See post #8. That will explain exactly how to test the Alternator.... ( the next time you can start it).
But you require a digital voltmeter... that will read AC voltage down to the millivolt range. The DVM also has to have block caps inside it, so when trying to measure AC ripple voltage, the block caps will block the 14.75 DCV from the battery. All Fluke dvms have block caps ( when using the ACV function).... but some DVM's don't have dc blocking caps...like my older B+K brand DVM. ( if block caps are not inside the DVM, when DVM placed in 'ACV' function..and test leads put across battery terminals...the DVM will indicate...14.75 vdc). Ideally the dvm should read...'zero volts AC'. IF it instead read several hundred Mv of acv, you got 1 or more shorted diodes.
lf 1 or more diodes fails..open, you got more problems.
It will take you all of 5 x secs to do the simple test...and while you are at it, measure the exact DC voltage on the battery terminals...another 5 secs.
BTW, what year is your car, and is it a manual or auto? What mods if any?
How old is the alternator ?
IF one or more the 6 x diodes inside the alternator has failed shorted, you will then be pumping RAW AC from the 3 phase alternator....directly into the battery...and also the ECU..and those 2 x items (+ a bunch more... but excluding stuff like headlights and horn etc) don't want to see anything but clean DC....IE: NO AC ripple.