Barry Freeman, Ph.D.*


Hearing aids (instruments) are entering a new era of rechargeable hearing aid batteries where consumers no longer have to deal with the hassles of replacement disposable batteries.  Just follow recent industry product announcements:

  • “Starkey offers rechargeable option thanks to ZPower”
  • “Free yourself from the hassles of disposable batteries with Phonak Audéo™ B-R rechargeable hearing aid.”
  • “Signia Cellion. The hearing aid that lasts a whole day on a single charge.”
  • Unitron announces “Moxi Fit R, the world’s smallest rechargeable hearing instrument”

The industry has had other defining moments related to hearing aids with the shift from body aids to BTEs (behind-the-ear) in the 1970s; from BTEs to ITEs (in-the-ear) and CICs (completely-in-canal) in the 80s-90s; a transition from analog to digital products in the late 1990s; a return to BTE style RICs (receiver-in-canal) in mid-2000s; and, of course, the recent adoption of wireless streaming hearing aids which now account for 88% of U.S. hearing aid sales. 

The industry appears to be positioned to witness a new trend of rechargeable hearing aids.  Recent evidence suggests adoption of rechargeable products will be welcomed with open arms by consumers.  When asked what compelling features consumers want in their hearing aids, the top answers were “a rechargeable hearing aid” and “rechargeable batteries” (MarkeTrak 9, 2015).  In a recent survey of hearing aid users, 70% indicated they wanted “a rechargeable hearing aid over disposable batteries.” 1   This appears to be confirmed by a recent report of “the initial success of the [Phonak] rechargeable solution where it far exceeded expectations and accounted for 50% of Audéo sales.”2

Rechargeable hearing aids are not new.  We know that companies like Siemens, Hansaton, and Persona have had rechargeable products available for more than a decade, and Dahlberg, Electone, and others, more than forty years ago However, for many reasons, they were not widely adopted and disposable zinc-air batteries have continued to be the dominant power source for hearing aids.  However, that now appears to be shifting as industry is taking a fresh look at rechargeable batteries (Figure 1). 

Figure 1. This provides an overview of various chemistries used in rechargeable hearing aid batteries. While zinc-air disposable batteries have offered an excellent high energy density battery, they are not rechargeable and an estimated 1.6 billion go to landfills every year. Nickel Metal Hydride (NiMH) is rechargeable and has had diverse uses ranging from the Toyota Prius to previous generations of hearing aids. However, NiMH has a relatively short operating time, especially with the new generation of digital wireless streaming hearing aids, and it has cycle limitations requiring replacement after an estimated 300 charges. (Courtesy of ZPower).


Lithium-Ion (Li-ion)

Lithium-Ion (Li-ion) batteries are widely used in consumer electronics, but recent recognition of their potential danger from toxicity and flammability are raising questions about their adoption for small products like hearing aids.3,4   Li-ion batteries have voltages approaching 3.6V as compared to zinc-air which is 1.2-1.4V, and it is also difficult to scale down the size of Li-ion to fit into hearing aids.  The smallest Li-ion size for hearing aids is a battery slightly larger than a traditional hearing aid #13 battery.  There also are shipping restrictions requiring special packaging and shipping labels.5   Due to these concerns, Li-ion batteries must be sealed in the hearing aid case and extensively wrapped to protect the user from leakage, flammability, and ingestion.


Silver-zinc (AgZn)

Silver-zinc (AgZn) batteries have been widely used in space craft, satellites, and by the military due to their high energy density, especially in small sizes.  They feature safety associated with their chemistry (which is non-toxic and non-flammable), along with their ability to be fully recycled.  Figure 2 shows a comparison of energy density by battery and chemistry.  While battery size is not shown, because the AgZn has a higher energy density, it allows for a smaller size. 

Figure 2. Comparison of energy density of AgZn, Li-ion, NiMH, and NiCad rechargeable batteries. Because the AgZn has the highest energy when compared to other battery chemistries like Lithium-ion and Nickel Metal Hydride, the AgZn allows for a smaller battery size.


