I've searched through the Service Manual, and there is no mention of a battery, aside from the six removable batteries that can be used to power the keyboard instead of using the power adapter.
By the way, if you're using batteries to power the keyboard, make sure they're fresh batteries. And make sure you remove all batteries from the keyboard if you are using the power adapter to power it.
What operating system does your computer use? Do you have the Yamaha Musicsoft Downloader program installed? If so, what does it show when you look at your keyboard's system memory and internal storage?
Have you tried clearing the backup memory (system memory), as described in the manual? Use Musicsoft Downloader to save a backup of the system memory first, in case it contains any registrations, User Songs, or User Style that you want to save. Also, I'd be interested in looking at the backup file to see if anything looks odd about it.
Sometimes clearing the backup memory will resolve an issue, but sometimes it's also necessary to clear the flash memory (internal storage). Again, use Musicsoft Downloader to transfer any song files or style files that are in the flash memory to your computer first.
If the Backup Clear and Flash Clear procedures don't resolve the issue, you can run the keyboard's built-in test program to see if it detects any problems. The test program is described in the Service Manual.
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