The BT-DN300 battery connects to the rest of the Di2 system using the EW-SD300 wires.
Is it backward compatible? Unfortunately not. Not only does it use a different port type (SD300), it also just does not have the firmware to handle 11-speed Di2 components.
Just like the previous generation BT-DN110, the BT-DN300 is also a seatpost mounted battery.
The Ultegra R8170 and DURA-ACE R9270 series are semi-wireless. Both derailleurs are connected to the battery using EW-SD300 wires, but the shifters communicate wirelessly. This battery has three ports, allowing you to connect wires to it without having to use any junction boxes.
Charging the battery through the rear derailleur takes about 1.5 - 2 hours using a USB AC adapter, and generally about 3 hours when using a computer's USB port.
Seatpost battery holder compatibility
These new batteries are almost identical to the previous generation ones, but because of the larger connector block, not every old battery holder is compatible.
I'll update this list as I get more information, but this is what I know so far:
These battery holders are compatible with the new battery:
Most pre-2022 battery holders are not compatible, or require some cutting to fit the new battery.
Attach dummy plugs to unused ports
With previous generation Di2 systems you could leave unused ports 'disconnected' or 'open', and the bike would work just fine. Sure, water could make its way in, and that can wreak havoc on the system... but that's the risk you take for not using dummy plugs.
New with 12-speed Di2 and the BT-DN300 battery is that each port must be 'occupied' - you must either connect an EW-SD300 wire to the port, or use a dummy plug.
If you don't, the system will not function.
This is what the manuals say:
When using the product, be sure to connect the electric wires to the center E-TUBE port. The product will not operate normally if nothing is connected to the center E-TUBE port.
Be sure to attach the dummy plugs to any E-TUBE ports that do not have electric wires connected to them. The product will not operate normally if the dummy plugs are not attached.
So if you've recently taken out the dummy plug - for example when updating your levers - and now the system doesn't work... this could be why.
Double-check that dummy plug and make sure each port has either a wire or a dummy plug connected.