When I'm on battery, Windows7 randomly reminds me to plug-in AC because my battery is low (i.e. 8%). But 2 seconds before it was i.e. 60%! If I ignore it it goes back to 60% after a view seconds.
If I'm at low percentage (lets say 30%) W7 starts to sleep my W520 randomly because it thinks its close to 0%.
When I'm on AC and I have the battery plugged in W7 reports that I plugged off AC randomly. Again: a view seconds later W7 reports that AC is plugged in again. Of course I'd never touched the cableling or unplugged from socket and there was no power loss at my location.
I've also noted that I have a number of Event ID 15 events in my logfile.
The Event ID 15 warnings in the system log are probably written when it happens. This may be caused by some temperature / fan speed monitoring program, such as Everest, AIDA64, TPFanControl, etc. running in the background which conflict with the EC/ACPI. The solution is to avoid running such programs in the background.
Additionally, it may be that the battery pack has a failing cell or needs to be reset to re-learn the correct charge / discharge curve and more accurately predict chage levels. If the Event 15 errors weren't present, and the events were only occuring on battery, this would be a more probable cause.
Power Manager has a battery maintenance option. It fully charges the battery, fully discharges it, then fully charges it again. If the events are only on battery, try the battery reset.
While noted in relation to a W520 system in this article, these symptoms could be possible on other ThinkPad models as well.