Cool...I haven't been around long enough to see those. Maybe do that on my Caad10 (take it from under the bb), but then I'll have to get inside the bike and change the wiring, right? How do you pull the wiring up through the seat-tube? Also, how small is the battery? Small enough to fit in 27.2 or no way? The second picture makes it look like...maybe! My battery looks like no way (Ultegra).
Yeah, it's heaps cleaner and makes sense to go internal. The battery charge lasts for a very long time. I actually got the Ultegra electronics kit as it had superior cabling at the time, but the battery is the same for DA.
Battery size shouldn't be an issue as you can separate the two cells (about AA in size), solder together and create a tube shaped unit for standard posts.
If you have a standard Di2 fit out, all you need to do is route the battery cable up inside through the seat tube. I used a piece of steel wire with a bent hook to grab the cable from inside the BB area. If your stock cable isn't long enough, you can order different lengths from a Shimano dealer. Just measure your seat tube and allow a bit of slack for connecting / disconnecting with the post removed. You will see that I actually took the socket from the stock battery holder and the socket from the base of the battery housing and they are connected together within the seat tube. This is important as the Ultegra unit has small circuitboards inside these parts which are required for the whole system to function so they must be retained. Secondly, they allow for easy charging as you can place the socket part from the original battery case into the Shimano charge unit to charge - without having to remove the battery from inside the seatpost.
You will need to be careful not to short the battery or even worse connect with wrong polarity as you can blow not only the battery - but the whole DI2 circuit (circuitry in the derailleurs I believe). Not for the feint hearted! I can try and post a guide if I get time.
Otherwise, wait a couple of month for Shimano to release the Di2 internal battery (tubular shaped designed for seat post installation) - part no SM-BTR2. Exactly the same as above but will probably sting you about $150 USD though.