Shows correct information upon startup, then no longer updates whatsoever. After a bit of research, it seems probable that it's because the plugin is trying to read it from /proc/acpi, which is no longer the location to look at (it's /sys/class). Note that acpid and acpi are installed and enabled, and acpi -b shows correct information.
A quick reading of the source code shows that it does at least try to use /sys/class/power_supply entries: (line 197 of http://git.xfce.org/panel-plugins/xfce4-battery-plugin/tree/panel-plugin/libacpi.c#n197 ) It's possible that this code is simply broken, but I would suspect that something else is wrong here. Unfortunately, I don't have any insight on what that might be, I just looked into it a bit because I was curious. Something to check might be to see if the entries in /sys/class/power_supply exist and are working. (on my system, /sys/class/power_supply/BAT0/charge_now )
It is working. I checked and procfs has no such entries, while sysfs does. Moreover, acpi uses sysfs, so if acpi shows correct information, it should be fine.
Additional information: I moved over to gnome3, their battery widget works fine, so this is definitely an xfce issue.
Report this to http://bugzilla.xfce.org/ and provide the link to the bug here, in the "See Also: " field
