Ever happened to be focused on something and miss the fact that your laptop is running out of battery? And to lose your work?
Yesterday happened twice! "Really? Hmm... I need to fix this as soon as possible."
I googled a bit and this stackexchange post  popped up. Nice, great! But being notified even when you're charging your laptop it's not nice and I had to change the solution a bit:

cat .notify-send_setup

#!/bin/bash
touch $HOME/.dbus/Xdbus
chmod 600 $HOME/.dbus/Xdbus
env | grep DBUS_SESSION_BUS_ADDRESS > $HOME/.dbus/Xdbus
echo 'export DBUS_SESSION_BUS_ADDRESS' >> $HOME/.dbus/Xdbus

exit 0

cat .battnotif

#!/bin/bash
export DISPLAY=:0
XAUTHORITY=$HOME/.Xauthority

if [ -r "$HOME/.dbus/Xdbus" ]; then
    . "$HOME/.dbus/Xdbus"
fi

battery_level=$(acpi -b | grep -P -o '[0-9]+(?=%)')
STATUS=$(acpi -b | awk '{print $3'} | sed -e 's/,//g')

if [ $battery_level -le 15 ] && [ $STATUS == "Discharging" ]
then
    /usr/bin/notify-send -u critical "Battery low" "Battery level is ${battery_level}%!"
    echo 'batt low' >> $HOME/cron.log
fi

echo 'ran batt' >> $HOME/cron.log