Well, I don't know anything really about sony's laptop bateries, but I do have some experience with Apple's older batteries doing similar things. Basically they stopped charging, or acted like they charged but only held a few minutes of juice. This was a known issue on one of the older line of G3 laptops and IIRC there was a program to reset some sort of memory in the battery that would usually fix the problem. I wonder if Sony has something similar or if you could just deep cycle the battery (hook up a 12volt bulb or two to it overnight) to discharge it completely and see if it resets.

-Mike