Notifies you if your battery is below a certain threshold and isn't charging. I'm using it for xmonad

low_battery.sh

#! /bin/sh                                                                         
#                                                                                  
# Script for notifying when battery is low                                         
# Set it up as a cronjob with                                                      
#                                                                                  
#   $ crontab -e                                                                   
#                                                                                  
# put the following in your crontab file to check every minute                     
#                                                                                  
#   * * * * * env DISPLAY=:0.0 <absolute path to script>                           
#                                                                                  

# get percentage of battery left                                                   
PERCENT=`acpi | cut -d , -f2`
PERCENT=${PERCENT%\%}

# only notify if percent is under WARNINGLEVEL                                     
WARNINGLEVEL=10

# don't notify if battery is charging                                              
if [ $PERCENT -lt $WARNINGLEVEL ] && acpi -a | grep 'off-line'
then
    notify-send --urgency=normal "Battery low!" "$PERCENT% left"
fi