It's FOSS - https://itsfoss.com/display-battery-ubuntu/
Brief: This quick tip shows you how to display battery percentage in Ubuntu 17.10 that uses GNOME desktop environment.
One of the radical new features in Ubuntu 17.10 is the introduction of GNOME as the default desktop environment. And since it is a whole new desktop environment, finding small things like displaying battery percentage on the top panel can be tricky.
In Unity, you could show battery percentage of laptops from the system settings itself. However, that’s not the case in GNOME desktop used by Ubuntu 17.10. You’ll have to use a dedicated tool like GNOME Tweaks. Let’s see how