Hoping that the problem was fixed, I retyped my post but still got the moderation message. Fortunately, I copied it to the clipboard this time.
Since I was able to post a photo before, I'm going to presume that URLs are the culprit so I'm including those as images rather than embedded links. I'm sorry to the reader that you would need to type the URL -- I don't know another solution.
I was indeed inspired by the iFixIt teardown and by this touchstone mod for the Epic 4G:
The Epic has more room inside, but the Nexus S has the advantage of a charging port positioned at the bottom of the phone. Here are the steps I performed along with caveats for anyone else who might want to try.
Move the touchstone charging back coil into the Nexus S back
- First, I tore apart a Pre inductive charging back. I removed the plastic film covering the inner working, then removed all the pieces (4 studs, coil and circuit) and pressed them into the adhesive on the plastic I had just removed.
- Next, I removed the NFC antenna from the back of the Nexus S. This might not be strictly required but I didn't like the idea of a large antenna coil sandwiched between a set of induction coils pushing 5V+. At this point I have no use for NFC so it was no loss for me.
- After a small amount of trimming, I found the location on the inside of the Nexus S back where I wanted to position the coil from the pre charging back. I positioned this twice before finding a location that I liked. I kept putting the back onto a touchstone to see where it would align relative to the table.
- I pressed the plastic cover on the charging coil into the back of the Nexus S. I didn't add any glue. The original adhesive was very good and in the finished product this is sandwiched in very tightly so it won't move.
Connect wires to the USB port in the Nexus S
- Not knowing exactly what gauge wire to use, I tore apart a USB cable and removed about 6" of the red and black conductors. You can probably get by with a lot smaller wire. I chose this wire in part because it was stranded. Now I don't have to worry as much about pulling a solder joint loose by flexing the wire when I take the back off in the future.
- Disassembling the Nexus S is a dream compared to the Pre. As shown in the instructions on iFixIt, remove six screws around the outside of the rear plastic shroud and it comes right off. There are no tamper-evident stickers.
- One thing not covered on iFixIt is that to get the rear plastic shroud apart from the screen, you need to pry the trim ring from around the outside of the phone, specifically the two long edges, so that you can see and release the tiny plastic clips that hold the two halves together. If anyone is trying this, I strongly suggest that you outfit yourself with a set of plastic/nylon cellphone disassembly tools first to avoid marring the phone. You can get a set cheaply from DX if you are the patient sort.
- Now I was looking at the guts of the phone. This is comprised of a single PC board and several floppy bits (camera, earpiece, etc) attached by small cables. All of this will come out together after removing three screws located in the top third of the phone and disconnecting two ribbon cables, one in the top left of the board and one at the bottom left, IIRC. The only trouble I had was the earpiece which is installed with adhesive so I had to pry more than I was comfortable with but it eventually came out.
- At this point I could see the USB port at the bottom of the phone and the wires connecting it to the PC board. I needed to find the 5V+ supply wire. Knowing that the outside two pins are 5V+ and signal ground, I checked continuity between the shield and each outside pin. The one that didn't ring out was the 5V+ supply. THIS NEXT PART WAS THE REALLY HARD BIT I tinned up one end of my red wire and soldered it to the 5V+ supply line coming off the back of the USB port. Had I messed this up I could have been looking for a new phone, but since it worked I knew I was home free.
- I then stripped about 1/4" off the black wire, tinned it, tinned one side of the USB connector shield and soldered them together. This is a very solid solder joint and very easy to make.
- Next I looked at where the plastic of the phone would mate to the PC board and routed the wires such that they might be sandwiched tightly but wouldn't increase the thickness of the finished product. This was pretty easy and there were several ways I could have routed them. I chose to come up into the battery bay via a little hole that Samsung had pre-drilled in the battery bay surround (take out the battery and you can see it). This was far easier than in the Epic mod in the Instructables link above. I reassembled everything as part of this process, checking tolerances as I went.
Finishing the construction
- The last and easiest step is to trim the wires to length, tin up the wire ends and contacts on the inductive charging back, and solder them together. To discover the polarity of the contacts, I set the back onto a touchstone and measured DC voltage across the contacts. You can just look at my picture.
- As I installed the back on the phone, I routed the wires so that they wouldn't cross each other or cross the circuit on the charging coil (the thickest part of the back at this point). I used some Scotch tape to hold them in place. If I did it again I would make the red wire shorter so that it would more easily nest under the black, but I made it work.
I wish I had taken more photos and if I ever have to take the phone apart or mod another one, I'll be sure to take more photos and post them. By looking at the photos on iFixIt and Instructables you will get a pretty good idea of what's in store.
Note: since the back of the Nexus S and the ring around the touchstone are both smooth, it doesn't grip as tightly as the Pre. If you are charging on your desk this is no problem at all. If you use a touchstone in your car, you will want to apply strips of electrical tape or vinyl stickers down the sides of the back to give the mating surface more friction.
Good luck luck to anyone else who endeavors to do this. I would be glad to answer questions if you still have any..
-darren