On a more serious note, and more in line with the topic, the only info I can add is that Yamaha stopped using battery back-up around 1997 with the PSR-8000.
Perhaps other manufacturers started about the same time, or even before?
Ian
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Yamaha Tyros4, Yamaha MS-60S Powered Monitors(2), Yamaha CS-01, Yamaha TQ-5, Yamaha PSR-S775.