It may well be an alternator output problem. First, inspect the battery cables and alternator wiring, particularly the small wire on back of the alternator. Clean the battery terminals and check the grounds. Then charge up your battery, start the car and measure the voltage across the two battery posts. You should see 13.7-14.1 VDC if the alternator output is good (temperature and load dependent). You can also measure the alternator output at the thick red wire on the back of the alternator and compare that value to what you measured between the battery posts (the difference should be very small-- if not, your battery cable is bad.

If the alternator output is low, you can replace the voltage regulator/brush assembly for much less than the cost of a rebuilt alternator.

Good luck!