CVO 9/16/2012That means your alternator isn't charging the battery. A battery in a car's purpose is to provide 12 volts at the time you turn your key and fire it up, after that the car is reliant on the alternator to maintain a constant 13 to 14 volts. If you have a bad alternator as it sounds, then that light will come on because the car knows it's losing voltage.
Local auto parts stores will test this vehicle charging system for free of charge.
Note: a 3/4 charge battery is 12.50 volts
A discharge battery is 11.75 volts.
A battery under a cranking load and it is reading less than 7 volts,it will be a recycled ( junk ) battery.
HouseCallAuto 9/16/2012In most cases it means that you have to replace the alternator. Have the charging system checked. Put a voltmeter across the battery while the light is on and with the engine running. The volts reading should be in the 14 volt range. If it reads 12 then the alternator is junk. If it reads correct voltage yet the light is still on, sometimes a cause is a wire that comes from the alternator is grounded. And in some cases the alternator is still the cause even when the charging voltage is correct.