OK, here's what I did:
Car just sat for a month. Put a voltmeter on it: 12.4 volts. Turned on headlights, down to 12.05 volts. Reved engine with lights on, up to 12.25 volts.
So I then changed to a somewhat smaller pulley: Started up at 12.3 volts, lights on, back to 12.04 volts; reved...