This program shutdown computer when low battery is reached. Found it safer that relying on some window manager setting.
If necessary, install psutil: sudo pip install psutil
Copy script where you like, for example in /usr/local/bin and set it as executable: chmod +x /usr/local/bin/lowbatt-shutdown.py
Test program as root to ensure it can access battery value. It should display battery level.
Add program to crontab to run every minute: run crontab -e and add this line:
* * * * * /usr/local/bin/lowbatt-shutdown.py
Change this setting to your liking
LOWBATT_SHUTDOWN = 20