Losing ALL power

I drive a '98 Jetta TDI and my battery light came on and then I lost electrical power about 15 miles later it sounded like a belt was going out and I lost all power. I changed the belt and alternater drove fine for about 30 miles and it went out agian. My battery did not test well so I replaced it. But when I started it up again the battery light came on agian. So I have no idea were to go next. Is there a relay that can help?

After replacing the battery, the battery lite came on may be due to the belt not sufficiently tight. Adjust the belt and see it lite goes away. If yes, your problem solved. If lite still comes on, I would remove the alternator and have it tested free at Autozone, Kragen, or PepBoys. You stated that alternator was replaced most probably with a rebuild one. It may be that the rebuilt was not good.

Alternator

If car will still run drive it to autozone or advanced auto. They will check charging system to include battery and alternator for free. You do not have to remove alternator to get it tested!

So the new alternater tested bad. The one I replaced was 120 amp the one that is on books from Auto Zone and we put in the car was 90 amp. So we blow it when we put it in and do I need a 120 amp Alternater?