When the alternator warning light comes on it's an indicator that something is going wrong with your charging system which could be either the diode in your alternator or even the alternator itself.
When your alternator warning light does come on you need to get your alternator checked out because the alternator warning light usually means that the alternator is either about to go out or has gone bad already.
When your alternator goes out your battery won't be able to charge back up and your cars engine won't be supplied with the proper voltage it needs to run.
Never ignore that alternator warning light because you could end up stranded somewhere.