The charge controller circuit board is fastened to the frame with two small screws and two pins at the sides. Remove the screws with the Philips #000 screwdriver. Carefully press fixing pins to release the board.

Battery is glued to frame with weak glue. Use prying tool or spudger to loosen the battery from the frame.

Pay attention to temperature sensor which is attached to one battery side with kapton tape. Don't tear it off on battery removal.

After component removal from the frame, the failure cause became clear. The negative battery terminal was torn off of the controller board. It may have gotten stressed on assembly and tore later on impact from dropping onto pavement or other hard surface. The battery was measured at 4.1V (nearly full).