Plasma 5: systray battery monitor incorrectly says "plugged in"

I am having an issue recently with Plasma 5 where the system tray battery icon and tooltip indicate that the battery is plugged in when it is actually discharging. However, when I actually click on the icon, the Battery and Brightness dialog correctly says that the battery is discharging. I haven't seen this before, but this is a new (to me, actually old) laptop, a Dell Latitude E6540. I'm not sure if the problem is intermittent or not.
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