BMW Repair: battery symbol, car battery, alternator

Question
what does the battery symbol mean?

Answer
Iris, the battery symbol means that the car is detecting an undervoltage condition with the engine running. Ironically the battery light is usually cause by a bad alternator not charging the battery and maintaining enough voltage for electrical system function. A fully charged  12volt car battery will have 12.65volts. The system should be charging between 13.0-14.5volts with the alternator spining (engine running). The light doesn't usually come on until 11volts is reached. Maybe more than you wanted to know but more is better right?
-Evan