Plasma notifies you if you have a broken battery at startup. This has regressed recently causing the notification to appear at random times (not during the startup) and also more than once. Reproducible: Always I think this is the commit that broke it: http://quickgit.kde.org/?p=powerdevil.git&a=commitdiff&h=b13feb5eccfadf6456105353dced0784d4e248c8
Could it be that kded crashed? Are you using an AMD graphics card? I haven't experienced such a message, although I changed the threshold to 98% for debugging so I would get those notifications.
Sorry after my exams are finished I will get back to you. (my main system is kde 4 and I need to recompile everything kf5 to test)
I have been using it for a couple of days and I can't reproduce.