A simple tool to give you Desktop Notifications about your battery, requires UPower.
$ upower-notify --help
Usage of upower-notify:
-critical duration
Time to start warning. (Critical) (default 10m0s)
-device string
DBus address of the battery device (default "/org/freedesktop/UPower/devices/battery_BAT0")
-report
Print out updates to stdout.
-tick duration
Update rate (default 10s)
-warn duration
Time to start warning. (Warn) (default 20m0s)
if you're using i3wm
or relatives, just chuck this or it's equalent in your startup script:
exec --no-startup-id "upower-notify"