why does my battery light come on?

Most probable cause is the altenatore is not putting out sufficient voltage. You will need to have the alternator rebuilt or purchase a rebuilt/new alternator. When replacing the altenator it's a good idea to have the battery tested as well. If the battery is near the end of its months rating I would replace the battery. The reason I push for battery replacement is if the battery is bad and is in need of constant charging the altenator has to work harder causing premature failure.