Alternator

Use an inexpensive digital voltmeter to measure battery voltage while the engine is running. It must be between 13.75 and 14.75 volts.

I would just add that under no circumstances remove the battery terminal while running the engine, some trey this old method to test the charge rate, but on EFI cars you can do serious damage as when striking back the terminal you can cause a voltage spike that can fry electrical equipment, do as caradiodoc says, alway test with a voltmeter.

Mark (mhpautos)