I would venture to guess that its a weak bettery. In order to over work your alternator in most mid-size cars, you'r going to need to be running about 1500+watts.
You can upgrade the big 3 (alternator positive to battery
positive, battery negative to chassis, and the engine ground to chassis) and supply about 25% more power. The alternator is capable of producing more power than the stock wires are capable of transfering to the battery in almost every car on the road.
read this for more info on the Big 3 Upgrade...
www .the12volt. com/installbay/forum_posts.asp~TID~73496~PN~1
Note: I put spaces in the hyperlink because the site wont let me post links yet.