2000 Dodge Intrepid Battery light on

My battery light in my dash board. Sometimes it doesn't come on until its been running for at least 15 minutes, then it comes on. I took it to the Autozone and have it battery check as well as the alternator. They said that the result was good and that the battery is good and the alternator as well. I don't know exactly what it is that cause to light my battery light on.
Could that be possibly the sensor light of the battery is out? Please, help.

Thanks,
sebastian