Between myself and Jim buying inks, we own far too many. This means I have some inks that are incredibly similar in color. And, because of that, I need to figure out which inks to keep and which to dispose of. Sometimes, I have samples of two or more similar inks that I'm trying to decide which to buy, or if I should buy one ink when I already own a bottle of a similar one.
Enter ink battles. I put the similar inks into pens with the same nibs (two Jowo #6 M, two TWSBI F, etc.), then test them out to see which I like better. Or, I may find out that, when used with the same nib, they’re sufficiently different to keep.
You can assume I have no problems with any ink that appears in an ink battle. I may find, when using the inks side-by-side, that there are performance differences, but the point of the battle is to choose a favorite ink, a "winner". These choices are often subjective, and you may disagree with me.
An ink battle — unlike ink dupes — is between inks I like. I enjoy using them enough to have them in my "for use" ink collection — as opposed to my repository of inks for testing dupes. If an ink performed poorly or caused problems, I wouldn’t keep it. And, if that were the case, an ink battle would be unnecessary.