Not entirely true. Your alternator takes some energy to spin. This is called parasitic loss. If you have ever had a car with a dead battery you will notice that when it starts it is working slightly harder. This is because the alternator is working at max to bring the batteries up to voltage, as well as to supply power to the ignition coil to keep the car running. On small 4 cylinder engines this affect is more dramatic. Sometimes when the alternator is working really hard it will cause the belt to chirp or even slip and squeal. Some cars have a circuit that cause the alternator to free wheel, as well as the AC clutch when you punch it. Minimizing the parasitic loss. On a lighter note, I love Dave's videos.