depends on the power and car. i know that regular batteries in my opinion arent the greatest things after 700 watts or so (typical car battery) i suggest a battery (like a agm kinetik) first unless they are going well over 1800 or more watts then an alt is called for (more than likely) benifits of the better battery are no leaking or corrosion and the ability to cycle better than a wet cell. (basically stays more consistant)
thanks audiolife. my car is nissan sentra 95 gle 1.6 liter sedan with stock alternator. my battery is 60 Ah with 100 minutes reserve capacity. I already have a 4-channel amp with 60WRMS x 4 @4ohms. I plan to add another amp for the sub, probably a 200WRMS x 2 @4ohms.