That is not an issue. PCB traces (and other copper foils stuck to surfaces) are capable of carrying more current than wires of the same cross-section area because the surface they adhere to carries away some heat.

A common adhesive copper foil is about the same thickness as a standard PCB trace, about 35 µm. According to for example this online calculator, a PCB trace of that thickness and 2 mm wide can happily carry 2 A.

A trace of that width and thickness gives the cross-section of 0.07 mm2. A standard copper wire with the same cross-section would have a 0.3 mm diameter and a capacity of about 1.3 A.

The point where the foil connects to the battery contacts is a different issue. You have to make that contact very reliable for the currents that the phone can take in heavy use. Just sticking the foil to the contact and hoping for the best will not cut it. This is why soldering is the best option.

To be honest, I tried it with copper foil only. It works but not reliable enough from my perspective. Each time you need to remove the battery the potential of destroying the copper foil at the original battery contacts is too high. So every 2-3 times I removed the battery the connection failed.

That's why I switched to cables, which might not be as elegant but it's very reliable.

How about doing the adaption with a flexi pcb? A quick look suggests it would cost approx $5 each in 20 off.
I can see a few wrinkles, but nothing too difficult to handle.
(I do a lot of pcbs for myself and for work, but I've never done a flexi though I've used them)

pichlo, MaemoUser, if you say it can cope with the heat, I think I'm going to try without wires...
MaemoUser, I don't know why you need to remove you battery often, but I don't need... Will see...
Thanks
p.s. can we solder only foil to the battery contacts or using any special glue for example: http://m.ebay.com/itm/290695765305?_mwBanner=1 ?

How stiff is the copper? Can you curl the part that will contact the battery in a similar fashion to the phone's contact pins, and then bend it to the required path. eg:

It's not stiff, you can bend it as you want...

I am often in different countries so need to change SIM Cards .

Of course if you are going to use a foil all the way then you need only one solder pojnt, to the original battery contacts. I know conductive glues exist but I di not know how good they are. I would be a bit concerned about how it performs under heavy load. The only way to find out is to try. Please report your results.

I suggest you bend the ends as per mrsellout's drawing. You want to attach the conductive side to the battery terminals, not the sticky side.

Yes, of course... This is exactly what I intend to do.

Hmmm, I have another little question. I still afraid a little bit about heating of the foil, maybe it better to put something (2 strips of isolation band) between battery and the foil ? What you think ?