During the past year, this column has presented a series of articles on batteries6,7,8,9,10,11,12,13,14,15,16,17,18.  It was learned, for example, that a battery is not just defined by its chemistry and size, but also by its capacity.  Capacity represents the energy measured in ampere-hours (Ah) in a cell.  It partially determines the run-time of the device it is powering.19  Consider this equivalent to the capacity of the fuel tank in an automobile.  How many gallons of gas does a car’s fuel tank hold?  This is the capacity of the gas tank measured in gallons.  From the articles, it was also learned that not all batteries are the same.  For example, with #312 size zinc-air batteries, it was reported that some may have 180 mAh capacity while others may provide substantially less capacity.  Just because batteries are the same size (e.g., 312), does not mean they have the same capacity.

Additionally, battery life is dependent also on the current drain of the hearing aid.  Staying with the automobile analogy, some cars get better gas mileage than others and some drivers may have better gas-saving driving behaviors than others.  Similarly, hearing aids have different current drains which are measured in milliamps (mA).  Hearing aids with multiple features activated, such as feedback and noise management will drain more current from the battery than hearing aids without those features.  Hearing aids streaming music or a phone call with 2.4 gHz will use more battery capacity than hearing aids using NFMI (Near-Field Magnetic Induction) and an intermediary device which has its own power source as depicted in Figure 3.

Figure 3. Comparison of current drains in mA of various hearing aids with and without streaming20.


It is clear that the batteries in the hearing aids represented in Figure 3 as Product C and Product D will last longer than those in Products A and B because they pull less current drain.  However, remember that Products C and D, because of their design to utilize conventional hearing aid zinc-air batteries, require an additional intermediary device that has its own power source, so streaming life may be more dependent on the battery life of the intermediary device than the hearing aid.


A next post will continue this discussion, but will concentrate on rechargeable batteries in hearing aids.



  1. Hearing Tracker. (2016).  Rechargeable hearing aid preferences: survey findings.  Posted August 9th
  2. Gola, M and Wieprecht, M. “Hearing aids: A sound route for growth in 2017.” Mainfirst Bank AG, January 2017.
  3. Sharpe, S., Rochette, , and Smith, G. Pediatric Battery-Related Emergency Visits. Pediatrics, Volume 129, Number 6, June 2012
  4. Samsung Report (2017).
  5. S. Department of Transportation, “New standards to improve safety of lithium battery transportation.”, 2016.
  6. Staab, W. My battery doesn’t seem to last long, what could be happening?  Hearing Health and Technology Matters. January 2016.
  7. Staab, W. Hearing aid battery life can vary widely.  Hearing Health and Technology Matters, February 23, 2016.
  8. Freeman, B. A. Battery life counseling – with consideration of rechargeable batteries.  Hearing Health and Technology Matters, March 1, 2016.
  9. Staab, W.J. Hearing aid battery life is not easy to predict.  Hearing Health and Technology Matters, February 23, 2016.
  10. Staab, W.J. My hearing aid battery life is longevity challenged! Hearing Health and Technology Matters, February 16, 2016.
  11. Staab, W.J. Why are there holes in my hearing aid battery?  Hearing Health and Technology Matters, March 15, 2016.
  12. Staab, W.J. Hearing aid battery – where are we?  Hearing Health and Technology Matters.  December 27, 2016.
  13. Staab, W.J. Hearing aid fuel cells?  Hearing Health and Technology Matters, January 3, 2017.
  14. Filips, G. Rechargeable batteries and hearing aids are a natural fit.  Hearing Health and Technology Matters, April 25, 2012.
  15. Schweitzer, H.C. Are you ready for the next major disruption?  Rechargeable hearing aids.  Hearing Health and Technology Matters, February 17, 2015.
  16. Hannan, G. What I didn’t know about hearing aid batteries.  Hearing Health and Technology Matters.  May 8, 2012.
  17. Klop, L. Hearing aid battery dangers.  Hearing Health and Technology Matters, April 19, 2016.
  18. Van der Ent, L. Market trends: the charge of recharge.  Hearing Health and Technology Matters, November 29, 2016.
  19. Buchmann, I. Batteries in a portable world: A handbook on rechargeable batteries for non-engineers. Cadex, 2016.
  20. Freeman, B., Powers, T., Perez, J. Battery Life: Counseling patients about the power consumption of wireless streaming hearing aids.  Presentation at Annual Conference of the American Academy of Audiology, Phoenix (2016).


*Barry A. Freeman, Ph.D. is Vice President, Business Development of ZPower Battery, LLC.  He is a past-president of the American Academy of Audiology and served on the Academy’s Board of Directors for six years.  He received the Distinguished Achievement Award from the American Academy of Audiology in 2006.

