Comment 3 for bug 1231550

so its probably related to the fact that I've explicitly told the battery not to charge while plugged in.
Thats accomplished by:

stop=80
start=40
bat_d="/sys/devices/platform/smapi/BAT0"
sudo sh -c "echo $stop > $bat_d/stop_charge_thresh && echo $start > $bat_d/start_charge_thresh"

More info at
http://www.thinkwiki.org/wiki/Tp_smapi
and that requires tp_smapi module loaded.