2002 Ford Focus with flashing battery light but not alternator or battery?

I need help with my 2002 Ford Focus SE with 130k miles on it. For the past 4 months the battery light flashes randomly while driving. The flashing and frequency of flashing doesn’t seem related to anything I can tell including temperature, length of time the car is running, speed or any variable. Seems like it should be an alternator, but that was replaced less than an year ago and it also has a new battery. The alternator always checks out fine at the mechanic and it will never flash for the mechanic. It went 1 month with no flashing and suddenly it will now flash maybe 20 times in a 5 mile drive. Yesterday it just died at a stoplight with no warning, but then after shutting everything off it started just fine and the battery light didn’t turn on for the 2 miles to home. Any suggestions that either I can look at (reasonably handy, but not much car experience) or tell my mechanic to look at?

Check the battery terminals for corrosion. Disconnect the wires, clean the connections, reconnect them. Then do the same to the other ends of the cables.

Also check the cables themselves, by checking voltage drop across the cable when trying to start the car. Voltage should be a few tenths of a volt. Then check the cable from the alternator to the battery.

Get something like this at an auto parts store and see what the voltage is doing while driving:

Don’t forget the ground cables. I don’t see where you live, but it’s very possible they need to be disconnected, cleaned and reattached. There should be at least one connecting the engine and the battery to the frame.

Is the light “flickering” as it would with a bad connection or does it do a steady on and off flashing as it would if the computer were sending a warning…

Another thing to check would be that the belt tentioner is working properly.

I had one two weeks ago that was so bound up it would not put enough tention to keep the belt tight. Loose belt means the alternator is not working 100% of the time.