To be fair, CCP themselves is on record as saying that Battleships are weak in the meta currently.
However this is not solely because frigates can kill them. It’s also because they die horribly to Dreads & Carriers at the same time.
However a complicated hard to understand & calculate on the fly mechanic is not the answer.
Simply increasing battleships base EHP would achieve the same result, while also addressing all the other issues surrounding battleships dying too easily to other things like dreads & carriers.
And a size locked secondary rack like discussed in one of the other threads would allow them to fight smaller ships effectively without making them overpowered against smaller ships. (Which simply increasing tracking on larger guns would do)