I don't know if I would go as far as a 999 SD rating for carriers. It might be a possibility to raise the rating to around maybe 20 or 30. That would give carriers protection against attacks by smaller naval vessels, but not against massed fire of surface ships or newer battleships.
I can't remember a time where carriers came out better in a surface battle with battleships. The Glorious succumbed to the gunfire of Scharnhorst and Gneisenau. The US escort carriers didn't fair well against the Japanese battleships at Leyte Gulf, and would have faired worse if the Japanese hadn't been scared off by escort attacks. I don't know of any more engagements between battleships and carriers off the top of my head. The fact of the matter is, if a carrier is caught by a battleship, its life expectancy isn't that good.
If we are going to modify the defence ratings for carriers, it should still be possible for battleships to kill carriers. IRC, destroyers take the brunt of attacks. An unescorted carrier should still be vulnerable to some sort of surface attack. It is something worth taking a look at. MDow