Good point and I beleve Villa may be correct and the plastic card may be a mistake. If after you test the starter on a car battery without the card stock and it is a little better than one of the wimpy and worthless box batteries, its worth uping the 'voltage'.

To at least the 18 volt Harbour battery. Its not clear exactly which starter you have but most I have found the switchs have handled the 18volt battery fine.

If your 'box battery' is the 12v 7 to 11 AH wet or gel variety then that combined with and older starter with worn berings, possibly poor brushes, long wires to a box and one with the small diameter contacts in the switch is a recipe for frustration.

Try at least the cheapie 18v you will not regrete it.

John