I had exactly the same problem for many years and nobody was able to find the solution (to be most precise they only offered expensive solutions such as replace the alternator etc) until I took the car to a car electrician with brains.

He found out two problems :

1) Although the output of the alternator was OK, the current did not reach the battery. Solution : he connected an extra cable from the alternor to the battery terminal.

2) There was a leak from circuit 13 I believe which is the internal lights circuit. After investigation the culprit was a bad connection of the afternmarket alarm/immobiliser to that circuit. He pinpointet that circuit by connecting the battery to the ampmeter and removing one by one the fuses.

Since then I had NO problems.

Hope this helps