Despite pretty impressive battery life - which is btw *nowhere* in the range of advertised 7500 page turns - over period of few months I have noted that the charging of the reader behaves kind of funny. Sometimes, even after being plugged-in for 24 hours, the battery indicator stubbornly stays on two squares out of four. Other times, right after connecting, it jumps right to full charge.
It is hard to really measure these things as the battery depletion is very slow on reading and I do not really listen to music on this device (that's what iPods are for). I am not the only person who noticed this behaviour - Steve Gibson and Leo Laporte, who enhanced my favorite podcast Security Now! to be an unofficial eBook Sony Reader news exchange :-) have seen the same thing (search for Sony). Btw, Steve and Leo - thanks for doing the eBooks plugs - I appreciate that a lot ...
One possible explanation is that the problem is more in indication, not in charging. The software running the Reader (Linux) may get confused about what is the actual state of the battery - because even if the charge shows one half, it does not seem like the battery was indeed half empty.
Maybe what would help is to create few charge-discharge cycles by running the MP3 player which should use the battery in matter of hours, not days. I will try it out when I get to it - unless the next software update fixes the flaw (if it indeed is a software problem).