Test the voltage at your cig lighter when you start the car... should be about 13.4 or something. If it's 12v or lower... the alternator isn't charging. Try reving it to 2.5 - 3k.... that will charge the field in the Alternator and it will then come up to 13.X.
If this is the case, then check small wires on the alternator. One of them is to charge the coil, if that is loose or corroded then it may not charge.
Alternatively it could just be your voltage regulator on the alternator... a separate part that can be bought for 20 - 30$.
Performance one step at a time.