(rms asked me to make sure his bug reports to the mailing list got into the bug tracking system)

From http://thread.gmane.org/gmane.linux.distributions.gnewsense.devel/1690/focus=1718

With an external battery, the system thinks it has unlimited power available, and uses it all fast.  We need a way to declare "pretend there is no external power".

This is now:

Bug 605235 -  No way to manually specify you are using an external battery pack

https://bugzilla.gnome.org/show_bug.cgi?id=605235

However I think there is actually some way to do this with the existing system - I noticed when I installed nutups (a UPS monitoring application), Gnome Power Manager suddenly had a new "on UPS power mode"; this mode is a lot like having an external battery (in fact they are logically equivalent, just different use cases).