Join Date: Jun 2006
Location: Eastern ON Canada
Car: 07 OBW 2.5i D-4AT
Feedback Score: 11 reviews
Not unusual for alternator regulators to develop an intermittent problem. If it works now, it might not work later. But I guess you could watch it for a while to see if the symptom reappears.
Back to the disconnected battery. The alternator could be applying higher voltage to the rest of the car's electrical system -- something in the range of 17-24 V, or more. That could damage critical functions, as well as stress simple things like light bulb filaments.
The alternator regulator works by controlling the current through the alternator rotor winding. As the rotor turns, the magnetic field generated by that current passes over the field winding, which generates an AC voltage and corresponding current that is connected through rectifying diodes to the alternator output terminal. The voltage from the field winding itself is not controlled; instead, the regulator turns the rotor winding current on and off fairly rapidly, with the on times longer when the battery is not fully charged, and shorter when it is. This results in a pulsed output from the alternator that goes between zero and somewhere above ~17 V. The pulses can be accompanied by spikes of voltage far higher. When the battery is connected, as was mentioned by others, the battery smooths out the pulses by absorbing the excess energy being output by the alternator. When the battery is disconnected, the pulses, and spikes, are not controlled.
Disconnecting the battery is not a way to test the alternator. A simple multimeter would be adequate to check if the alternator is working.