What about the 820 cover?

I think, the 820 cover will do fine. As far as I remember, it outputs 5V. The coil is removable easily (under heat) and very thin. It also comes with something like a shield, I guess*. I tried this before for another phone.
I was always going to try this when I get my Jolla. 'In picking' currently, I can report the result when it arrives.

*<1mm thick, flexible but quite heavy for its volume. Larger than the coil. Can someone tell what this is for? Is it to block the rays going beyond the coil?
btw: Could the electromagnetic field harm any of the other components? I think the field must be pretty strong to charge the battery. What happened, if AC was generated in other parts?

It's quite possible, actually. I've never seen it so can't say for sure but my wild guess would be it's there to increase efficiency rather than protect something. Magnetic circuits such as transformers (transformers not transformers - wireless chargers are nothing more than the former in disguise) work best if the magnetic flux leak is kept to the minimum. This is usually achieved by directing the flux through some kind of magnetically conductive material.

It is possible in theory if the field is powerful and there are circuits in its path with coils inside them, oriented the wrong way. Even a badly designed PCB track can act as a coil. It is unlikely in practice though. Besides, any metal bits (such as the battery compartment in the N900) would divert the magnetic field and act as a shield.

OK, thank you for that assessment. I uploaded two pictures of the 'disassembled' 820 shell.
The parts are easily removable. Back then I used a hairdryer to heat the glue.


EDIT: OK, sorry guys, I was wrong! The 820 shell outputs 6.95V, just measured it again...

No worries. You just saved me €9

First of all, thanks a lot rainisto for your work and for posting it here.

I gave the touchstone wireless charging kit a try.
While idle the output voltage is about 5.64 V.
While charging this drops down to about 4.53 V.
I think this is due to the kit not being capable to deliver the needed current.
From past experiments with the N9, it seems that the touchstone kit is not that powerful.

Nevertheless, I gave it a try and charged for about 10% of the reported capacity.
It generally seems to work (despite of the relatively high drop of voltage from idle to charging).
However, the touchstone kit doesn't seem to be optimal for this.

I attached some photos of the internals and of the phone on the touchstone charger.
Personally, I quite like the "floating look" of the device when it is attached to the charger.

PS: During these experiments no Other Half was harmed.
The receiver is held in place by its own glue (After having it pulled out from a Palm cover.).
The connection "pads" are held in place with hot-glue that can be removed completely without causing any damage.
By the way, I created the "connection pads" by forming a bit of the wire to a flat circular shape and then used tin-solder to fix it.

PPS: Is there a way to monitor the charging voltage and current on the device (e.g., via sysfs, proc, or something)?

I made some more experiments.
Generally, charging via the touchstone kit seems to work quite stable.
With my old N9+touchstone hack the setup was very sensible with respect to how the phone is put on the charger.
So far, this seems to be no issue with the Jolla phone.

I also measured some charging times.
These values are pretty rough but should serve as an indication of what to expect.
In about 30 minutes the device charged approximately 14% of its capacity (from 43% to 57% absolute charging state).
In about 2 hours and 14 minutes it charged approximately 52% (from 43% to 95%).

By the way, I "modded" the charging unit a little with one of the stickers I got with the phone.
So, now I have my (semi-) official Jolla wireless charging kit.
I attached some photos of this.

That is badass, wish i had that wireless charger! I also wish i had a Jolla!
Anywho, nice work guys

Today I've ordered receiver (S4) and charger from www.dx.com and in 6 days I'll let You know how I managed to fit everything.


http://dx.com/p/metrans-mwt03-super-...-black-226778#

I just measured the charging current with the "csd tool":
http://talk.maemo.org/showthread.php?t=92189

With the touchstone it reports a charging current of around 430mA.
When directly attached to the USB charger it reports around 960mA.