Shadow Rear Derailleur (12 speed)
The RD-R8150 rear derailleur is part of Shimano's new 12 speed system, allowing for both wired and wireless communication between the derailleur and shift levers.
It is used with the newer battery, the BT-DN300.
This derailleur features a charge port, LED, button and an EW-SD300 type port used to connect the derailleur to the battery. Both the rear derailleur and the front derailleur connect to the battery using a wire. The shift levers send their shift commands to the rear derailleur wirelessly.
These 12-speed derailleurs use the same charger as the new Di2 power meters, and perhaps even better: the port cover can be replaced when broken.
The low sprocket maximum is 34T and I expect a lot of bikes to be built up with 11-34T cassettes.
Cross-chaining is not blocked
11-speed Di2 had a feature called Gear Position Control that effectively locked out the smallest two cassette cogs when in the small ring at the front. This feature is not enabled for 12-speed Ultegra and 105 derailleurs.
If you're on 12-speed Ultegra or 105, you can shift into the smallest cassette cogs when you're in the small ring. There is no way to change this setting.
LEDs and button cheat sheet
Keep having to go back to the manual to read what the LEDs mean or what the button does? Download my Cheat Sheets, print them, and keep them close to your bike. That way you'll never have to look for the manual again.
Crash Mode Reset
Since the main junction functionality has moved to the rear derailleur, Shimano also changed the method of getting out of crash mode. Pressing the button on the rear derailleur would be hard during a race, so you can now shift out of crash mode.
To get out of crash mode on 12-speed road bikes, shift up and down the entire cassette. This video by Rides of Japan explains it well: