Fitting a Shimano Di2 battery has gotten easier thanks to PROs Internal Battery Holder. They make two versions of a mount that allows you to secure the battery inside your bike.

The first version, and the one you expect, is the one that fits inside round seatposts. These are designed to work with PROs 27.2mm and 31.6mm diameter posts, though they should work with most other manufacturers round posts. The part is simple. Its two halves of a cylinder. You put the battery inside the halves, with the cable side down, and push it in. PRO also includes a tool to pull out the battery for those few times you might need to.

The second version is one that fits inside a forks steerer tube. Yes, the fork. Like the seatpost mount, its two halves of a cylinder and should fit in most round 1 1/8 steerer tubes, with the ribs crushing slightly to accommodate small diameter changes. The Di2 battery is 152mm long and 16mm in diameter, so youll need a steerer tube thats 2cm longer to accommodate both the battery and the mount. youll also need Di2 cables, a Di2-compatible stem and top cap, and Di2-compatible handlebars (these have tiny ports for the Di2 wires.