the only thing on your car producing electricity is your alternator. you can get another battery or a cap or whatever but guess what thats just gonna put more strin on the alternator you have batteries etc. are just electrical storage devices they dont produce more current so if possible get a better alternator. what car are you putting this in?
a capacitor may help since it will use minimal strain on the alt since the amp wont be drawin constant power just whenever the cap draws it but if your puttin in a decent size system then the Alt would be more ideal but for peoplem with budgets the cap is a temp solution!
i was ignorant on this subject when i started installing my system and i was worried about my lights dimming so i went ahead and got a cap and guess what my light still dim i am on a really tight budget so i have to wait to get a ho alt but thats one of the things on my list of things i need get an ho alt it def is worth it
see if you cant sell that cap to a friend or something. i did the same thing, it didnt help my car at all. only bought it cuz the guys at BestBuy said it would solve my dimming problems. we are all noobs at one point. just learn from this and try to sell that cap to someone who doesnt know lol