Batmon is a simple Python script that monitors battery usage on Linux and sends desktop notifications when the battery is low. It's intended to improve the UX when using window managers like XMonad that don't bring all of gnome's conveniences. It uses notify-send to send notifications to the desktop.
pip install batmon
Next, find your battery with ls /sys/class/power_supply/. Usually it's BAT0.
Then, in your .xmonadrc or similar, batmon [BATTERY] &. That's it!