[jhv48]

When mine did that the first time, I found the wire to the alternator had broken at the alternator post. The second time this happened, it was the internal voltage regulator. Finally had to replace the alternator.
Do as others have suggested. With the engine running, check the voltage at the battery. If no charging is showing, put the tester on the one wire post at the alternator. If no charge, you've found the culprit. Either rebuild or replace it.