Hi,
Had this odd problem for a few months. I think the battery level voice announcement has somehow got confused with disk, or something. When the battery hits 50% I hear "disk size 50%" instead of "battery 50%". When it hits 25% I hear "disk size 25%", and so on. The disk capacity is not changing, only the battery is running down.
The problem persisted even across an upgrade to the latest stable Rockbox version last week.
Jim.