As a follow-up from how fast does the alternator charge the battery?, how powerful are alternators supposed to be at idle?

Specifically, if I have a lot of OEM accessories in my 2008 Jetta 2.5L SE, that are all turned on (e.g., heated seats, AC, defroster etc), or, if it's really hot and humid outside, and I have AC running at full speed, and the engine sensor is so hot that it turns on the engine cooling fans at full speed, too, and voltage immediately drops and stays below 13.2V (e.g., below 2.2V * 6), but is still above 12V, is there a problem with the alternator/voltage-regulator (mine's a combined part), or is that by design? (When the car is just turned on, or the power-hungry accessories are turned off and the engine cooling fans aren't running at full speed, then the voltage is between 14.4V and 13.5V, depending on how long the car has been running.)

2 Answers 2


Alternator output is dependent on the speed of the alternator (which will be proportional to engine speed, but usually faster). [This source][1] states that a "typical" alternator has a minimum speed of around 2,400 RPM, max output at around 6,000 RPM, and a never exceed (redline) speed of 18,000 RPM.

IIRC that engine redlines at 6,500 RPM, which means that at idle the alternator is probably turning a bit below 2,400. Based on that the behavior you describe sounds pretty normal. You should see the voltage climb as the engine speed comes up to normal operating speeds.


From my experience the alternator does not necessarily start with the engine immediately because it drains power while cranking.

It can ramp depending on the outside temperature the current battery charge and so forth. So by design alternators may not run at full speed immediately after start and the vehicle components may be starting prioritized.

There are requirements at high class cars that the radio and headlamps shall not flicker while cranking, therefore other consumers must be rergulated... if there is no second battery.

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .