Hi there! I wanted to get your advice first before I go and have it repaired, replaced with a surplus or brand new.

Here's my story: I left the car on idle while I go get something inside a store. However, I never thought it would take about 15 min. because there was a line. I didn't turn off the car because my friend was inside with the everything on (A/C, radio, etc.). And when I came back and drove off it was all fine until about a minute of driving, the whole A/C turned off, the radio and also the dash, but the engine was still running so I immediately pulled to the side. I called my mechanic and said it could be the alternator. He help me charge my battery to full, plugged it back in and drove the car to his shop.

In his shop he replaced the voltage regulator and the carbon something, I can't remember what it was called. However, when we turn on the car with the repaired alternator the voltages are fine even with the A/C on and it charges the battery, but after a few minutes or after the car warms up, the voltage goes down and the whole car just runs on battery as per my mechanic.

Is this a bad alternator or could it be something else or worse... both? Car is a Toyota Vios 2003 or Belta or Yaris in other parts of the world.

Thanks in advance for your time!