The Ultegra Di2 front wire kit connects the Ultegra STI dual control levers to the control module/battery indicator which is part of the front wire kit. To check the battery charge level, just press and hold and shift button. To fine tune the rear derailleur adjustment, hold the button on the control module until it turns red. Then just press the shifter until the rear derailleur is aligned perfectly over a cog. Please note that an Ultegra Di2 system requires four connecting wires in addition to the front wire kit.
Ultegra Di2 wiring kits represent an improvement over first generation Dura-Ace Di2 wires because Ultegra Di2 use smaller, 2-core wires instead of the 4-core used on first generation Dura-Ace. Ultegra Di2 wires are compatible for both internal and external routing so there is no need to buy separate internal or external kits. The Ultegra Di2 wires are lighter with smaller connections so there are smaller holes required in the frame for internal routing. Connections on the Ultegra Di2 are water tight without the requirement to install a shrink wrap seal like on Dura-Ace Di2. Please note, Ultegra Di2 wires are not compatible with Dura-Ace Di2 components.