Hello,
I'm a also a user of plasma desktop version 4.10.
I found out that removing the battery and reinserting it while the user is logged in actually works as expected (shows that the battery was removed and reinserted).
the problem appears when logging in without the battery (with AC plugged only) and then trying to reinsert the battery back to the computer actually does NOTHING to the battery monitor and it doesn't recognize that the battery was inserted - though the computer uses the battery as usual (when unplugging AC). 
As a result the system doesn't warn about critical battery charge and, well, after a while the whole work will be lost because of sudden shutdown.

I tried working this bug around by inserting the battery, logging out, logging in again, and here it is the battery monitor is back and fully functional.

Hope that helps solving the bug ;-)
And btw I've worked with lower versions of KDE and didn't have that bug...
Thnx

Reproducible: Sometimes

Steps to Reproduce:
(Steps 1,2,3 probably not necessary) 
1. log in with battery in laptop
2. remove battery while charging
3. log out the user
4. log in with no battery (only AC)
5. reinsert the battery
Actual Results:  
the widget doesn't show that the battery was inserted - shows that the PC is running on AC even when unplugging out the AC cable (the computer stays powered using the battery)

Expected Results:  
the widget indicates the the battery was inserted and show the remaining percentage of the charge (and other data)