Install the Di2 battery in the fork steerer

Updated January 16, 2024 by BetterShifting Terry

Part no longer in production

While this guide still applies, and the steerer battery holder works with 12-speed Di2 batteries too, Shimano/PRO has stopped making these parts. They are no longer in production. You can probably find the steerer battery holders online if you're lucky though!

Ever since the release of the SM-BTR2 Internal Battery years ago, people have been installing their Di2 batteries in the frame. The seatpost is the ideal place for this battery and several brands make seatpost battery holders to securely hold your battery in place.

However, what do you do when you cannot put the battery in the seatpost? Because your bike has a suspension seatpost or perhaps a dropper post? Where do you put the battery?

Some manufacturers put the battery in the down tube - like Canyon do on the VCLS seatpost equipped bikes. Others sometimes use an external battery instead.

Both of these are not ideal. Putting the battery in the downtube generally means that you have to take out the bottom bracket to get the battery and this can be a real pain. It’s not often that you need to get to the battery, but personally I prefer to have it somewhere easily accessible.

There’s also the SM-BTC1 of course - the bottle cage battery holder. Unfortunately this is a really chunky unit. I have one on my indoor/commuter bike and it works just fine… it’s just really big.

Put the battery in the fork steerer

My new favourite solution is to put the battery in the fork steerer. While originally intended to be used on mountain bikes, this works on road bikes too - several PRO stems have an opening allowing you to route an EW-SD50 wire from the fork steerer through the stem, into the handlebar.

Sure, this isn’t for everyone and you should always put the battery in the seatpost if you can, but this is a nice alternative.

Let’s take a look at the different methods of installing the battery in your fork steerer.

PRO Battery Holders

Besides wedging it in place yourself using a makeshift solution, there are two options available to you. You can use either the PRO Di2 Battery holder (PRAC0085) or the PRO Gap Cap Expander Di2 Battery holder.

pro steerer tube battery holder

The first one looks a lot like a regular seatpost battery holder, except you use it in the fork steerer. It is placed below the top cap (and star nut) and will work in standard fork steerers.

More interesting is the PRO Gap Cap Exp. Di2. This is basically an expander plug that will also hold the battery in place securely. There is a non-Di2 version available too, without the battery holder, so make sure you get the right one.

PRO steerer battery holder product shot

There is a general version, meant to be used with all stems (PRHS0110) and a Tharsis specific version (PRHS0111) - this last one comes with a Tharsis-specific cap. Other than that, they’re identical.

When you take the Gap Cap Expander out of its packaging you’ll end up with the expander/battery holder, top-cap and bolt, three tie-wraps, an O-ring and the manual.

PRO steerer battery holder parts laid out


In order to safely and securely store the battery in your fork steerer, it needs to be at least 180mm long or high, as measured from top of the fork to the bottom. Now.. on this specific bike I got lucky. I cut the fork steerer a long time ago, and it turns out it’s only 185mm long.

So why is 180mm the minimum steerer length? If yours is any shorter than that you run the risk of having the battery come out the bottom of the fork and get damaged… or worse.. And besides that, it’d look a bit silly having your battery come out the bottom of your fork.

If your fork currently has a star nut installed in it, that’ll have to go. I’m not a bike mechanic, but there seem to be two obvious ways to remove the star nut.

You can either take a long, old screwdriver and hammer the star nut down until it comes out the bottom - or you can use a drill to take out the center of the star nut, after which it should collapse and fall out by itself.

I used the second method, because it seemed more fork friendly. The internet is a bit divided on the subject though. Just use the method you think works best for you.

If you do take a drill to the bike, make sure you wear safety glasses to protect your eyes. Small bits of metal can shoot up when the star nut collapses and you do not want those in your eye. Some cutting oil can help too.

With all that out of the way, let’s actually get it installed.

Installation Process

The expander/battery holder comes with a set of tie wraps, an O-ring and a top-cap. It also comes with a manual, which is surprisingly useful. It actually shows how to install the expander and the battery:

I get that posting the PRO instructions might defeat the purpose of this page, but stick with me for a while and you’ll see there are some things the instructions manual does not mention.

