So my 2002 fiesta 1.4 petrol with 40,000 miles recently through up a battery light, it would come on and off and on and off. Altenator was checked and was fine so I assumed the battery was faulty, but as the car was driving perfectly I drive thousands of miles with this light coming on and off and nothing happening :) I then noticed one night my lights were dimming and flickering, this continuedoes for about a week, and one night I went to get in my car after work and the battery was completely dead. So I replace the batter the next morning and it starts perfectly :) Since then I have had absolutely no flickering or dimming and the car still drives like a dream BUT no matter what I do the battery light will not go out, it is on 100% of the time, I put it on a computer and deleted it and it comes on again when I start the engine, I'm stumped cause the car drives perfect.. and ideas? 

Thank you :) 

It could still be the alternator intermittently failing.  Just because it was working fine when they did the test, doesn't mean it isn't faulty when the light comes on.