Having extra batteries is a big deal, but only when the stock battery and alternator cannot support the added draw of the system. And, when a car is idling, the alternator is not producing the current, nor voltage (both matter significantly and people always leave it out), so it needs to be spun/revved up to actually produce some larger amounts of "usable" current. Batteries over the HO alternator are what matter more, because many of us (like myself) have a system that will draw WELL more than what our alternators can produce by themselves. Having more batteries creates essentially "a buffer," from the system's major current draw from killing the alternator by drawing from it solely, once the stock battery per se, is drained to a point below usable voltage can be provided by it.
