Is this still the original controller to which you
connected the battery in reverse or did that controller have to be replaced?
As there is now no spark whatsoever when you connect the battery
(and the BAC-601 Display is still powered up and working) I would say that the controller and its wiring harness may need to be replaced.
It might be worth removing the controller and checking the continuity between the battery connections and the soldered joints on the controller's circuit board, and also looking for obvious signs of burning or physical damage on the controller and its wiring harness.
Unless you can find something obvious that can be repaired or re-soldered etc., I think a new controller is likely to be the next move.
Alan