Battery is a AGM Battery and is supported by insulation to prevent vibration. Looks like PO fabbed up a piece of sheet metal to hold the battery.
If you already have all the answers, why did you start the thread?
The sheet metal bracket is attached directly to the frame, and the battery is in direct contact with that. Any frame vibration it transmitted directly to the battery. Didn't you *just* have to replace that battery?
Come to think of it further, what holds the battery in place, the closed seat foam?
The setup is an engineering disaster, or afterthought at best. I wouldn't consider living with it the way it is. But then, I wouldn't have bought it with those mods, for fear that other things were tampered with in similar fashion.
Having said that, I'm a sucker for a cheap rescue project. Were it free, I'd probably collect it to rescue and make whole again.