Tech Topic | September 2016 Hearing Review
The authors contend that silver-zinc rechargeable batteries are an exciting innovation in hearing aid power sources. This new battery technology for hearing aids can operate a wireless streaming hearing device for a full day, recharge in a matter of hours, last at least one year, and can be used interchangeably with zinc-air batteries because it does not need to be sealed in the hearing aid case.
Hearing aids are one of the few premium portable electronic devices that still use disposable and not rechargeable batteries. Battery life in hearing aids is getting shorter as the features to enhance listening experiences are added to new hearing aids, and a battery that used to last a few weeks now lasts only a few days. Unfortunately, we don’t hear a lot of hearing care providers talk about rechargeable solutions, even though they currently are available.
How Does a Battery Work?
Battery capacity is like the gas tank in your automobile. Batteries have defined capacities measured in milliamp hours (mAh) the way your car’s gas tank might have a capacity of 20 gallons.
Similarly, your hearing aid has a current drain measured in milliamps (mA) just like your automobile has a fuel consumption rate (miles-per-gallon) which is dependent on how fast you drive, driving conditions, engine size, and your car engine’s efficiency. A hearing aid without wireless streaming features, for example, might have a reported current drain of 1.0 mA. A 312 zinc-air battery with a functional capacity of 126 mAh would have a predicted battery life of 126 hours (Figure 1). However, if the hearing aid had certain features activated, the current drain could be higher with battery life correspondingly lower.
Figure 2 shows battery current drains measured on a commercially available hearing aid with 2.4 GHz streaming features. Current drain at idle with no features activated was 2.0 mA, but increased to more than 6.0 mA with streaming.
The shorter battery life and need to change batteries more frequently may not be the fault of the disposable batteries, but may result from the enhanced performance and features of the hearing aids.
Example of Hearing Aid Battery Use
John wears his new hearing aids 12 hours per day with wireless streaming to his i-Phone an estimated 3 of those hours. During normal operation without streaming, the current drain of each hearing aid is 2.0mA and while streaming it is 5.5 mA. Therefore, he drains a total of 34.5 mAh everyday (2.0 mA x 9 hours + 5.5 mA x 3 hours = 34.5 mAh of daily current drain). If he uses a disposable 312 zinc-air battery with a functional capacity of 126 mAh, he will have an estimated 3.6 days of battery life (126 mAh/34.5 mA per day = 3.6 days).
The obvious conclusion is that there are associated increases in costs to operate the hearing aid, including the need to change batteries much more frequently, purchase batteries regularly, and dispose of zinc-air batteries in landfills since they are not recyclable. Why, then, is there not a greater interest in rechargeable hearing aid batteries?
In surveys of hearing care professionals, they noted, “patients don’t ask for rechargeable batteries.” Yet, individuals with hearing loss identified rechargeable hearing aids and batteries as among the “most compelling features they sought on hearing aids.”1 Why is there a disconnect between consumers and providers? Perhaps, it is related to the poor experiences that professionals have had with rechargeable products which, historically, have been found “to not provide a full day of use for users” (submitted paper; Freeman B, Dueber R, and Renken T, 2016).
Nickel Metal Hydride Rechargeable Batteries
Until recently, commercially available rechargeable hearing aids almost exclusively used Nickel Metal Hydride rechargeable batteries (NiMH). These batteries have worked well in the Prius and other hybrid automobiles, and also served their purpose with non-wireless streaming hearing aids. However, just like hybrid automobiles, these batteries by themselves are not sufficient to run a full day strictly on the power of the battery.
NiMH batteries have been available in selected hearing aids for more than a decade. The authors contend that, unfortunately, NiMH can’t provide enough energy when fully charged to operate many of the newer wireless hearing aids throughout an entire day; thus, hearing aids with NiMH rechargeable batteries have experienced limited adoption by manufacturers and hearing care professionals, other than as a niche-product for wearers with severe dexterity and sight limitations who have difficulty handling disposable batteries.
Lithium-ion Rechargeable Batteries
Many consumers want to know why hearing aids do not use the lithium-ion batteries that are widely used in portable electronics like our cell phones and even some cochlear implants. While some hearing aid manufacturers are adopting lithium-ion and lithium-polymer batteries, the authors believe there remain some major challenges associated with them, including:
- Safety. Lithium-ion batteries can be dangerous and even life-threatening if accidently swallowed. Battery ingestion fatalities and severe esophageal injuries have been associated with lithium-ion coin cell batteries.5 Sharpe et al in 2012 stated that “If a lithium-ion button battery lodges in the esophagus, surrounding tissue injury can occur in just 2 hours.”6 Although hearing aids powered by Lithium-ion are sealed to protect a patient if accidentally ingested, chewing on a Lithium-ion powered hearing aid by a pet, such as a dog or cat, could prove fatal to the animal.
