I'm having a problem where the N900 says 'battery empty' and shuts
down, then, after I wait a moment and start the phone back up, the
battery still has around 40% charge left (as determined by "lshal |
grep perc", which may be a Klingon command).

The batteries I'm using are oldish, but I doubt that alone is the problem.

This routinely happens when I'm browsing online - talk about annoying - though that may be a coincidence.