Originally Posted by djgizmo
Yea, and those 'most' people don't know what their doing with car audio, let alone electronics. Upgrading the alt wouldn't be needed if adding a second battery using an isolater / relay. Each of those would 'switch' the top off charging to the other battery periodically. One thing that I was informed about a isolator is that it drops 2 volts (lol, not 2 amps), so at 14.4, you're back @ 12.4ish and its barely charging especially if its running your audio system at the same. Relay switch I believe doesn't have that issue. All in all, he's better off with a good cap.
Isolators are made up of several high current diodes which restrict each battery from charging the other and eventually draining both batteries. The diode voltage drop is between .4 and .6 volts - not 2 volts! The isolators on my solar power system run at 13.7v between 4 optima marine batteries.
If the relay is opened and the alternator is not operational then both batteries will drain each others voltage as where an isolator can operate whether or not the alternator is running..