First let's get to know the issue. A retuning customer has a design a product that is basically a cylinder with an 18650 Li-ion battery running down the center. The engineer is evaluating the BQ51003 / 51013B and designed the charging coils to be coplanar and positioned at the “bottom” of the device with the coils below and away from the battery and circuit board. For aesthetic reasons the mechanical engineers moved the coils so that the battery now runs down the center of the coils and the PCB is just on the outer radius of the inner coil positioned in between the RX and TX coils. Now that the molds have been made the customer is reaching out to me to see if we can salvage this configuration.
Now let’s give you some details on the configuration. The coils are coplanar as described above, the RX fitting inside the TX, so far so good. The inner RX coil is approximately 1.3 to 1.5 inch outer radius with 6 or 9 turns of 22 to 26 ga wire with the coil being three turns long and two or three turns thick depending on the configuration. The outer TX coil has approximately 2+ inches inner radius using 16 to 20 ga wire again with the 9 turn configuration similar to the RX coil. The design is being used at frequencies of about 180KHz to almost 400KHz again depending on the configuration and tuning components. Unfortunately I did not get the inductances of all the various coils used in their experiments.
So now we get to the heart of the matter. I suspect that it is the Ni in the casing of the Li-ion battery that is really messing with the performance as it is ferromagnetic (if anyone can expand on this premise I would be grateful for an education). I think the PCB inside the coils will also play a roll but it does not seem to be nearly as substantial as the battery. Anyway while I was experimenting with the position of the battery a thought came to me, “ Let’s try some ferrite plates and see if we can direct the flux away from the battery “. So I took a battery and placed it against the coils (as opposed to through the coils) and found that I could get a significant reduction in coupling efficiency, though not as great as when the battery was inside the coils, but this configuration would allow me to pull off the next trick. I placed a 1/16” ferrite slab between the battery and the coils and the influence of the battery disappeared. So now we can direct the flux through the ferrite and back around to the coils effectively shielding the battery, Cool. So here are the questions;
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Would a “sleeve” of ferrite (kind of like a thin toroid) allow the flux to pass without being effected by the battery if inserted between the battery and the inner radius of the RX coil?
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How thick should such a magic sleeve be?
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How long should the sleeve be with respect to the coil length?
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What permeability would be best ( low / med / high )?
Best - Dante
