My suggestion: I suspect we're only talking about a couple of feet between the battery box & new junction box, but voltage loss is voltage loss and the cost of a short cable like that shouldn't be a show-stopper. So, within reason the thicker the wires (one black and one red?) from the battery box to your new junction box, the better. For starters, I'd suggest they should be at least as thick as the main house wires that go to the battery box right now, and maybe one gauge thicker. What gauge is that wire?