Ford Fusion Fan Belt Problems

battery light comes on about 30 seconds after start up and stays on,car has had a new alternator and battery fitted, checked the 3 pin plug to back of alternator which is all ok and volt meter shows car to be charging ok. HELP!!!!!

Yesterday, having been driven about a mile, the battery light came on. Green Flag came out and said the battery was OK, the alternator was OK and the fan belt was OK, so it must be a "glitch" and low risk to drive. The diagnostic thingy i...