All 2 to 7 place helicopter operators owners are faced with a run down or dead battery some time and need an Auxiliary Power Unit (APU). With this switch built into their "two battery truck" (possibly their helicopter fuel truck) they have a readily available and inexpensive APU all charged up ready to go all the time.
We hook these switches to the truck batteries with #2 insulated copper welding cable. We solder copper connectors to the ends of the cables after they have been cut to the right length. You may have a better solution for the connections. However, it is something to consider when making the switch as there needs to be room to hook up some heavy duty cable to the switch connections.