As a Sony user for almost a year now, and a satisfied one, I have heard people complain about battery problems before. Anticipating a future problem I purchased a replacement battery for my 505 from a third party source. Having read through the tech manual it appears to be a 5 minute job with one small screwdriver to replace the battery so I thought it a good pro-active measure. But my battery life seems to just get longer and longer the more I use the reader. I was happy getting two weeks of daily reading (leaving the unit on in between reads) but these days I seem to be getting even more than that!
I think I paid about $25 USD for the battery. While Sony certainly should have helped you in a better fashion, rather than sell the unit, why not put in a new battery from a third party source. That way you aren't even dealing with Sony again and your reader is functioning properly.