I read in this answer that to set the configuration to suspend we need to type:
gsettings set org.gnome.settings-daemon.plugins.power critical-battery-action 'suspend'
However I want just to turn off the screen. The reason is that I believe the battery meter is broken, but the battery has power to last about one hour.
Meanwhile I'll simply disable any action when the battery is critically low typing:
gsettings set org.gnome.settings-daemon.plugins.power critical-battery-action 'nothing'