Full wireless Di2 skips the frame-mounted battery and buries the power source inside the rear derailleur itself, which is exactly what the BT-DN320 is built for. Shimano designed this small lithium-ion cell to tuck into a sealed cavity on the XTR M9250, XT M8250, Deore M6250, and GRX RX827 wireless rear derailleurs, where it drives the shift motor while staying out of the way of rock strikes and trail spray. If you're building up a new wireless drivetrain from individual components — or replacing a battery that's finally aged out — this is the only cell that fits those derailleurs.

Capacity is 305 mAh at 7.6 volts, or 2.32 Wh total, which Shimano rates for about 1,000 kilometers of riding between charges depending on how often you shift. Real-world range varies with terrain and shifting habits — a technical enduro lap burns through charge faster than a mellow gravel loop — but most riders can go weeks between top-ups. The battery pops out of the derailleur without tools and charges through the EC-DN100 dock (sold separately), which runs off USB-C and takes roughly an hour for a full charge.

Weight comes in at around 26.5 grams, so the on-derailleur battery placement doesn't add meaningful mass compared to older wired Di2 setups that used a frame-mounted cell and internal wiring. Weather sealing matches the derailleur body itself, meaning the battery is rated for the same wet and muddy conditions the derailleur is designed to survive. One item to note before ordering: the wireless derailleurs don't ship with a battery in the box when purchased as standalone components, so if you're piecing a system together rather than buying a complete groupset, plan on adding the BT-DN320 and EC-DN100 charger to the same order.