I met the same problem in a bought 180c, which in due course I was able to estimate had been left for more than ten years with the battery in it, but not on charge.
Tomlee59 saith sooth. There is no remedy (because the case halves will not come apart while the battery is inserted) but to 'winkle' the battery to-and-fro and inward to break up what you should pray is not massive, battery-induced corrosion of the inner metal RF shields, battery terminals on the logic board, pins of ICs on the logic board and daughter card, and ... need I depress you further?
If the boards have suffered, you may be able to clean them carefully with water containing a very little vinegar (acetic acid), but the battery contact posts may be beyond salvation. We shall all close our eyes and wish very hard for you.
de