Here is the Crontab that I use
Notify if battery is full (equal to 100%)
*/1 * * * * if [ $(cat /sys/class/power_supply/BAT0/capacity) -eq 100 ]; then XDG_RUNTIME_DIR=/run/user/$(id -u) notify-send Hey "Battery Full"; fi
Notify if battery is low (less than or equal to 10%)
*/1 * * * * if [ $(cat /sys/class/power_supply/BAT0/capacity) -le 10 ]; then XDG_RUNTIME_DIR=/run/user/$(id -u) notify-send Hey "Battery Low"; fi
Example Notification Image: