I think the aluminum will not interfere with the induction charging coils.
Looks like that magsafe DOES have a ferrite element in it to keep the back side field away from the aluminim shell.
Hi guys. The bottom line is that Al is not desirable, but can be used if you keep it far enough a way from the drive coil. The currents induced into a metal like the Al are called Eddy Currents. These circulate around the AC magnetic fields in conductors and flow in the opposing direction of the currents in the charging coil. Hence, yes they tend to heat the metal, but probably insignificant for the application. They also tend to cancel the field lines out that you are trying to get into the pickup coil. Hence, this heat power is used up and not much power provided to the appliance.. at least not very well. You are better off if you have a magnetic material surrounding the coil, but not if it is conducting. This thing is really just a form of a transformer where you are going to take the power delivered and store it into the appliance battery. Wiki explains Eddy currents a bit.
https://en.wikipedia.org/wiki/Eddy_current
In 60 Hz transformers that you are familiar with the metal is a conducting Si-iron magnetic material, but it is constructed of layers/leaves which has breaks in it separated by non-conductors, usually polymers/varnish/paper, to prevent or limit the induced Eddy currents to small distances. As the frequency is increase the thickness of these layers must be made smaller and smaller which becomes impractical. Hence, for higher frequency devices like this one they use non-magnetic materials like ferrites (a special form of iron oxide). I am not for sure what frequency this device is using but I am sure it is at least in the 10s of kilohertz range if not higher. The reason high frequencies are used is because the power delivered occurs each cycle. The more cycles, the more power delivered. Anyway, the magnetic component is only there to try to increase (focus) the portion of the magnetic field at the pick up coil of the watch or phone etc. Efficiency is important and so I suspect that they try to match the inductance/capacitance of the drive circuit to that of the pick up circuit. This way you can operate near resonance and get a little more power transfer. If I were building one of these and did not have access to a custom piece of ferrite, I would embed the drive coil in plastic (epoxy) but keep it as close to the surface as possible so that it would be near the receiving coil. "Close" is relative to the coil diameter. From a geometry of the fields stand point, I would also try to sort of match the transmitting coil diameter to that of the receiving coil diameter.
So the ideal geometry of the ferrite is sort of like below , however the hole in the middle is not required and this shape is actually made for transformers so that wire coils can be threaded on to them. The charging coil is wrapped around the portion labeled as diameter D and so sits in this slot between the outside of cylinder D and the inside of can shaped cylinder A. The magnetic field lines circulate along the center D portion then along the back side flat, then up the outside A portion and sprays out into space where the receiving coil is located. The fact that the material is an easy to magnetize (soft) ferrite causes the field lines to be trapped in the ferrite. They would not reach a metal that was located on the back side. Hence, field lines are focused out into space in the area where the letter A and E are shown, which is where the pickup coil needs to be placed. I took this sketch off of a web site that actually sells ferrite embedded into plastic to try to make a simple shape for phone/watch chargers. It is not as good as this shape, but it works... somewhat. Check out the pictures and links at their web site..
https://www.caracoltech.com/wire
In the most efficient charging system, there would be second piece of ferrite embedded inside the watch/phone with a similar shape but now the pickup coil is inside the this second piece of ferrite. By the way, the hole, C, in the Center is to provide a place for a bolt to go to clamp two these two ferrite pieces together, one with the primary coil and the other with the secondary coil. Hence, the construction builds a transformer. The slots on the circumference are for the coil wires to feed inside.
