Fdpowermon is a simple perl script that installs an icon in a system tray compatible with the freedesktop.org specification.

Every three seconds, fdpowermon calls acpi to find out what the current battery level is. It will set the output of the acpi command as a tooltip text, and will update the used icon as appropriate.

In addition, fdpowermon can optionally call perl subroutines when the power reaches a given level. No such subroutines are provided or enabled by default, however.