Ya...like dim his lights more. The alternator has to run the car and maintain the battery. Typically, batteries draw about about 7A of current and need 13v< to charge. The flickering lights means the alternator cannot keep up and the voltage is stepping down to the battery. In case you don't know; a battery is around 12.5v and the alternator produces around 14v. When the voltage dips b/c of current demand, you're running off the battery.
I don't care if you have a stock battery or 15 aftermarket batteries; the lights are still going to dim.
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To fix this issue, I'd like to know more about the OP's system and vehicle. In normal daily drivers (<1kW), a "Big 3" upgrade is a waste of time and money. A 4ga run that's <3' can carry A LOT of current with minimal voltage loss. Spending who knows how much on 1/0, connectors, loom, and a few hours doesn't make much sense if you're going to gain .1v at the back of the car. Likewise, it's probably a bad idea to toss HO alternators and whatnot at the car if the problem can be fixed by finding a better grounding point and/or turning down the gains a bit. Hell, a good cap will probably fix the problem but I'd prefer a "free" solution...wouldn't you?
A big 3 upgrade can help voltage by up to 20%. there are some of the most knowledgeable people in car audio on this board, do you really think that the big 3 would be such a popular recommendation if it showed minimal results? Im not trying to be a dick but its a proven way to help voltage drop.
its also shown some cars perform better, get more mileage and even make another pony or two with it done. as for wanting a free solution. you get what you pay for.