Measuring a hearing aid to ANSI (American National Standards Institute) Standards should be a first step before any programming of the hearing aid is attempted.  While it is assumed that the hearing aids meet ANSI Standards, as measured and sent by the manufacturer, confirmation of the hearing aid’s basic foundation is necessary to ensure that the eventual programming of the hearing aid starts from a solid and known foundation1.  Unfortunately, such measurements seldom, if ever, are made before a hearing aid is programmed.

Is it “okay” to assume that hearing aids meet ANSI Standards when they arrive? A recent study by Holder, et al (2016), as discussed in a previous post, reported that of 73 hearing aids tested, results showed that none were within the allowable tolerance for every specification, with the exception of Max OSPL902.

To be clear, ANSI does require a set of conditions to be met for ANSI measurements.  Specifically, the settings are as follows:

Basic settings of controls

The hearing aid shall be set to have the widest available frequency response range, the greatest available HFA-OSPL90 and, if possible, the greatest HFA-FOG. Where possible, the AGC (automatic gain control) function of AGC hearing aids shall be set to have minimum effect for setting the gain control to RTS (reference test setting) and for all tests except those of section 6.15. Other adaptive features such as some noise suppression and feedback reduction systems, etc., which may affect the validity of the measurements made with steady-state pure tone signals should be disabled. For the tests of section 6.15, the AGC function shall be set to have maximum effect. The settings used for testing shall be specified by the manufacturer by providing either a test program, a set of programmed settings or by reference to physical control settings. For the purposes of this standard, expansion shall be considered as part of the AGC function3.

Is this important measurement step taken routinely?  This author doubts that it is seldom, and never taken by most who dispense hearing aids.  Figure 1 shows the results of measurements taken on 12 premium hearing aids to determine how closely they meet ANSI Standards.  These were all received as normally-purchased hearing aids directly from manufacturers.  All were RIC (receiver-in-canal) instruments.

Figure 1.  Twelve premium hearing aids measured according to ANSI Standards.  The pink-colored areas represent measurements falling outside ANSI tolerances.  Blue represents measurements better than the tolerances.  The yellow areas, in question marks, indicate that no determination could be made because of insufficient information, or in the case of one instrument, it was rechargeable and the battery could not be removed for current drain to be measured.


Comments Relative to Measurements in Which Tolerances Are Involved

Note that not all ANSI measurements have tolerance requirements.  These will be identified in the discussion that follows.  Reference is made to Figure 1.  All measurements were made with factory level test equipment, the Frye Electronics 8000 Hearing Aid Analyzer.

OSPL90 – ANSI tolerance is to not exceed the manufacturer’s specification by +3dB.  None of the hearing aids exceeded this value.  In fact, all were below this number ranging from -0.3 to -6 dB, or an average of -2.85.  It appears that manufacturers take exceeding this value seriously, keeping the OSPL90 (output saturation pressure level at 90 dB signal input) lower by approximately 3 dB.

HFA-OSPL90 – None of the hearing aids exceeded the ±4 dB tolerance for this measurement (high-frequency average output saturation pressure level with 90 dB input signal).  All but two were under the published value, which might be expected based on the OSPL90 measurements.

FOG Peak – No tolerance is required for this measurement (full-on gain) from that which is published.  However, the range from published data was rather wide – from +11 to -15.

HFA FOG – This measured value is to be within ±5 dB of the published specification (high-frequency average full-on gain).  Two of the instruments fell outside this area, both on the minus side.  Only two of the instruments fell above the published value, while all the others were below, ranging from -6 to -1.1 dB.

Current Drain mA – Current drain (in milliamp) tolerance is the published value, plus 20%.  Half of the hearing aids exceeded this tolerance.  One of the instrument’s current drain could not be measured using traditional procedures because it used an internal rechargeable battery.

Battery Life in Hrs. – No tolerance figure exists for this measurement.

EIN dB – Equivalent Input Noise (EIN) measured value is not to exceed the published value by more than 3 dB.  Of the twelve instruments, two fell outside this tolerance range.  However, half of the instruments (six) measured lower than the published EIN.  Having been involved in writing hearing aid specifications, that the publshed data is higher is not unexpected.  It is better to have EIN measured lower than published than the other way around.  This is most likely intentional on the part of the manufacturer.  Overall, the average EIN in dB was 20.4, with a range of 17 to 26.  The lowest measured was 13.2  and the higherst at 40.5 dB.  This essentially represents the noise floor of the hearing aid.

