I am experiencing a strange issue, likely because I do not fully understand the technology I am trying to make play nice together, but maybe someone here can shed some light on it.
I have an HTC Evo 4G and a Duracell Powermat inductive charging kit. The kit is designed to charge an included battery to externally connect to USB devices, and also comes with an iPhone 4/4S case which the phone would dock into and allow it to wirelessly charge. I had the idea to remove the inductive coil from the iPhone case and install it in my Evo, and happened accross this tutorial here: http://asmd.wordpress.com/2011/09/26/evo-4g-wireless-induction-charging-modification/
The induction coil from the iPhone 4 case wasn't exactly the same, however I did find the +5V lead, connected it to the phone in the way described in the above tutorial, and everything works great - that is, until I put the case back on the phone. The coil is taped directly to the inside of the back case of the phone, and by itself (connected to the phone or not) it works fine. The problem is that when the case is on the phone, OR the phone's battery (which is located directly behind where the coil would be with the back of the phone attached) is closer than about 1/4" of the coil, or directly against but covering more than about 1/2 of the coil horizontally, the Duracell inductive transmitter makes a 'ticking' sound for a second or so and then nothing happens. Normally, it would make a 'bleep' sound and then begin charging.
After some mild testing, it seems like it's just the proximity of the battery to the coil that is the problem, however in the tutorial I linked to, the setup is nearly identical but apparently that person didn't have this issue. Is there some type of interference or something that could be causing this? Would placing some kind of shielding such as a wire mesh between the coil and the battery help? (Haven't tried this simply because I don't have any handy - though I tried a thin sheet of metal and plastic, and neither worked unless the battery was again at least 1/4" away from the coil).
Perhaps I just don't understand the physics behind it but it seems like it should work and I'm stumped as to why it wouldn't. I don't see anything that could be causing a short or anything, so I'm really at a loss. Thanks in advance for any ideas or explanations as to why this might be happening.