GM-GMC: Battery light staying on, cigarette outlet, auto parts stores

Question
My battery light came on today an would not go off.I have a 1999 GMC Jimmy.My battery is about 3yrs old and recently I have been using the cigarette outlet to charge my phone Also my SUV has a feature that when you turn the car off, you can still play the radio for up to 20 min and I often use that but when the 20 min are up I turn the car on then off again to use another 20min.Could this be the reason my battery light has appeared?
Also I recently had to replace my alternator a month ago but everything has been fine since and know this.. What does it sound like it could be?? **PLEASE HELP**  

Answer
Your radio should not cause this problem, unless you have one of those "road thumper" systems that use a lot of amps.  They can run a battery down and put a very high load on the alternator.

I think your new alternator has stopped working correctly.  If you have access to a volt-ohmmeter, set the scale to 20 volts DC and measure the voltage at the battery terminals with the engine idling.  The meter should read between 14 and 14.4 volts DC.  If it is lower than that, the alternator needs to be removed from the vehicle and tested.

If you do not have a volt-ohmmeter, many auto parts stores will test your battery and alternator for free right in their parking lot.