I have a 2003 Ford Focus SE. 4 Cylinder. The battery light has been coming on for a few months, it will come on, and go off. I can hear a difference in the engine when the light comes on.I took it to a auto parts store and had them do a diagnosis on it, the guy said something about the diodes not charging/holding a charge. Last weekend I took it to a mechanic who did another diagnostic test and said the battery is fine, the starter is fine, and the alternator is fine. They were unable to tell me what the problem was because the battery light wasn't ON when I brought it in.I've had the car about 7 years and i've never changed spark plugs or O2 sensors, which are things the mechanic said it could be. Can you shed any light for me?