negative for 24V/48V and stepping down to 12V

[deeperblue]

If I convert my motorhome to 24V (or 48V) battery bank, there are still many loads need to be powered by 12V. (light, jacks, slides, generator starting current, etc). So I need either a 24V (or 48V) to 12V step down converter, or set up an 12V battery to provide the 12V power. My question is:
Should both the 24V/48V and the 12V all connect their negative post to chassis? Or only one of them should be connected to chassis?

Thank you very much!

 

[meetyg]

It depends on the step down converter.
Some of these have a shunt resistor on the negative side, to measure the current flowing through.
This current measurement is important for the internal control loops (if the converter has some sort of current limiting or over current protection). If you connect both negatives to chassis, and your loads are also chasis-grounded, you are in effect bypassing the negative side of the converter, which bypasses the mentioned current measurement.

I actually blew up some fuses on a step-down converter, because I thought input and output negatives could be connected through a common negative.

However I think Victron (Orion) has a non-isolated DC-DC converter (the one with only three terminals) which uses a common ground, so that could work for you.

 

[HRTKD]

See my build for how I'm doing step down from 48 volt to 12 volt. Mine is isolated.

48 Volt System Toy Hauler Build

Since I was remiss in documenting my 12 volt RV build, I'll begin documenting the new 48 volt build from the day that I received the cells. The RV will be a 2025 Northwoods Desert Fox 27FS. This is a four season bumper pull toy hauler with a GVWR of 12,800 Lbs with a theoretical cargo capacity...

diysolarforum.com

As far as chassis connection goes, it depends on if you use an isolated or non-isolated component. See this discussion on the Victron forum:

Non-Isolated vs Isolated Orion-Tr - VictronEnergy

community.victronenergy.com