It could be that when the intelligent charger shuts down, the 505 is sending current fromt he battery back to the charger. I have seen this happen with my 505 once when I left it plugged into the USB after I told the computer to disconnect it.
Try charging your 505 via USB again just to make sure it really is your AC adapter. But if you find out you really do need a new battery, go here and get one.
http://www.batteryship.com/htmlos/ht...item=LIS1382-J