Ive had the same prob on my 2001 1.8 zetec petrol ,ive traced the wires back to the ecu plug ,done continuity test on the wires and all were ok,all that the wires do that come from that plug on the alternator is for the ecu to read the out put voltage of the alternator ,and the ecu puts the battery light on or off , my light comes on intermitant ,the prob is there fore in the ecu ,i know all this cos i contacted the diagnostic doctor in a certain car mag ,there are loads with the same prob ,i got fooled by it and fitted a new alternator at first (110 pound down the drain) so long as you know your battery and alternator are ok you could just live with it ,my light has gone off and not been on for few months ,but other times it flicks on and off ,i dont know if there is a fix for thr ecu ,hope this helps (14+volts at the batery with engine running is correct).