Why does my battery light stay on all the time in my audi a3?

hi , i hav a audi a3 1.8t sport 1998, last week i replaced the battery with a new one from halfords and was ok til i did 10mile then the battery light came on and has say on!! i was driving the day and the light is still on and them the ABS light is now on and a big warning light (!) has came on. can any one tell me why this is? And what i must do to fit it. many thanks.Why does my battery light stay on all the time in my audi a3?well it might need to be reset is all but have you had the charging syystem check... only real way of knowing if you needed a battery or alternator ... belt might be loose ...it check... go auto zone have it scan they will help you out