sudo apt-get install xbacklight

#!/bin/bash

lowBatLevel=25 #battery percent considered low
desiredBrightness=25 #brightness level to lower to ( 0 to 100 )
batLevel=`upower -i /org/freedesktop/UPower/devices/battery_BAT0 | grep percentage | awk '{print $2}'  | sed 's/.$//'`
curBrightness=`xbacklight -get`

if    [ $curBrightness -gt $desiredBrightness ]  && [ $batLevel -lt $lowBatLevel ]  ; then #change 25 to your desired battery %
xbacklight -set $desiredBrightness
fi