Thanks very much Pete. After waiting about 2 months for mine to charge spontaneously, I finally opened up the unit, used a 5v 1a charger for 5 sec on each battery (still connected within the unit). Worked like a charm. It is now charging for the first time in its life -- up to 4% after ~10 mins. Again, THANK YOU for a genuine solution for my formerly depleted NOS keyboard.
Update: The unit fully charged that same day and functions now as it was originally intended.