Campagnolo's EPS Power Unit is the reliable brain of electronic shifting. Inside sits a 3-cell, 12-volt Lithium-ion battery that delivers up to 1,200 miles on a single charge. It also houses the electronics necessary to deliver pinpoint shifting precision to the front and rear derailleur mechanisms (sold separately). If you're cross-chained, the system immediately senses it and sends a message to the front derailleur to move ever so slightly to avoid chain rub. Plus, the one-piece, ultrasonically-sealed unit is impervious to water or road vibration.