This is my favorite one from that site Finkster posted. So many people think that this is the case with alts.
Myth
Driving a car will fully recharge a battery.
Battery
Truth
The alternator is not a battery charger, but rather, a battery maintainer. It is true that a lightly discharged battery may regain full charge under normal driving conditions. However, a severely discharged battery must be recharged by an auxiliary battery charger. Failure to do so may result in damage to the vehicle's alternator and/or charging system.