If the battery has enough charge to start the car the alternator should keep that light off even at idle. I would check the battery voltage, it should be about 12.6 with the engine off. With the engine running it should between 13.2 and 14.5 volts. If you don't have a multimeter you can display the voltage on the Odometer display. Go to the FAQ & DIY section of this forum and look for an article "How to show RPM, Fuel Level, Speed and more on your Odometer Digital Display".To clarify. The battery light comes on for no more than 2-3 second at slow speed or idle. Has done this about 10 times today on a trip into London. When the car is cruising along it does not illuminate.