Create a battery report to check the status of the battery. Losing that amount of charge overnight may indicate that it is failing to hold its charge and needs to be replaced.
When you have the report compare the Design Capacity value against the Full Charge capacity value. With a good battery they should nearly be the same.
If the battery is failing/faulty here’s the Ifixit HP Compaq Presario CQ56-110US Battery Replacement guide that shows how to remove/insert it. Ifixit sells replacement batteries. Just click on the Parts link in the guide above.
Have you checked that the cd drive works OK i.e. opens and closes when you press the eject button and that it still reads discs OK?