Normally in a car's DC installation the negative is on the car's body. Is it just to make it cheaper by not having to provide two wires everywhere, but just one and use the car body as negative, or is there another more important reason it is done that way?
I ask, because I want to make a secondary DC installation in the car, with another battery, different voltage, to be used for my electronics, computers, etc. The second installation will have a separate battery, but that battery will be charged off the main installation - not directly, but through a charge controller, basically a DC-to-DC converter with current limit, which will have separate positive and negative terminals for both input and output.
How do you recommend to wire the negative of this secondary DC circuit, should I keep it separate, isolated from the car body, or should I also connect its negative wire to the car body, so use a common ground for both installations?