Background: I'm designing a portable appliance (24 DC, ~30 amps, air conditioner) and need to have a switchable power source between vehicle-supplied power (alternator) and a separate dedicated battery (LiFePO4). Both are 24v. This battery is NOT connected to the vehicle in any way, it is dedicated to powering only the unit when selected.
Can I simply leave the ground terminals of both vehicle and battery connected at a common grounding point, and only switch the positive line via a single relay? From my limited knowledge and reading other power supply/ground related questions I would tentatively think the answer would be "yes" but would feel much better if someone would confirm for me. :-)