I have a 2006 V6 mustang. 130k miles on it. This is my first post on here, and if this is in the wrong forum or something I am sorry. My battery light came on a few weeks ago, but it would just come on for like 15 seconds then go off. I went over to Autozone and they tested it and said alternator and starter were fine, but battery needed a charge. So I charged my battery, and the next day it came on again. Well eventually battery completely went dead. Luckily it was under warranty and I got another battery. I got it yesterday and battery light came on again for 15 seconds. It makes me think my alternator is going bad, but I am not sure since it tested good at the store. Thanks for anybody that can help.

 

Yes, when the battery light comes on, it's telling you that the voltage is low, usually from a failing alternator. Test it yourself with a simple volt meter. Should be approximately 13.0 to 14.0 at idle.