You will most often find it installed in the seat post and for a good reason: this way it is close to the derailleurs (12-speed Di2) and junction B (11-speed Di2).
This guide was originally written for the 11-speed BT-DN110, but works just as well for the 12-speed BT-DN300. The dimensions of these batteries are the same, and the only place they differ is the connector block.
Di2 battery washers adapters
Despite many manufacturers advertising “Di2 compatible” seat posts, you can install the battery just fine in a regular seat post – as long as it’s round. All you’ll need to install the battery in a non-Di2-compatible seat post is a seat post battery holder.
Feel like skipping ahead? Use these links:
- Install the battery into a "Di2 compatible" seatpost
- Install the battery using a seat post battery holder
Note: If your frame is equipped with an aero seatpost please consult your frame manufacturer for Di2 battery installation guidance as regular battery holders won’t do.
Install in a Di2 compatible seat post
If you have a Di2 compatible seat post like the PRO Vibe (carbon), you won’t need any additional seat post battery holder.
Your seat post will have come with a Di2 battery collar (holder). It will probably already be installed, but if it isn’t, install it.
Now take the two black adapter plates that came with your battery and put them around the battery and slide the battery into the seat post collar until the adapters slide into place.
Next you’ll have to install the washers. First install a flat washer, then the wave washer, and finally the second flat washer.
To fix the battery in the seat post and finish its installation you’ll have to insert the snap ring. Take a set of snap ring pliers with a claw diameter of 2 mm or less to mount the snap ring and place it on top of the final washer.
Your battery installation is now complete – all that’s left to do is to connect the battery to the rest of the system using an electric wire.
Install the battery with seatpost battery holder
You could try to install the battery without a battery holder if you really wanted to, but make sure you secure it well – you do not want it to slide out or start rattling whenever you ride across bumpy terrain.
As far as I'm concerned a Di2 seat post battery holder is the only real option. All the major brands make these, both in the 27.2 mm and 31.6 mm sizes.
So now that you have your seatpost battery holder of choice, it’s time to install the battery. The images we’ll use on this page show PRO Di2 battery seatpost holder, but the process is similar for other brands’ holders.
Take the rubber adapters and slide them around the battery as shown above. Make sure you can still access the e-tube port. Now would be a good time to remove the dummy plug from the port.
Then slide the battery and rubber adapters into the seat post. This will fix the battery in place.
That’s it, you’re all done! You can now connect the rest of the system and finish installing your Di2 components.
A note on removal - or uninstalling
Some Di2 seat post battery holders work really well. They work so well that they can make it next to impossible to remove the battery from the seat post. In these cases you can usually get the battery out by taking a J-bend spoke and inserting it into the seat post, hooking the bent part of the spoke behind the battery holder. You should now be able to pull it out.
I've written a page on doing exactly this. Interested? Read more on removing a stuck Di2 seatpost battery.