I've searched and searched and my brain has melted trying to follow the 200+ post discussion on tank circuits, so I'm asking for a definitive answer regarding my setup.

I have a 65A alternator and a 45Ah main battery. I'd like to add a gel-cell second battery (between 7-10Ah) to run my PC in the garage whilst updating software and the like.

I understand that wiring them in parallel with a diode causes a voltage drop which, with my small alternator, could be quite significant. Adding the fact that a proper isolator is a LOT of money here (about $180) is the relay method the way to go? I use an Opus 150W and my draw is less than 5A, so crank is not really the issue.