The light means that more electricity is being taken out of the battery than is being put back in by the alternator. If you know the battery is good then I would suspect the alternator. In my experience these usually last about 75k miles. You can check the alternator by measuring the voltage at the battery. With the engine running you should have 14volts or so, with the engine off it will likely be 12.5 - 13v. If the fault is intermittent check for poor connections to the battery and also the earth connections (battery and engine)
Richard, January 2009
