Okay, what you need to do. Is go to an Autozone and have them check your battery / alternator. They do it free while both parts are still on the car.

but basically they will be putting a load onto the battery. your battery should be holding a charge of 12volts, anything above 10 will get Er' done but anything below is considered a bad battery.

Alternator should put out 16v, but.... go have them check it brah.