Have to agree with the Kat....
A fully charged battery will read 12.6 volts with a voltmeter across it...Have someone crank the car and notice voltage.....should not drop below 10 volts....
Next let the car start and run....on a cold engine, the voltage should be around 14 volts and as it warms up and the rpms drop, so will the voltage, to around 13 volts....
Now turn on the lights and rear defogger and fan blower on high, and voltage should drop to around 12 volts.....rev the engine to 2000, and voltage should increase to 13 volts.......
If you are not getting these voltages, then it looks like an alternator.....however with the engine now off, and using a test light, not a voltmeter, attach the negative end to the bat neg terminal, and the probe end to the bat pos terminal......notice the brightness....now take the probe end and touch the large red cable attached to the back of the alternator....should be the same brightness.....if much less or not at all, you probably have a fuseable link problem....
Many times when an alternator is removed, it tests out ok....because it is cold....alternators tend to test bad on hot engines, as they warm up, and under a load....