In any case:
https://github.com/koreader/koreader/pull/1630
As for the wonky stalls/jumps to 100%, as @tshering confirmed, that's on HW quirks; the battery percentage in KOReader's top menu is always refreshed on demand.
If you want a pretty lightning bolt icon, enable CRe's full top statusbar, it has a graphic battery meter that switches to a lightning bolt when charging.
Personally, I like knowing the exact charge, even when charging, so I went with a simple '+' prefix when charging instead of switching the percentage value to a simple "Charging" string.