When my family gave me the PRS-700 for Christmas, I was a little concerned about the battery life of the device since they had spent so much on it. I checked out the Sony reader support page and found this in the repair section:
[If the battery life still seems short, service may be required to replace the battery. The SonyŽ eService Web site at
http://eservice.sony.com/ can assist you in obtaining service from a Sony Service Center or locating a Sony Authorized Servicer.]
That led me to believe that it is possible to replace the battery. I wasn't able to find out how much it would cost, though. You have to fill out a form, etc. and I figured I had a couple of years before I needed to do it, so I didn't bother. I hope this helps!!
Kaz