I would get 6A, 50V Rectifier Diodes (4-Pack) Model: 276-1661 | Catalog #: 276-1661 from radio shack $2.99. I would put 2 of them in series so they would drop 1.4 volt in the power cord. That would drop 14.2 down to 12.8. If battery are at 12.4 that would drop to 11 which is still above the 10.8.

http://www.radioshack.com/product/index.jsp?productId=2062591