The adapter might have some sort of protection inside, but it will not work (and possibly it will fry) if you reverse the polarity. I bet you have this kind of plug:
The tip is the positive contact while the side plates should be connected to the negative pole of the battery.
For extra safety, and since you are working with a somewhat big battery, add a fuse in series with your adapter: if you mess something up the fuse blows and the wires don't catch fire. The fuse should be rated a bit above the maximum input current that should be written somewhere on your adapter. Please note that it's well possible that there already is a fuse and that it's inside the above plug.