Sure, it seems easy enough, right? Stick the battery in the holder, slap the tie-wraps on and insert it into your fork steerer. That’s it.

This is what it looks like with the tie-wraps installed. You are meant to cut the top two short, but leave the bottom one as is. The manual itself seems to suggest you cut all three, but this doesn’t match the next image - and the PRO instruction videos on YouTube say you keep it long too.

With the last one left intact, this is what it looks like:

The next step is to attach the EW-SD50 electric wire and slide it into the frame.

Before you actually do that though, try pushing it in without the wire attached. This will help you do a couple of things:

  • You can check the length of your steerer tube and required spacers
  • Figure out the wire length you need and wire routing
  • Make sure it actually fits in the fork steerer

This is as far as it would go:

If this happens to you then there are two things you can do: find some smaller or low-profile tie-wraps, or modify the ones you have.

In this case, I decided to do the latter. I took a file to my tie-wrap and took a fair bit off. This is what it looked like when I was done:

With this done I gave it another go and this time it went straight in. Next up is figuring out what wire length you need. This depends a bit on what you are connecting the battery to, where that component is located, and what the wire routing is like.

You’ll need at least a 250mm wire, since the battery is positioned with the connector at the bottom and the distance from bottom to top is 180mm. On this mountain bike I’m using an SC-MT800 display which is located right next to the stem. I installed a 300mm wire and that seems to get the job done just fine.

After deciding on the wire length it’s time to insert the battery holder and the battery in for real. Before you do so though, slide the O-ring on at the bottom. It’ll help keep the wire in place.

Another quick tip: position the tie-wrap ends or locking mechanisms on the same side as the wire. This will help it go in easier.

Which side is that you ask? Well.. the expander part has a big groove in it. Large enough for three EW-SD50 wires.

If you’re installing this in a full carbon fork steerer you’ll want to apply some carbon paste to the expander and then slide it in gently. Note: if gently doesn’t do the trick, hitting it with your fist is perfectly acceptable too in my book.

The top part of the expander can rotate, so make sure its orientation matches that of the expander itself. The F should be on the Front and the R goes on the rear. The rear also has the torque spec on it - 7Nm.

Next, add any spacers and use the top cap to eliminate any headset play. If you’re thinking of using a different top cap - you’ll probably have to shorten (cut) the bolt. The one provided is relatively short and there is a perfectly good reason for that. There just isn’t a whole lot of space for longer bolts, due to the battery being in the way.

Route the wire through the stem.. or not

So if you’re looking at this battery holder then you may want to route the electric wire through the stem and into the handlebar. As you have perhaps noticed from the pictures, my MTB stem is only 4cm and there is no way you would be able to route a wire through that.

Besides that, my Cube handlebar does not have a hole facing the stem so there would be no way for the wire to enter the handlebar. All of the PRO Di2 compatible stems and handlebars do have these holes and these would be the perfect handlebar/stem to use with this battery holder.

The alternative I’ve chosen is to use a file to make my own cable routing hole.

You can then route the wire through the hole, out of the fork steerer. With the top cap installed and the wire connected to the SC-MT800 display, that looks like this:

PRO steerer battery holder and top cap installed

I think it looks pretty good, even though the wire isn’t routed 100% internally. I’ll post a full building-this-bike post soon and I’ll link it on this page.

This is what it looks like from the other side:

For now though, this should help you decide whether or not the fork steerer battery holder is worth it or not.

If you have any questions or comments, please send me a message or leave a comment below. For more information on installing Di2 components, visit the Bike Building Hub or the Installation Guides section.

BetterShifting Terry

About the Author - BetterShifting Terry

I enjoy playing with bike tech - both bike building and wheel building, bike maintenance and of course, Di2. Besides writing content and working on the technical side of BetterShifting, I also work as a Software Developer in The Netherlands. Read more on the About this site page.

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