That's what I was saying about trying to turn the pulley by hand with the belt off. You WILL feel the resistance, which I did not say is the magnetic field, soory for not being totally clear on that. Sorry, but that's wrong. Using a scan tool, I can read the amount that the computer is telling the alternator to output, and it WILL change. Care to guess how many amps those little headlight bulbs pull? It's usually 45-55 watts each, divide that by roughly 14 volts, which is what the typical charging systems maintains, so 90/14 is about 6.5 amps. That doesn't seem to be a lot, but on many newer cars, just running the engine pulls 30 amps, add the heater blower, radio, and other stuff, you're getting close to the capacity of the charging system really fast, therefore the headlights DO put more of a load on it, as it will be noticed more when the system is already loaded! Dual batteries and higher output alternator, along with monster amounts of torque in the lower RPM range. It won't drop noticeably A new alternator will usually say to NOT use it to charge the battery, to make sure the battery is fully charged before starting the vehicle, to prevent damage to the alternator. The high load, all at once, upon startup will overheat the thing, killing the diodes or regulator.