Bad alternator?

[ShiftTheDrift]

I've noticed something lately where the voltage indicator drops significantly when doing something as simple as rolling up the window or turning on the headlights. Could this indicate a bad alternator.

The battery was replaced brand new 3 months ago.

 

[wytstang]

Mine does that but I have u/d pulleys. If you have a voltmeter while the car is running hook up the voltmeter to the batt. It should read atleast 14.2 volts that means the alt is charging.