This is not necessarily a technical fault.
Edit - This link seems to address the issue http://forums.lenovo.com/t5/IdeaPad-Y-and-U-series-Laptops/Z560-Battery-80-available-plugged-in-not-charging-Energy/ta-p/309028
Check your PC manual, the Lenovo support site, the Lenovo folders in Start menu All programs & the BIOS for a battery utility. Several PCs have utilities with names like "Battery life extension" that will only charge the battery when its charge falls below some threshold such as 80%.
If you search this forum for battery not charging, you'll see several examples of this. Review the search results for ones concerning Lenovo PCs to get you to a solution more rapidly.