Correct use of diodes in car stereo
As it is difficult to determine exatly how the batteries are charged from the alternator it is not possible to be totally certain what would be the best way to proceed.  If there is a double (diode bridge) output alternator or some other existing battery disconnect system then diodes later may not be needed as the head unit could connect to the Aux battery all the time and if this runs flat you would have to start the engine to charge it and this may automatically connect the two batteries already.

If you want to operate it from the main battery with engine not running you couldconnect the ACC line to control a relay that will supply Main power (NO) to the head unit (COM) when the ignition is on and then feed from the Aux battery (NC) when the ignition is off.  You will have a power glitch when you cycle the ignition.

If the batteries are connected together at the alternator then none of the extra stuff afterwards realy matters as both will drain together.

You can have a look at the large selection of solutions that have been invented for your situation in the list of pictures blow.  There are smart products and DIY solutions, some of those solutions will not work and you can ask here if you want to confirm a specific case.  The important thing is to establish how the two batteries are currently separated from each other at the moment when not charging and if the second battery would be nice to have available for starting if you let the main battery run flat with the headlights illuminating your campsight. 

Some of these are intended for high currents and others for slower charging only.  Some will sense the main battery voltage and only hook up the other if the main battery if mostly charged.

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