Occasionally, when logging into Plasma5 on Mageia 6, PowerDevil fails to initialize and KDE's Power Management services are rendered inoperative (i.e. no battery icon on the system tray and opening the respective component in System Settings presents a nasty error message).
I couldn't verify one way or another if upstream is even aware of this issue or not, but the folks over at NixOS certainly are and their Thomas Tuegel offers the following simple workaround:
"One way to mitigate this temporarily would be to disable session saving. In System Settings > Startup and Shutdown > Desktop Session choose Start with an empty session under On Login"
I've been using it for several days now, and it does seem to do the trick.
To read the full NixOS bug discussion and analysis see here. I've also filed this (with the same text) as Bug 21773 on our own Bugzilla.