Hi Georg3200
the link is
http://wn.com/Replacing_The_Sony_Por...RS-505_Battery
I Googled for a replacement battery and found a number of sites selling them for a variety of prices including the guy who made this video and he includes the tools you need. Not sure which country you are in but he is happy to send it from the States to the UK. I managed with the usual mini screw drivers and being careful. It worked for me. Good luck!
The PRS 600 is much easier to do but then wouldn't it just be our luck to have the 505!