Electronic & Hydraulic Shifting (Di2, Ui2, customisations)

Backstory: At the end of last year I treated myself to a couple of Cervelo framesets, a P2 and an S2. I was planning to fit the S2 with a SRAM Force groupset I had spare, but that ended up going on my Croix de Fer as I decided that the S2 was an ideal opportunity to try electronic shifting. However, I wasn't so keen with the idea of having a fugly great big battery strapped onto the aero sleekness of the S2 frame, so decided to hack the Di2 system to place the battery in the seatpost.

At the same time, I rather fancied the idea of having satellite shifters on the tops of the handlebars for climbing, but didn't like the look of the similarly fugly Shimano satellite shifters, so decided to hack them to make small shifter buttons which could be hidden underneath the bar tape. The difficulty with hacking Ultegra Di2 is of course that while the original 7900 Di2 used a nice simple 4-wire interface (power, earth, signal, signal return), Ultegra Di2 uses a two-wire interface where the signals are sent over the power wire using a distributed network system similar to CANBUS. So I decided simply to concentrate on repackaging the Shimano components but to retain essentially all standard Di2 parts.

First off, the battery. The aim here, obviously, was to reduce the physical size of the battery to allow it to be inserted into the rather slender S2 seatpost. The battery has three connectors - positive, negative, and a third one which is only used for charging. I assume it's for balancing the two cells in the battery. The battery is held together with three screws and lots of black rubber-like glom.

I carefully removed the three screws (they have one-way only heads so they can be tightened by not loosened as standard) by cutting slots in them with a Dremel and then just winding them out. However, the outer casing of the battery is also held in place by a lot of glom, so I ended up using the Dremel to split it in two and then levered the two halves off the internals. This is what I was left with:

There are two cells in series, a sensor wire running from the junction between the two cells, and a small PCB at the end, one side of which is connected to the -ve and +ve terminals of the battery and the sensor cable, with the other side being connected to the three contacts in the end housing of the battery. As far as I know the small PCB is just a charging and overload circuit and doesn't contribute towards the Di2. However, given that the plan involves having 2 LiPo cells inches from my gentleman's parts, I decided that having the wee PCB in place wasn't such a bad idea.

With the white plastic casing holding the cells together out of the way, the tab holding the two cells together could be bent back on itself so that the cells are in a straight line. For the middle sensor wire, I simply bent it roughly into shape and cut it shorter. With the charging/overload circuit at the end, sideways on, all that's needed is some flying leads to connect everything together.

Soldering done, and some tape to hold it all together, it looks like this. The connector's a model aircraft servo cable - it's light, cheap and reasonably robust.

I also added some hot glue around each of the contacts and between the two cells just to provide a bit of strain relief and physical resilience.

Once that had hardened I covered the whole thing in heatshrink (I must find my hot air gun - waving lit matches underneath charged LiPo cells is not something I enjoy doing) and then some insulating tape. All up weight is 45g, which compares to 71g for the standard battery. You could save a few more grams by using lighter weight cables, just having a single layer of heatshrink, and possibly using smaller cells, but the S2 isn't intended to be a particularly light bike, so 45g is fine for me.

Next up, modifying the charger to suit and trying to charge the modified battery.