anyone know how high of voltage i can run into these? the manual says 15v, but is there more head room? i run my 6v d8 on 9v dvd bats w/no prob. will 2, 7.2 dvd bats wired in series w/5400mah, last longer than, if it can be done, 2 9v dvd bats wired in series w/5400mah?? any help would be great. ed