that is, after some... years... of my oold faithful keyboard telling me 'internal battery low' - i haven't used it that much anymore, indeed - and a couple of previous (failed) attempts, today i tried again to remove the backpanel (lots of screws!). Such operation never seemed easy, but today the bottom plate slipped away in a second, and voilà, all the circuitry was looking at me.

 

Now i'd just need a new battery... (until 2020)