With the launch of Dura-Ace 9100 came new firmware offering road riders Synchronized Shifting options between front and rear derailleurs. A short while later this technology was made available to Ultegra 6800-series and Dura-Ace 9000-series riders who have the latest Di2 battery.
The new battery (BT-DN110) contains a memory chip that can handle the processing power required to deal with the multiple shift patterns and customizations that Synchronized Shifting brings, offering you the choice to suit your gear positioning to your personal riding style.
If you’re planning on putting this battery in your seatpost you will probably also want a seatpost battery mount. You can easily find those on Amazon, Chain Reaction Cycles or eBay, All major manufacturers make them.