I just got parts for my Power Half from Sparkfun. I'll be using 2000 mAh 3,7 V LiPo as a external battery.

Power Cell will be used for charging the Power Half and converting the 3,7 to 5 V. I'll take the JST connector off and solder the battery straigth to the Power Cell. It's possible to add a SPDT switch into the Power Cell to control its state. Total cost for this set is about 30 eur.

There's microusb port in the Power Cell so the Power Half will be charged via it.

I have also some 5 mm Copper Tape to connect the power into Jolla.

Thickness increase will be about 5.5mm. LiPo at the left side in the picture is 1200 mAh.

I don't have enough time this week for prototyping (except taking pics of the parts ) so you have to wait for next week

finally! someone who is gonna give us a battery toh!!

IIRC didn't Jolla say that the phone can supply power to the other half? If so couldn't the phone's USB be used to charge the battery making it a lot simpler?

It can, but output is 3,3 V and the current which you can draw from the phone is pretty low (300 mA max so it would take ~7h to load the 2000 mAh LiPo). And LiPo requires still a special charging logic so this is the simplest and imho thinnest option (of course custom made LiPo can be slimmer).

I have also started the project, too
http://talk.maemo.org/showthread.php?t=91895&page=3

The problem with the story is the following:
The board what I bought, the charge can not keep up.
However, if you use 2 boards, then the charge is very good and quite fast.

If we compare times your board with my:

Power Cell: 3.3V input
Pulolo: 2.5 v-5v input

Cell Power: 5V @ 600mA max
Pulolo: 5V @ 400/500mA

The smartphone itself has a shot of 1000mA. That's why when I connect only a board, he comes after about 30 seconds, no longer with the charging

Therefore, the Board is very hot and overheated accordingly. When 2 boards that I use, charging is perfected, but still very hot.
Now it would be nice to know if the Power Cell is also very hot and only with a board charging perform continually.

The interesting thing about the Power Cell is that he has a Undervoltage lock with it. That is missing in the Pulolo boards.

I'll report here how it performs when I have tested the board.

CSD tool shows about -780 mA and the board gets pretty hot but continues to charge. However, I'll probably find somewhere a boost converter which is capable to ouput 1 A. I have charger-only boards from Adafruit too so I'll just need the 3,7 -> 5 V (1A) boost converter.

Power Cell would work if the current could be limited via Jolla but as far as I can see that is not possible. Of course one can use it, but it gets hot and probably has some efficiency problems at those output levels.

Edit: I think this will do: http://www.ebay.com/itm/New-1pcs-1A-...-/191014690726

Anything able to throttle down voltage when approaching overload or overheat should work well...

i order two now two from them from HK, shipping will take 2-3 weeks

Wasn't there problems with the charging via the pogo pins? With charging going on and off all the time?