This is a copy of reply in an old thread post regarding battery drain on MEP-002a/-003a. This reply was by KeithJ:
"Leaving batteries connected on these sets will drain them, something in the voltage regulator puts a small parasitic load on the system. When the batteries are connected, there is a considerable spark like a capacitor charging. This will ruin batteries, I remember a PS article covering this with recommendation of disconnecting the batteries if being stored for more than a few days."
Upon further searching & reading its just the initial charging of a capacitor that was discharged over time when left disconnected and it does not pose much of a drain. Never-the-less, I still disconnect my battery disconnect switch just to be safe. If by chance rodents were to chew through the insulation of wires while genset is not in use it could cause a problem if left always connected is my thought for disconnecting.