Freq. Lower – No tolerance value exists for this feature.  The test equipment frequency measurement range started at 200 Hz.  As a result, any response below this frequency could not be recorded, other than to indicate that the response was lower than 200 Hz (e.g. <200 in the data).  Of the instruments, six had a higher measured frequency range than provided in the manufacturers’ specifications by an average of 115 Hz.

Freq. Upper – No tolerance value exists for this feature.  The test equipment frequency measurement range extended to 8000 Hz.  As a result, any response above this frequency could not be recorded, other than to indicate that the response was higher than 8000 Hz (e.g. >8000 in the data).  Of the instruments, nine had a lower measured frequency range than provided in the manufacturers’ specifications.  One was lower than the published specifications by just over 3000 Hz.  For those instruments publishing a 10,000 Hz upper range (three instruments), one measured below 8000 Hz (6777 Hz).  Of the ten hearing aids where measurements were below 8000 Hz, the average response was lower than the published specification by 1106 Hz.

THD – Total Harmonic Distortion (THD) was measured at the frequencies of 500, 800, and 1600 Hz.  The ANSI standard calls for a tolerance of no greater than +3% of the published value.  One has to wonder, with today’s digital hearing aids, if this tolerance factor is even an issue.  For example, 3% of 1% THD is 0.36.  Added to the value of 1%, this means that the allowable tolerance is 1.36%.  Based on calculating these percentages for each of the three frequencies used for THD measurements, it is not surprising that a number of the hearing aids had tolerance measurements outside the acceptable values.  But really, what is the perceptable difference between 1 and 1.2 THD?  But, this value exceeds the tolerance acceptance.  Of the instruments, only one (hearing aid #9), a rechargeable unit had THD values of perhaps some significance.  Most would agree that THDs at 500 Hz under 2% are not worthy of concern.

Response Curve – One measurement not calculated was that of the response curve.  A cursory observation of measured response curves relative to the manufacturers’ specifications, suggests that some of these instruments would fail this test.  This calls for ±4 dB within the curve from 200 to 2000 Hz, and ±6 dB from 2000 to 4000 Hz, or 0.8f2.

Input/Output – Not all of the hearing aids showed a linear input/output function, which would be expected if the hearing aid is set to linear.  The Standard does require that the aid be set to linearity or to its lowest AGC activity, but some of the hearing aids still showed non-linear function, including three that showed expansion.

Time Constants – Attack and release time tolerances (not shown in Figure 1) were seldom met.  The most common deviation is represented by the release time graphs of Figure 2.  In some hearing aids the instrument never “stabilized” within 1000 msec., way beyond its published recovery time.  What seems to be more commonly seen is the inability of the instrument to stabilize during recovery.  It is not clear what the circuit action is attempting to do that creates such long instability during recovery. 

Figure 2. Attack and release times for three of the twelve hearing aids testing, showing the instability of adjusting to the new level, especially for the release time.


Should Hearing Aid Professionals Check The Hearing Aids They Fit Prior to Programming?

The results of hearing aid measurements suggests that this would be good to do.  A problem with this, however, is that most clinic/office hearing aid measurement units do not have calibrated microphones of the range and sensitivity to make such measurements.  Factory instruments do.


The results of this investigation into hearing aids meeting ANSI Standards shows that one cannot blindly assume that all new hearing aids are in compliance.



  1. Frye, G., and Staab, W.J. Hearing aid analyzers…factory floor testing as a start. Hearing Health and Technology Matters, January 24, 2017,
  2. Holder J, Picou E, Gruenwald J, Ricketts T. (2016). Do modern hearing aids meet ANSI standards? J Am Acad Audiol 27(8):619–627.
  3. ANSI S3.22-2003. (Revision of ANSI S3.22-1996). AMERICAN NATIONAL STANDARD Specification of Hearing Aid Characteristics. Accredited Standards Committee S3, Bioacoustics. Standards Secretariat Acoustical Society of America 35 Pinelawn Road, Suite 114 E Melville, NY 11747-3177.