Ubuntu is not showing the notification if there is a critically low battery.
I already tried this reinstalling libnotify-bin notify-osd, but that didn't help.
I am using a dell Inspiron 15 3537 laptop.
Please note that by default battery notification should be enabled in your system. Try reinstalling the packages of NotifyOSD which was the source for notification alert in Ubuntu and then check the reported issue, If you have not already tried with NotifyOSD
Command to install NotifyOSD
sudo apt-get --reinstall install libnotify-bin notify-osd
Hope this helps!