Certain electronic devices on the vehicle maintain a small continuous current drain (parasitic load) on the battery. A battery that is not used for an extended period of time will discharge. Eventually permanent damage will result.
In order to maintain the battery state of charge while storing the vehicle for more than 30 days:
When the battery cannot be disconnected:
A battery that has remained in a discharged state for a long period of time is difficult to recharge or may be permanently damaged.