Credit: @9bots a million thanks for the battery chafing discovery! I can see the same effect though my battery still works. Battery problem: the battery cover chafes the battery. Image 1 shows the same damage described by 9bots, and image 2 shows that in addition my battery pack shows wear in all the places where it has touched the ridges in the casing. Image 3 shows my solution: I glued strips of an old mouse pad on top of all the ridges. And taped the worn places on the battery pack.