Cant do synchro shift without this battery, and its worth the upgrade just to get synchro shift. I'm not a lazy shifter but it is convenient in a race to not have to think about the gearingcross chaining as you meet hills. Just shift!
WesternBikeWorks is a great place to buy (especially in a case like this) and offered to allow me to return the Shimano products!
After talking with the Shimano USA Customer Support I was stoked to get a bike with Ultegra 6800 Di2 and upgrade it to get Synchro Shift. As required I ordered the BT-DN110 battery, EW-WU111 Wireless Transmitter, and an E-Tube Wire. After I set this up I discovered that the Synco Shift on the 6800 (or 9000) only allows down shifting the front derailleur after you reach either the innermost or second innermost sprocket on the cassette (I've been riding bikes with front rear derailleurs for over 40 years and as a rule try to avoid big ring - big sprocket combinations). I spoke to Shimano USA Customer Support team and they did not know about this undocumented limitation. After several interactions they were either unwilling or unable to work with Shimano to address this defect (which is likely a software coding mistake in the e-tube project app).
I'm terribly disappointed in Shimano USA. They don't stand behind their products or take the time to address a situation where they have under delivered product capabilities.
I replaced my older BTN2. I was upgrading my derailer and I wanted the features on the e-tube app so figured now was the time. Straightforward unplug and plug. It is not clearly stated so I was confused if the new battery alone support blue tooth or I needed to add the EW-WU111. This is not the case. The BT-DN110 WILL pair with your phone and you can use the etube app. This DOES NOT require the EW-WU111.
Also, the new batteryderailer combo supports synchro shift which I unexpectedly like very much.
Does what it's supposed to...but the real joy of the new DN110 (vs BTN2) is the ability to allow the slimmer D-Fly replacement Di2 transmitter AND the ability to support syncroshift on Ultegra Di2 (as of 32017) with firmware upgrades (you are regularly upgrading your firmware, right?).