Adding Battery to Melex 512 for Aux electronics

[cartmann32]

Hey guys.
Just recieved a Melex 512e
No clue on what it is exactly or what year it is; but I plan on tearing it down, redoing all the stuff that's wrong on it, some body work; maybe lift it if I can, then do a full audio system and lights to show off my business.

That being said, what's the best way to add an auxillary battery to this without messing with the overall voltage of the 6 batteries to are used to run it?

I will more than likely put an isolator switch on it, so it doesn't drain the cart when it's off; but after that, I'm kind of lost.

If there are any helpful threads on here that are Melex specific, you can just direct me there; or if my setup is comparable to other major brands, that would help too.
I've already started looking at vintage golf carts for parts and whatnot, but is there any other cart parts that are NOT Melex that will fit this thing?

Thanks in advance, I know, lots of questions.

Thanks!!