Hi,
1. Let us know if you have any trouble finding the settings, but check the Windows Advanced Power Options to find out what the settings are for your Low Battery Action/Notification as well as Critical Battery Action/Notification and set them accordingly (if you notice the system powers off when your battery reaches 15%, set the warnings and actions to occur at a higher battery level).
2. Consider contacting your local ASUS support about either replacing the battery or sending in the notebook for repair especially if you are still under warranty.
http://support.asus.com/Hotlines.aspx?SLanguage=en3. If you can get your hands on another S46C notebook or battery, consider swapping the battery between the 2 units to find out if the problem follows the battery or notebook.
Good luck!