The BMW alternator is the pumping heart of your electrical generation system on your vehicle. Without this part functioning properly, your battery will only carry the electrical load so far before the engine ceases to operate.
Most BMW owners are aware that their car runs on a 12 volt, positive ground electrical system. What they usually are not aware of is that their alternator produces from 13 to 15 volts when working properly. This is engineered on purpose so no matter what the load is on the engine from the drivers input, there will be sufficient electrical energy available to respond appropriately. It will also help charge the battery of your vehicle faster than if it was just 12 volts.
The testing of a BMW alternator is a simple procedure. A meter has to be used with one probe on a ground and the other on the output wire of the alternator. At idle the voltage should be near or just below 12 volts. When the engine is throttled upwards, the voltage will increase until it plateaus. If this reading is not above 12 volts to at least 13, then the replacing of the alternator should occur.
For the BMW owner that is worried about the excessive amount of voltage their alternator is generating, no damage will occur. This is because of the voltage regulator will control the amount of voltage that enters your car's electrical system including the wiring harness.
Another simple test of your BMW's ability to generate electricity is to turn the light on. If upon throttling the engine up the lights dim, then you should consult with a technician to determine the source of the reduced electrical input your vehicle is experiencing.