Bionic: Block Notification: laptop Battery Critically Low
I found these commands did the trick:-
~$ dconf write /org/gnome/settings-daemon/plugins/power/use-time-for-policy false
~$ dconf write /org/gnome/settings-daemon/plugins/power/percentage-low 0
you can check the dconf edit worked like so:-
~$ dconf dump /org/gnome/settings-daemon/plugins/power/
[/]
percentage-low=0
sleep-inactive-ac-timeout=3600
sleep-inactive-ac-type='nothing'
use-time-for-policy=false
@vanadium provided the correct keys and values to update. The keys were missing from my dconf but adding them seems to have solved the problem.