So Boeings answer to the fire risk is to surround the battery with a fire proof case. One small problem, the main a/c battery is on board for a purpose, to provide power to various systems when all else fails.. If the battery ends up as a molten mass then that protection is lost. All sorts of systems come off the Hot battery bus. I guess the APU battery could be less of a problem, but it certainly would effect ETOPS.