There is some easily available information on finding the status of a battery, or weather it\'s charging or not. (GetSystemPowerStatus API or System.Windows.Forms.SystemInformat
I would just get a UPS and programatically tell it to cut all power... most should have an interface for doing this. Otherwise, as someone already said - a computer-controlled power strip would do it ^^
I've actually played with this idea when I was testing/writing about way too many new laptop models a while ago and the battery testing was annoying to set up, monitor and analyze.
I wrote an app that would do exactly everything (setup, listening, measuring, reporting) except unplugging the power and then replugging it and starting the computer again...