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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. :-)

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    \$\begingroup\$ Yes, that will work fine. \$\endgroup\$
    – pjc50
    Sep 30, 2014 at 21:05
  • \$\begingroup\$ Connecting the negative terminals together should be ok and is common and usually preferred, however the current you expect to draw is notable. For this reason I would suggest you pay some attention to where you will connect the two grounds together. The volt drop in the wires when your device is drawing the full load may affect sensitive circuits. Ideally you should not share the ground wires with other devices unless they also share the same supply wire or will tolerate the likely ground bounce. Also a single common point is advisable. \$\endgroup\$
    – KalleMP
    Oct 2, 2014 at 7:03

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Yes, that will work.

Current can only flow through a closed circuit, so if you use a single switch-over contact relay with a high current rating and decently high DC voltage rating you are always sure only one can be connected at once. In a 24V vehicle, I'd go for 100VDC and 2.5* your peak current, because you don't want transients and load dumps to cause problems.

Note: To relays DC voltage is never the same as AC voltage! if it says "125V" only, it's likely AC voltage and DC voltage might be as low as 30V. Automotive relays from a decent manufacturer rated for 24V automotive DC have the proper contact finish to switch on and off vehicle DC often and without fail.

If you want to be completely sure of the safety of your battery from the alternator system if it's a device you want to produce, make sure the relay is an integral part of the Device-Battery system with only the one contact going out to alternator +.

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