Mechanical Battery Piece

This is one of three drops that have to be collected before the minecart upgrade can be assembled, and Skeletron Prime is the boss holding it. It only exists inside a treasure bag, so a Classic world never sees any of this chain. The good news is that there is no drop rate to fight: one kill hands it over, and farming Prime for it is wasted time.

The kit these parts build upgrades a cart you already own rather than handing you a new one, so the payoff only shows up if you have laid real track across the world. If you travel by wings and hook, park the piece in a chest and forget it.