Currently the only options I can see with the battery plasmoid are to always show charge information or never show it except on mouseover. Probably the most ideal options which is surprisingly missing is to have it show charge information *only when discharging on battery*. Does this option exist, perhaps as hidden in a plasmoid configuration file to be added?

As far as I know, no hidden option exists. You may wish to file a feature request at bugs.kde.org.

Done. https://bugs.kde.org/show_bug.cgi?id=244283