Originally Posted by c1apton
One alternator would be more than adequate (provided it's a fairly high output as the majority of BMW alternators are) The marine dual battery systems use one battery for starting and normal ignition functions and the deep cycle handles all the excessive accessories and the duties when the motor is not running. They come with an automatic switcher that sends current to one or the other or both batteries (when needed) So in your case you'ld hook up all your audio gear to the aux or secondary battery. No big deal and you avoid the headache of trying to fabricate a second charging system.
Alrighty... so make sure I've got the 90amp alternator, and a deep cycle battery to run the sound system off of. Think I should replace the existing battery with a deep cycle as well?