The lights dim because the voltage drops, not the current (what you call amperage). Batteries only supply power when the voltage drops down past their resting voltage. Normal systems run at about 14.4V and good batts sit at about 13ishV. Meaning that between that 14.4 and 13 all the power has to come from the alt. The dimming comes in because once the voltage drops to 13V your batts don't have enough power stored to keep it from dropping further and the headlights output is directly related to it's incoming voltage (i.e. less voltage, less bright (dimming)). Adding more batteries will definitely help keep your voltage at least at the resting voltage but it puts more strain on the alternator because it has to try and charge them back up in the low demand times (in between bass notes). Running extra batteries on a stock alt can cause it to die prematurely. When I pulled the stock alt off my first car it was completely black inside, I was kind of surprised it was still running at all.