Hoping someone can reassure me that I am replacing the right part.

While driving the car on the highway this past sunday the battery light came on. Once i had arrived at my location a couple minutes later i noticed it had went off. Didnt think too much of it but kept an eye out the rest of the day and no issues. Yesterday I take the car out and the battery light was illuminated at idle. A small rev to the car would make it go off and then would re-illuminate back at idle and over 3k rpms. I took it to autozone to have the battery and alternator tested and wouldnt you know it at that point the light was off and both tested fine. Today i drive the car to a few errands and the last stop the car wont start. I get a jump start and head home where the battery completely drained and the car died about 10ft from the driveway. Normally i would say alternator but know our cars are quirky and could be something else. The battery is about 8 months old, The terminals have no corrosion and are connected fine, battery and alternator tested ok yesterday but something is obviously not right since today i had to get a jump and then died, and the belt shows no signs of wear or oil on it. Is the alternator bad or should i be looking at the voltage regulator as the culprit?