- Sealed case. Lithium-ion batteries must be sealed in a case, meaning they are not removable. In instances when the battery does not last the full day or users forget to charge them, the hearing aid user must go without their devices. It also means that, when the battery completes its life-span (generally about one year), it must be returned to the manufacturer for battery replacement.
- Higher voltage. Lithium-ion’s output voltage of 3.6 V is well above the maximum limit of most hearing aid electronic circuits, which are designed to operate closer to 1.2-1.4 V. Although there could be positive developments associated with higher voltages, adopting this chemistry adds considerable size, cost, and redesign of the hearing aid architecture.
- Insufficient energy and size limitations. To date, it has not been demonstrated that lithium-ion has the capacity to operate wireless streaming hearing aids a full day unless the hearing aid uses external streaming accessories that have their own power supply. Plus, lithium-ion typically is not scalable in size to small hearing aid battery sizes like 312 or 10.
- Flammable. The contents of these batteries are highly flammable and explosive.
- Disposal restriction. Due to their flammability and lack of recoverable materials, the batteries are problematic as waste materials. Since they are sealed into a case, they are not meant to be removed.
Silver-zinc Rechargeable Batteries
The newest advancement in rechargeable batteries is a commercial spinoff from NASA. The US space program used silver-zinc batteries, and NASA’s Spinoff magazine will soon highlight “NASA technologies that are benefiting life on Earth in the form of commercial products.”7 Silver-zinc rechargeable battery chemistry is now in the commercial sector with a current focus on rechargeable batteries for hearing aids.
Silver-zinc batteries have the highest theoretical specific energy and energy density of all commercially available rechargeable battery technologies (Figure 3). These batteries utilize an aqueous (water-based) electrolyte, which reduces the flammability hazard often associated with some Li-ion batteries and are therefore safer for both the user and the environment.
Figure 3 shows a comparison of the published literature values for the silver-zinc battery energy density and specific energy with respect to other commercial secondary battery chemistries. In the figure, it can be seen that silver-zinc batteries have the highest specific energy and energy density ranges when compared to the other rechargeable chemistry solutions available on the market, including Li-ion.
In recent years, improvements in battery technology have focused mainly on large-format batteries for motor vehicles and energy storage. At the opposite end of the size spectrum, relatively little attention has been placed on small and miniature batteries for electronics and medical applications.
Miniature batteries are where silver-zinc offers distinct advantages over every other rechargeable battery technology. As we know from experiences with our cell phones, rechargeable batteries (eg, lithium-ion) hold their charge for only a few months before users begin to notice shorter battery life and longer charge times. This is not atypical, as it is known that lithium-ion cells usually deliver the rated capacity for only the first few cycles (ie, charges), and then rapidly fall to between 90-85% within the first 100 cycles. Furthermore, the capacity of the Li-ion cells generally plateau between 85-80% of their advertised rated capacity before the 200th cycle. In comparison, the silver-zinc button cells developed for hearing aids and wearable devices maintain greater than 98% of their advertised rated capacity for over 300 cycles. This represents a significantly greater capacity density and cycle life performance over current rechargeable batteries.8
Silver-zinc also has fewer safety risks than, for example, lithium-ion as noted above. Ortega9 recently evaluated various batteries with an artificial saliva test in response to concerns about battery ingestion fatalities and esophageal injuries reportedly associated with lithium-ion coin cells.5,6,10 Comparing different batteries and chemistries, they found lithium-ion to have the largest increase in artificial saliva pH (Figure 4). The larger the pH increase, the greater the damage would be to an individual’s esophagus for someone who swallowed the battery. Li-ion’s higher voltage of 3.6V results in greater damage faster, while the lower voltage of silver-zinc and zinc-air results in only moderate pH changes. They concluded that the lithium-ion cells would have done substantially more damage to human tissue than any of the other battery chemistries.
Deaths attributed to coin and button cell batteries has attracted the attention of groups such as the Consumer Product Safety Commission, IEC, WHO, and ANSI, and has prompted the industry to develop standards to improve battery ingestion safety through multiple approaches:
- Warning language and symbols;
- Education for parents and healthcare professionals;
- Child-proof packaging, and
- Battery compartment design.
