It isn't actually a 'battery' light, it is a charging indicator light and as such usually indicates a fault with the alternator or the charging system. It may appear to be charging OK at present, but might indicate a developing fault that would require an alternator repair/replacement in the near future. The varying brightness and flickering could suggest a poor electrical contact or a loose or damaged wire.
I would start by checking that the connectors on the alternator are secure and making a good contact, and check that the battery earth straps are also secure and making a good contact (ie. not loose or rusted) where they attach to the inner wing and bonnet landing panel. Also check the power and earth lead connections to the starter motor.