Lithium Ion Cell/Batteries Electromagnetic Field Reduction in Phones for Hearing Aids Compliance
Authors: Maleki Hossein , Howard Jason N , Hallmark Jerry A 
Published: February 2020

The Hearing Aid Compatibility Act of 1988 (HAC Act) requires that the Federal Communications Commission (FCC) ensure that telephones sold/used in the US and its territories are hearing aid compatible. “T-HAC” (Tele-coil Hearing Aid Compatibility) requirements are to ensure that inductive (magnetic) coupling from cell phone does not interfere with the hearing aid’s audio quality. “T-HAC” requirements include three areas:

1)     Signal Level

2)     Frequency Response

3)     Signal Quality (Signal-to-Noise Ratio)

Batteries occupy a large percentage of a phone total volume and that leaves limited options to place  the phone speaker in locations with minimal EMF interference. The EMF is generated by electrical currents in the phones’ circuit boards and components, including the battery. Therefore, it is important that batteries to have no/limited EMF emission in order to ensure phone speaker audio quality.

Li-ion cells, typically, are packaged using either an aluminum-can (prismatic) or laminated layers of Aluminum and polymers, known as Li-ion polymer (LIP). Prismatic cells, generally, consist of jelly-roll with +/-tabs placed on opposite ends of cathode and anode, respectively, before being wound into jelly-roll. The LIP cells, on the other hand, are built using either jelly-roll or cut-and-stacked cathode and anode layers. A polymer membrane electrically insulates cathode from and anode. Organic electrolyte provide Ionic conducting medium for Li-ion to shuttle between anode and cathode during charge and discharge. Therefore, spread/shape of EMF emission depends on the cells construction (stacked electrodes or jelly-roll, +/- tab position on electrodes, and external connectors’ distance) as well as charge/discharge current loads. However, it is important that cell design changes be compatible with the cell mass manufacturing processes.     

 Here, we have investigated design changes leading to EMF emission reduction from Li-ion batteries consisting of a single Li-ion cell or two Li-ion cells contented in parallel. Changes include: 1) cell’s internal +/- tabs locations and length on cathode/anode layers; 2) cell’s external +/- connectors’ spacing, and 3) parallel connected cells arrangement. Fig. 1A shows a schematic of the experimental setup used for measuring the test samples’ EMF emissions under GSM pulse current in frequency range of 12Hz-20 KHz. Fig. 1B shows EMF emission contours along a jelly-roll length (X-direction), with copper foils wound representative of anode and cathode. Fig. 1C shows a schematic of samples with +/- tabs positioned on the opposite, center and same ends of the representative anode and cathode layers: black arrowed lines emphasizing +/- current flow directions. Table-1 shows EMF emission values (maximum and minimum) for the samples in X (length), Y (width) and Z (13 mm above the sample top surface) directions.

For this article, we tested EMF emissions on a collection of prismatic and LIP cells with wound electrodes (jelly-roll), LIP cells with cut-stacked electrodes, and two prismatic cells connected in parallel, from three different major cell manufacturers. Results show that the cell’s internal +/- tabs location on cathode/anode, and spacing between the cells external +/- connectors play critical role in reduction of batteries EMF emissions. Effective EMF emission levels of a multi-cell battery (MCB) assembly are impacted by its cells’ arrangement and their external +/-connector current paths. Furthermore, proper EMF arrangement of parallel and/or series connected cells (placed side-by-side or on top of each other) for reducing EMF emission, requires clear understanding of each cell’s internal structure and external +/- current paths.

Figure 1