Relays and Dual Batteries

hello, I'm wiring some accessories into my car and I'm wondering if I will have any issues with utilising trigger and source from the seperate batteries? I want to wire the relay to be triggered across the coil from the main battery +12v to energise the relay terminals from the Aux battery +12v to then power accessory load. Should I be aware of any issues? Do I need to ground either circuit back to battery terminals or is the shared -12v chassis ground ok. Should I be using a diode relay? Thanks for any help!

While you can switch it using power from your main battery, if you are running a stereo, amps, etc. off an auxiliary battery to keep from draining your starting battery, consider powering the toggle switch from the second battery. Use a battery isolator rather than a standard relay and make sure the wire that connects both batteries has either a fuse or circuit breaker at both ends. Also, check the plug on your alternator for a wire that's only powered when the engine is running (either 5 volts or 12 volts). Connect a wire to that and using a transistor, trigger the battery isolator. (You'll have to do some math to find an appropriate value base resistor.) This way the alternator will keep both batteries charged. Some people will say to just use the ignition wire to trigger the isolator, but if your key is left on without the engine running for any reason, this will drain both batteries. Chassis ground will be fine.My ex once told me I have a perfect face for radio.