BTW OP -
It is actually far from useless, but really does it only shine when running big power; to where the stock grounds and cables cannot support the added draw of current. Most people go an install some garbage that will only draw say about or less than 40 amperes, so no modifications need to be done really. However, the Big 3 cannot hurt, ever. It was actually something that increased power output and fuel mileage on a lot of Subarus '03-'05. Crazy huh? //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif
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. Unfortunately there is no alternator out there that will supply the ~800A of current draw that is capable of being pulled by my system, so the batteries need to make that up, and even though my alternator is capable of producing about 250 amperes, it will only do so when spun up more than idle. However, a smaller pulley remedies that some. Err, just ask if I left anything open-ended or unfinished with an explanation, I'm in class writing and attempting to pay attention simultaneously.