Dartin for Divorce

Hit your local auto parts store and have them test the battery and alt in the car. If there's a problem, it'll show up pretty quick.

If you have a multimeter, you can also do some tests at home. Measure the voltage at the battery during cranking, it should stay above 10.5v. Then check volts running. It should measure 14-15v. At rest the battery should read 12-12.6v but resting voltage means nothing without voltage under load (during cranking).