You need to find your way to your controller compartment under the battery. In there are the thick red and black wires. you need too solder on two more thick red and black wires long enough to reach your rack. Drill a hole if you need to to get them out of the compartment, and then solder on a suitablle connector for the battery. Anderson connectors will be best because they don't have exposed pins, and this connector will be live when your other battery is on. It would be safest to have the additional battery at the same voltage. Then you can use them at the same time. If you want to experiment with a higher voltage battery, you have to make sure that you have your normal one switched off when you connect the new one otherwise severe damage will occur. Likewise, you mustn't leave a flat battery switched on when you connect a charged one.
A safer idea would be to bring your existing battery wires from the controller compartment up to the rack with a female connector. Put a female connector on your new battery and then run a male connector back to your controller, so that you can connect either, and/or make a "Y"connector to connect them in parallel (if both charged and same voltage).
