This is by design on some Lenovo laptops, including yours. Charging your battery to about 50% will increase its lifespan compared to a full charge. You should keep this setting if your laptop is on mains power most of the time, and when it's not, you don't need it to work on battery for a long time.
In order to disable this setting:
- run Lenovo Power Manager (usually a double click on the battery icon will do)
- switch to advanced mode and go to Battery tab
- click "Battery Maintenance..." button
- select "Always fully charge" option
If you don't have or don't use Lenovo Power Manager, you should be able to set charging thresholds via the registry. For Windows 8, the procedure is described in the following topic on Lenovo forums:
Find the registry key
You should see 4 entries that are currently set to 0:
The first 2 entries,
ChargeStartPercentage, change the starting charge threshold.
The second 2 entries,
ChargeStopPercentage, change the stopping threshold.
The values of these entries are logical values and decimal percents, respectively. For example, to start charging your battery at 40% and stop charging at 80%, you should set
ChargeStopControl to 1,
ChargeStartPercentage to 40 and
ChargeStopPercentage to 80.
Don't forget to restart your computer to make the changes effective.
Be aware that this method does not seem to work for all users.