do i need a capacitor

first you get a battery. and a cap..if your still dimming-shouldnt be. then you get a bigger alt.  

I had this problem in my 79 civic..rigged a system up..put more wattage head lights...power was sevely drained...i got a bigger battery..problems solved.  stock alt works fine and charges right.

cant edit....

Should have put this in but i just remembered..you need to calculate your draw and match that up to your alt's output.  IF its higher..then yes time for a bigger alt.

What kind of amp do you have and what kind of car is the system in?  An upgrade to your alternator and charging system can become expensive especially when not necessary.  First look to see if your lights are diming when you air is on at night, if not you are fine.

well to save some money try this first up grade all power and ground wires so the match if you are running 4/g use it on everything.

but for the dimming upgrade your ground wire on the alternator it will get rid of almost all of it if not all.

cuz a stock alternator i only like 10/g maybe 8/g if you do that it will help out alot and save you alot of money