So ... got out my 20 year old MZ-R900. Works perfect other than not charging battery. Tried new battery, no better. Tried using a strip of copper wire ...ta-daa! working. One thing I noticed on my device is that there is a crack in the battery cover that means there isn't full pressure on the battery terminal. I cleaned the copper terminal, cut a 10mm piece of copper wire and bent it into tight U shape. Then hooked it under the spring section of the terminal in the cover (so it stays in place). Now it charges!