The lithium ion battery weighs just 71g, which is also the difference in weight between the mechanical and Di2 system, about the same weight as an energy bar. Officially, you can get between 500 and 1000 miles of riding from a full charge and even when it drops to 25% you can still ride about 155 miles. Unofficially, testing has yielded as many as 2500 miles on single charge.  Works with both Dura Ace 7970 and Ultegra 6770 Electronic drivetrains.