2006 Ford Focus Battery Light On and Off?

I have a 2006 Ford Focus and the battery light has been coming on and off for the past 8 months. I'm going on a long road trip so I feel I really need to address this issue. I had my battery, alternator and started check and they are all fine. Also the light comes on for what seems like a every 20 minutes or so for a couple of minutes. Another thing is that I feel like it starting happening after getting an aftermarket stereo installed.

If the aftermarket stereo included an amplifier call the maker of the amplifier and ask them how many more amps the alternator will need to operate all the aftermarket toys.

There's your problem! People always have to install aftermarket junk. Remove that!