% sudo shutdown nowThe password required is that of your account. At the end of shutdown you are in single user mode. Now you can wait until the battery will completely be discharged. You can accelerate the process typing fsck repeatedly.
When the computer turns off, put it in charge mode again. It may be that using this technique more than one time can increase the battery duration more and more. Sorry for my bad English!
[robg adds: I'm sure my [insert any non-English language] is worse than Marco's English! A friend points out that you can replace the fsck with yes > /dev/null, which simply eats processor time, as described in this hint.

