This is a simple battery warning script. It uses i3's nagbar to display warnings.
Let this script run as a cronjob.
Open your crontab.
$ crontab -e
Add the following line to check battery status every minute
*/1 * * * * env DISPLAY=:0 /PATH/TO/YOUR/SCRIPT/i3batwarn.sh
in the cron tab (before calling this script).
I usually use my web browser and terminal in fullscreen, so I've added i3-msg to disable fullscreen and show the warning.
Added a pidfile where the pid of each (one max) nagbar is saved and a check to kill the nagbar if the battery is above the limit. This usually works but sometimes the nagbar has to be closed manually.