What is the meaning of BAT○ in btop++?

On top (of the default layout) I have the CPU graph box. The title line of this box says on the left : "cpu", "menu", "preset 0", and on the right : BAT○ 98%, and 2000ms. Everything is self-explanatory, except BAT○. Clicking it does nothing. I noticed BAT and ○ don't have the same font. An extensive search of the web and of their github returned nothing. There is no man page. The included help is not searchable. After extensive searching, I suspect this may be related to the bat command (alternative to cat), or this may be the name of the font in use, maybe specific to my system. btop aka btop++ is an alternative to htop