The Proton X (プロトンＸ?) is a treasure exclusive to the Japanese version of Pikmin 2. It is a National Hi-Top AA battery. It is found in Submerged Castle on sublevel 4. It is very similar to the Proton AA found in the other versions of the game.
Note: Olimar and the Hocotate Ship's notes for this treasure are the same as the ones used for the Proton AA, except in Japanese.
English Proton AA notes: “This object resembles the first treasure I found on this planet, so it holds certain sentimental value to me. However, I've just noticed that the size it slightly different. Sometime, I should take the time to read my previous treasure log book. It's good to reflect on the past sometimes...”
English Proton AA notes: “Some enigmatic energy unknown on Hocotate is trapped inside: who couldn't use more energy?”