Obviously what you have is a DIY job gone wrong that you inherited from previous owner. If turned sideways, it must be a gel battery not wet cell acid. The simple replacement is buy the largest 12 volt deep cycle battery that will fit in the space...a size 27 or size 29 if it will fit. An Autozone or Walmart battery will work fine, but more expensive and better batteries are available if you want to pay more for higher quality.
Some prefer multiple Trojan-Brand 6 volt golf cart batteries tied together to give more capacity at much greater expense.
Originally Posted by Scott619
I have a used BT 5290 and I need to know what the house batteries are supposed to be. It now has one 12v turned sideways in the rack secured with a piece of rope. From the rack I suspect that it may be a pair of 6 volts. If someone could tell me what group of batteries it should be I would appreciate the help.