One of three things. First your battery can be bad. Alternator can also be defective or have loose connections. If the charging system is working properly, battery's up to snuff and all connections solid and the battery still goes dead overnight you've got some sort of power drain from some defective electrical component.
Easy way to find out. When the battery is fully charged disconnect the negative cable. It it stays charged but starts loosing power when connected back up again start searching for the source. Good luck.