As we enter a new generation of hearing instrument technology, professionals need to be cognizant of the ramifications of these advancements on their patients.
Silver-zinc rechargeable chemistry in hearing aid batteries appears to be among the most exciting advancement since we first transitioned from silver-oxide and mercury batteries to zinc-air. The silver-zinc rechargeable microbattery offers about twice the energy of NiMH rechargeable batteries (Figures 5-6) and provides patients with exceptional battery life even with wireless streaming hearing aids (Figures 7-8).11
Figure 7 presents average wear times by a consumer wearing a wireless streaming hearing aid with a silver-zinc #13 battery. This patient wore the hearing aid an average of 16-17 hours per day on a single charge, and the battery recharged in approximately 3 hours.
At the completion of every day, the silver-zinc rechargeable battery had a 60% average remaining capacity, suggesting that the patient could have worn the hearing aid an additional day without recharging.11
Similarly, Figure 8 presents a patient wearing a wireless 2.4 GHz streaming hearing aid with Size 312 silver-zinc rechargeable battery.
At the completion of average wear time of 15 hours/day with an estimated 2 hours of daily streaming to a smartphone, the patient had 10-12 mAh of remaining capacity—enough for the patient to wear the hearing aid with streaming several more hours.
Silver-zinc rechargeable batteries appear to be an excellent innovation for the hearing aid market. The battery can operate a wireless streaming hearing aid a full day, recharge in a matter of hours, last at least 1 year, can be used interchangeably with zinc-air batteries because it does not need to be sealed in the hearing aid case, requires no handling throughout the year, and is a fully recyclable green energy.12 Currently, an estimated 1.6 billion zinc-air hearing aid batteries go to landfills every year. A single silver-zinc battery will take the place of an estimated 100 zinc-air batteries. Providers finally can give patients the rechargeable technology they are requesting with a space-age rechargeable battery that was good enough for NASA astronauts and certainly should be good enough for hearing aid users.
Abrams HB, Kihm J. An introduction to MarkeTrak IX: A new baseline for the hearing aid market. Hearing Review. 2015;22(6):16. Available at: https://hearingreview.com/2015/05/introduction-marketrak-ixnew-baseline-hearing-aid-market
Staab W. My battery doesn’t seem to last long. What could be happening? Hearing Health Matters. January 2016. Available at: http://hearinghealthmatters.org/waynesworld/2016/my-hearing-aid-battery-doesnt-last-long
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;April 2016.
Linden D, Reddy T. Handbook of Batteries. 4th ed. New York: McGraw-Hill;2010.
Poison Control National Capital Poison Center. Fatal button battery ingestions: 49 reported cases. 2016. Available at: http://www.poison.org/battery/fatalcases
Sharpe S, Rochette L, Smith G. Pediatric battery-related emergency department visits in the United States, 1990-2009. Pediatrics. 2012;129(6):1111-1117.
Dueber R. Rechargeable hearing aid batteries draw from NASA research. Spinoff Magazine. 2016; In press.
Ortega J, Dueber, R. Energy density comparison of silver-zinc button cells with rechargeable Li-Ion and Li-Polymer coin and miniature prismatic cells. Battery Power. 2015;19(4)[Winter]:21-22. Available at: http://www.batterypoweronline.com/main/articles/energy-density-comparison-of-silver-zinc-button-cells-with-rechargeable-li-ion-and-li-polymer-coin-and-miniature-prismatic-cells
Ortega J. Design and testing of the safe high energy density silver-zinc battery chemistry for hearing aid devices. Presentation at: International Battery Seminar and Exhibit, Fort Lauderdale, Fla; March 21, 2016.
Poison Control National Capital Poison Center. Nonfatal button battery ingestions with severe esophageal or airway injury: 200 cases. 2016. Available at: http://www.poison.org/battery/severecases
Freeman B. A new door to rechargeable hearing aid battery solutions. Hearing Review. 2015;22(8)[Aug]:22. Available at: https://hearingreview.com/2015/07/new-door-rechargeable-hearing-aid-battery-solutions
Freeman B, Marincovich P, Madory R. Making Audiology green. Audiology Today. 2016; 28(1)[Jan/Feb]:50-54.
Correspondence can be addressed to HR or Dr Freeman at: [email protected]
Original citation for this article: Freeman B, Ortega J, Dueber R. What’s the State of Rechargeable Batteries for Hearing Aids? Hearing Review. 2016;22(9):28.