The standard power unit of battery that brings the Dura-Ace Di2 and Ultegra Di2 systems to life. The lithium Ion battery is housed in a waterproof (and bombproof) case that can withstand abuse from the daily rigors of cycling. One full charge can last for many, many miles.
All Di2 components on the bicycle require the same firmware version. Using Di2 components with different firmware versions may cause malfunctions. Use Di2 system checker to update component firmware from http://e-tubeproject.shimano.com/