters, but they are not chargers and, as noted in the datasheets, are not intended for use as chargers. I believe Safiery and Sterling Power both have DC-DC chargers in the 12/48 variant, or otherwise just wait a bit until Victron has a DC-DC charger solution.
I have done this but you need to get an isolated 12V to 48V converter and then input it into a Victron Orion-Tr 48/48-8A. This worked fine for me because I had the 12V system far from the 48V battery and then running 48V to the Victron Orion-Tr 48/48-8A needed a very small cable compared to running 12V so was economical as well. You might even be able to use a higher voltage boost non-isolated up to 70V but not sure how that would perform. All I did in the Orion was change the start voltage to just under 48V to get it to go.