Battery Monitor for Ubuntu 20.04
I have Ubuntu Budgie 20.04 and this worked for me:
cd /etc/UPower sudo nano UPower.conf
If you set UsePercentageForPolicy=true then edit the percentage lines to your liking, such as:
If you prefer a time based approach then set UsePercentageForPolicy to false and set the time notification options to your liking, such as: TimeLow=1200 TimeCritical=300
Hit ctrl+X to save the UPower.conf file and close out of nano editor.
Reboot computer or
sudo systemctl restart upower for changes to take effect.
By doing this I was able to get the low and critical battery notifications. The pop-up notification only displays for about 2 seconds. Here is what it looks like: