Battery Life: Counseling Patients about Their Wireless Streaming Hearing Aids

This text course is an edited transcript of a ZPower webinar on AudiologyOnline.

Learning Outcomes

After this course learners will be able to:

• Explain how to counsel patients about variations in battery life from wearing wireless streaming hearing aids.
• Describe the impact wireless features and streaming have on battery life.
• Explain the choices to patients of the advantages of different battery chemistries including rechargeable batteries.

Introduction and Overview

ZPower started in the 1990s as a spinoff of NASA. NASA was using particular battery chemistries in their space shuttles and in satellites, and then the military adopted them. ZPower wanted to devise a way to miniaturize this battery chhemistry, which is silver-zinc, and make it rechargeable. In 2008, they decided to explore applications for the hearing industry.

The data that I'm presenting today was primarily collected in the ZPower electronics laboratories, and also in clinical field trials with thousands of patients who have been wearing the ZPower rechargeable battery systthem with their silver-zinc rechargeable battery. We've done field trials everywhere from Vanderbilt University, to University of Pittsburgh in Dr. Catherine Palmer's labs. We've worked with Kaiser Permanente in northern California, as well as with independent practices and private clinics around the country.  ZPower is primarily a company of scientists, chemists and engineers. I'm the only audiologist in the company. When we started to explore the need for rechargeable batteries in the hearing aid market, we looked at our battery to determine what kind of capacity and sizes were required to meet the needs of patients. We discovered that there are a lot of factors, both product- and patient-related, that were unclear to us, and not discussed much in the literature. For example: What is the impact of wireless streaming on battery life? We know that battery life used to be 10 days to two weeks, and all of a sudden, we're getting three to five days on a battery. What about product features? We've learned that features like feedback management systems, wireless programming, and ear-to-ear communication all have a dramatic impact on the battery life. This led to our study and the results that I will discuss today within the framework of counseling patients. 

Historical Perspective and Timeline

Some of the first powered hearing aids were created back in the early 1900s. Miller Reese Hutchinson produced early vacuum tube hearing aids, and they had huge current drains. It took a lot of power and energy to operate these hearing aids. Then, transistors were introduced, which drastically reduced battery drain, as well as the size of our products.

I entered this field in the very late 1960s after coming out of the military. I worked in a pediatric center in Massachusetts in the Boston area. At that time, we were fitting children with body hearing aids worn on their chest. Furthermore, most people had monaural fittings; there weren't many binaural fittings. As we moved into the 70's, we started to transition to behind-the-ear hearing aids, some of which were pretty large. Sometimes, hearing aids were put into eyeglasses. If a person needed hearing aids, but had perfect vision, they would wear glasses with clear panes of glass, with the hearing aids mounted on the frame behind the ear. That's when people like Sam Lybarger and Cy Libby started publishing work on earmold acoustics. As we moved into the 1980s, hearing aids got smaller with transistors, in-the-ear hearing aids came along, and more fittings were binaural. The 1990s saw the introduction of CICs, and we moved into programmable hearing aids and early digital hearing aids. In the 2000s, wireless hearing aids with streaming capabilities were introduced. 

New Era in Hearing Aid Rechargeable Products

Today, we're entering a whole new generation of products as we move into using rechargeable batteries. Soon, we may no longer have to counsel our patients about batteries, or how to install batteries, because of the transition to rechargeable products. Recently, Starkey and Unitron both introduced new rechargeable products with the ZPower silver-zinc batteries. We know that Phonak and Signia have introduced rechargeable hearing aids using lithium ion cells. With other consumer products, especially in the same price range as hearing aids, can you think of one that requires a replacement battery every three to five days? Can you imagine if you had to do that with your cell phone, and change the battery every three to five days? In the same way that we witnessed the transition from analog to digital hearing aids, or from body aids to miniature styles, we are about to witness the transition from disposable batteries to rechargeable batteries. This is becoming a greater need since we've introduced wireless streaming hearing aids. According to some recent data from the Hearing Industries Association, wireless streaming hearing aids now account for 88% of the hearing aids fit in the United States. 

Battery Overview

​ Let's review types of batteries, and the power sources that are available to us in the hearing aid industry. 

Zinc-air.  Zinc-air batteries continue to dominate the hearing care market. They have great energy. They work well with our hearing aids. They are disposable, but not recyclable. It is estimated that 1.6 billion zinc-air hearing aid batteries go to landfills every year.

Nickel metal hydride. In terms of the rechargeable market, nickel metal hydride is a battery chemistry that is rechargeable. Over they last decade, they have been available in rechargeable products from Siemens, Hansaton, and Persona. They were working great, except that as we started to move into wireless streaming products, the nickel metal hydride did not have the capacity to last a full day for patients.

Lithium ion. We often see lithium ion batteries in many consumer products, such as cellphones and other electronics. One downside of lithium ion, as seen in recent news headlines with Samsung phones, is that there are issues with flammability and toxicity. Because of those safety issues, they have to be sealed in the case. The new batteries that you see from Signia and Phonak are tightly sealed in their cases because of the concerns about flammability, as well as ingestion. A study in the journal Pediatrics (Sharpe, Rochette, & Smith, 2012) raises a concern about lithium ion button cells, and the increase of U.S. emergency room visits over the last decade due to children swallowing lithium ion batteries. Compared to traditional hearing aid batteries, which run about 1.2 to 1.4 volts, lithium ion tends to be around 3.6 to 3.7 volts. If a lithium ion battery gets swallowed (either by a child or a pet) and subsequently caught in the esophagus, the chemicals combined with the voltage can cause serious injury. Lithium ion batteries are non-recyclable. They also have size limitations.  Currently, you can't scale them down smaller than what I call a "pregnant 13." Lithium ion is slightly bigger than a 13 size battery; after they are wrapped and sealed for protection purposes, they are slightly larger than perhaps the smallest 312 or 10 size batteries.

Silver-zinc. Silver-zinc is relatively new to the hearing aid industry, having been introduced within the last few years by ZPower. Silver-zinc is the chemistry that was used in the Apollo space mission. It is used in satellites and in nuclear submarines. It is a very safe chemistry. It's non-toxic, non-flammable, and fully recyclable. It has very high energy density. It can be used in smaller battery sizes. In contrast, with lithium ion, the battery will last longer, but due to the safety issues and the requirement that it be housed in a casing, lithium ion will result in a larger sized battery. As we move toward small battery sizes, we have tremendous advantages in terms of the capabilities of silver-zinc.

Understanding Battery Performance

Two vital components of battery performance are: battery capacity and hearing aid drain. Wayne Staab recently has presented a series of articles related to batteries on his Hearing Health Matters blog. A battery is not simply defined by its chemistry and size, but also by its capacity. Capacity represents the amount of energy measured in milliamp (mA) hours in a battery. It partially determines the run time of the device that it's powering.

As a comparison, consider the capacity of an automobile's fuel tank - how many gallons of gas does your tank hold? That's the capacity. In the same way, we think of capacity of hearing aid batteries. How much energy is stored in the battery? This becomes important because not all batteries are the same. If we buy a 312 battery from several different companies, they may have three totally different amounts of capacity. Similarly, you can buy a car that holds 10 gallons of gas, or one that holds 18 gallons of gas. In the same manner, one 312 battery may have 180 mA hours of capacity, while another one may have 120 mA hours of capacity. Having this knowledge about capacity will be helpful as you counsel your patients. They may complain that they purchased batteries at a discount store for a great price, but they don't seem to last very long. You can advise them to look on the back of the packaging and see the capacity of the cheaper battery cell, as compared to those that you carry in your clinics. Just because they're the same size, does not mean they have the same capacity.

In addition to capacity, the concept of hearing aid battery drain is important. Battery life depends on the current drain of the hearing aid. Staying with the automobile analogy, some cars get better gas mileage than other cars. In fact, some drivers may have better gas saving driving habits than others (e.g., driving more slowly, turning off the air conditioning, etc.). The same holds true for hearing aids. If the person is using ear-to-ear technology, feedback management, or if they are streaming, these features will quickly impact the battery drain. Hearing aids streaming music or a phone call with 2.4 GHz technology are going to use more battery capacity than hearing aids using near field magnetic induction (NFMI), which uses an intermediary device with its own power source. Knowing how these factors impact battery life will inform your patient counseling. 

Battery Life

We know that battery life is getting shorter. According to the Hearing Industries Association (HIA), in 2016, 88% of new hearing aids had wireless capabilities. We know that hearing aid features and wireless streaming consume batteries. How much streaming are patients doing? How much are they using ear-to-ear communication? Are they using CROS and BiCROS, and what is that doing to the battery drain? We know that our patients are complaining about short battery life, and the frequency with which they need to change the batteries. These are some factors that we need to consider.

Comparison of Rechargeable Hearing Aid Batteries

Figure 1 provides an overview of various chemistries used in rechargeable hearing aid batteries. We've talked about nickel metal hydride, lithium ion, and now silver-zinc. While the disposable zinc-air batteries are a good, high energy density battery, they're not rechargeable, and they all end up in landfills. Nickel metal hydride is rechargeable, and has had a diverse history. The Toyota Prius, for example, uses nickel metal hydride batteries. Nickel metal hydride batteries were used in previous generations of hearing aids, including products by Rexton, Hansaton, and Siemens. However, nickel metal hydride had a relatively short operating time, especially with the new generation of wireless streaming hearing aids, because it has cycle limitations. Cycle limitations are the number of times the battery can be charged before it needs to be replaced. Generally, with nickel metal hydride and hearing aid batteries, it's around 300 charges. In other words, after about 10 months, you have to replace the battery. Lithium ion batteries are widely used in consumer electronics, but we also know some of the potential dangers that we've been reading about, and we just don't have an experience yet in the hearing aid industry. They haven't been on the market for very long. They do have the high voltages running at around 3.6 volts, compared to the 1.2 to 1.4 of the other batteries, but it's also difficult to scale down to below size 13 batteries. Also, if you are dealing with lithium ion batteries, there are shipping and labeling requirements. You're required to use special packaging and shipping labels by the U.S. Department of Transportation. Plus, there is concern about leakage, flammability and ingestion. Silver-zinc batteries have been widely used by NASA and military. They are non-toxic, non-flammable, and they have the highest energy capacity when compared to other comparable rechargeable batteries.

Figure 1. Battery comparison.

Consumer Perspectives

MarkeTrak IX, published by HIA, revealed the top four most compelling features sought by non-owners of hearing aids (Abrams & Kihm, 2015). It was anticipated by hearing industry professionals that patients would choose wireless streaming as their top choice, in order to listen to music through the hearing aids, or to talk on the phone. However, the results were surprising:

The number one feature that patients wanted was volume control, and two of the top four items were related to rechargeability. Patients want their hearing aids to be rechargeable.

Another recent study of consumers commissioned in part by ZPower was conducted by Abram Baily of HearingTracker.com.  Hearing aid users around the world were asked questions via an online survey. The results indicated that 70% of patients wanted a rechargeable hearing aid over disposable batteries.  In addition, all-day power was the top requested feature of rechargeables, and 85% wanted the flexibility to use disposable batteries with their rechargeables when needed. 

ZPower Study

As we looked at these issues, we conducted a study of our own.  The purpose of the study was as follows:

• Measure power use and battery drain of a sample of wireless hearing aids.
• Compare reported battery drain from data sheets to actual measured battery drain.
• Report measured battery drain in varying listening situations and streaming scenarios.
• Understand the performance of hearing aids in different listening situations with different batteries to enhance patient counseling. 

As part of patient counseling, it is important to stay on top of smartphone technology and updates. For example, iPhone changed their software such that when an iPhone user was scrolling through their photographs, it would make a "whooshing" sound each time they swiped to the next picture. This "whooshing" sound was causing a significant drain on hearing aid batteries, and patients complained that their batteries weren't lasting very long. A knee-jerk reaction might have been to provide new batteries to the patient, when in reality, it had nothing to do with the battery.

Manufacturer Datasheets

One of the first things we did to begin to understand battery capacity was look at the manufacturers' datasheets. These data sheets indicate the current drain of the hearing aid so that you can provide estimates (Figure 2). Product A and B were 2.4 gHz products, and product C and D were NFMI streaming products. Regardless the type of streaming approach, the typical current drain listed on the manufacturers sheets are around 1-1.3 mA. As a simple example of how current drain is calculated, if you had a battery with 180 mA hours of capacity, and it drained at one mA per hour, you would divide 180 by 1 mA, which would translate to 180 hours of use. 

Figure 2. Manufacturer data sheets current drain.

The question we have to ask is: How realistic are these datasheets? Are patients getting that kind of capacity and run time on these hearing aids in real-life situations? Joergensen and colleagues (2013) also wanted to determine the accuracy of manufacturer datasheets as compared with what happens in the real world. According to their findings, there is “…little correlation between datasheet figures and measured real-life current consumption, even when hearing aids are used without the wireless streaming options activated.” They further stated that, “Relying on the datasheet is difficult and, in many cases, not sufficient when answering a hearing aid wearer’s questions with regard to battery life.” 

Datasheets are based on ANSI performance standards. According to ANSI 3.22 (specification of hearing aid characteristics), you need to turn everything in the hearing aid off: all adaptive features (noise reduction, feedback suppression, anything wireless) should be disabled. Essentially, the ANSI standard tells us to run the hearing aid in airplane mode. The published data, especially when it comes to battery current drain, is not pertinent to usage in real world situations. 

As stated previously, capacity is the amount of stored energy, and consumption is based on the usage habits of hearing aid-wearing patients. Again, similar to driving a car, the capacity is how much gas can fit into the fuel tank, and consumption is how far the car will travel on a full tank of gas. In the same way, we can determine how many milliamps per hour, and how much capacity is in the battery, so that we can figure out how many hours of battery life we can estimate with our hearing aids.

The standard industry formula used to calculate the run time for a hearing aid is: battery capacity divided by current drain (in milliamps). For zinc-air batteries, it is recommended that you multiply the result times 0.7, because it's estimated that a zinc-air battery will lose up to 30% capacity after you pull the tab off. Some zinc-air battery manufacturer representatives state that the drain is not that much. However, the battery literature indicates that once you pull the tab off of the zinc-air, you begin to lose capacity. Furthermore, we know from experience that if we leave a tab off a battery, over time, that battery will tend to die off or lose capacity. If we have a zinc-air 312 battery with a 180 mAh capacity, and a current drain of 2.0 mA, the hearing aid run time is calculated as follows: (180/2.0) * 0.7 = 63 hours or, at 12 hours/day, ~5 days.

For a zinc-air battery, you must multiply the result of 90 hours by that constant of 70%, because zinc-air is continually draining. You'll get a total of 63 hours of run time; or, if the patient is wearing the hearing aid for 12 hours a day, they will get about five days of use out of the battery. Keep in mind that every hearing aid is going to be a little bit different, and the batteries that you're using are also a little bit different. If you are not comfortable with using this formula, you can use Wayne Staab's graph on his blog at Hearing Health Matters. It's important for us to understand exactly how batteries operate, so that from a business perspective, we can plan and order appropriately in our practice. Additionally, from a counseling perspective, providing accurate information to our patients will build trust and foster confidence. 

At ZPower, we investigated the actual current drain of hearing aids, with and without the features activated, and with and without streaming. The dark green bars in Figure 3 represent the hearing aid drain that is reported by manufacturers on their datasheets. For products with 2.4 GHz of streaming, you'll notice that before any streaming occurs, only using features such as feedback and noise management, the actual run time is closer to 2 or 2.4 mA of battery drain. With products running NFMI (which use an accessory device for streaming purposes), without streaming, the battery drain is not as high as it is with 2.4 GHz technology, but it's running slightly higher than the published results. When you start to stream with 2.4 GHz products, the battery consumption goes up dramatically, because the current drain is so much higher. You'll see the drain is less with NFMI products, because of the accessory device, although it is still higher in all cases than the published data. 

Figure 3. Typical product current drain.

Rechargeable Batteries Size 312

What would this mean for battery life? Let’s look first at size 312 rechargeable batteries and the hearing aid operating times (Figure 4). A 312 silver-zinc (AgZn) battery has an effective capacity of 37 mAh. Lithium ion (Li-ion) is not available in a size 312 (the smallest lithium ion batter is slightly bigger than size 13). Nickel metal hydride (NiMh) is available in a 312, but has a lot less capacity of 22 mAh. Silver-zinc has not quite twice the capacity of nickel metal hydride, but significantly higher capacity.

Figure 4. Rechargeable batteries, size 312.

Let's look at an NFMI hearing aid device with a 312 battery, and compare nickel metal hydride to silver-zinc without streaming.  In this case, battery life with silver-zinc will be about 25 hours (Figure 5), dropping down slightly over time with three hours of streaming. In contrast, the nickel metal hydride drops down dramatically after three hours of streaming. Those of you that have fit the older nickel metal hydride batteries from Hansaton or Siemens know that by about four o'clock in the afternoon, your patients complain that the nickel metal hydride batteries start to fade and die. You don't get that with the silver-zinc batteries, because of the high capacity.

Figure 5. NFMI battery life in hours (312 rechargeable)

Rechargeable Batteries Size 13

Size 13 batteries are available in lithium ion, nickel metal hydride and silver-zinc (Figure 6). The silver-zinc capacity is significantly higher than lithium ion and the nickel metal hydride. In other words, you're going to get a longer run time on the silver-zinc batteries because of the high capacity, as compared to lithium ion; but the silver-zinc and lithium ion are fairly comparable.

Figure 6. Rechargeable batteries size 13.

Lithium ion batteries have what's called capacity loss. It's well known that lithium ion batteries tend to lose their capacity over time. Lithium ion loses 20 to 30% of its capacity after 200 to 300 charges. We experience the same thing with smartphones. When a smartphone is new, the phone may run all day on a charge, but after several months we notice that we have to recharge it more often because of this capacity loss. We don't have this experience with lithium ion with hearing aids yet, but we can anticipate that over six months to a year, your patients may inform you that they are not getting a full day of use on one charge. Using lithium ion, it is possible to do a "quick charge," where the battery will recharge in 30 to 60 minutes, and get patients through the rest of the day. However, this will require that patients take their hearing aids off, and put them into the charger for an hour during the middle of the day. In some of the studies that Hearing Tracker conducted, patients didn't like that concept. 

With size 13 rechargeable batteries after 300 charges, nickel metal hydride goes dead (Figure 8). We're not using NiMH much anymore in hearing aid industry. Lithium ion will go from 42, down to potentially around 30 mAh of capacity. 

Figure 7. Size 13 rechargeable batteries after 300 charge cycles.

With a size 13 battery, battery drain will depend on hearing aid usage, wear time and streaming behaviors (Figure 8). After 200 charges, the silver-zinc always recharges back up to full capacity. In fact, one of the characteristics of silver-zinc is that it always charges to capacity, but over time, rather than taking three hours to charge, it may take longer. Lithium ion tends to lose its capacity, so you'll get a reduction in wear time by our patients. 

Figure 8. Size 13 rechargeable batteries: new vs. post-200 charges (NFMI).

Battery Current Drain: Special or Unique Cases

Counseling CROS Patients

In some of our clinical field trials, we found interesting data and surprising patient complaints that we had trouble initially understanding. These difficulties could potentially impact patient counseling. We looked at CROS and BiCROS hearing aids, and we learned that the transmitter power supply does not last as long as the receiver. It takes more energy to transmit than to receive. If your patient is experiencing a short operating time, be aware of the fact that it might be the transmitter battery that's going down.

Binaural Wireless Streaming

Another issue that we found involved patients who wore binaural wireless streaming hearing aids. We received patient complaints of short battery life when one of the hearing aids was either not working, or the patient didn't put on the second hearing aid (e.g., it was out for repair). It appears that when both of the devices are not worn at the same time, the other hearing aid spends a lot of time searching for its matched pair. When that happens, it is streaming all the time, searching for the other device. For patients who wear hearing aids with ear-to-ear communication, and one of those hearing aids is not working or switches off, you need to "tell" the other hearing aid to stop searching for its matched pair. Otherwise, you're going to have very short battery life.

A recent study by Joshua Alexander and colleagues as reported by Staab (2016) looked at this issue.  The study found that an ear-to-ear hearing aid spent around 20 minutes searching for the missing partner aid, before returning to a baseline current drain of 2.0 mA. When the partner aid was present, the aid maintained a current drain of 2.0 mA. After 20 minutes of searching, a message was sent the other hearing aid to stop searching for its partner. Be aware if you are using hearing aids with wireless ear-to-ear communication, that if the other hearing aid is not there, that could be causing significant battery drain. When features were turned off, the current drain was around 1.5 mA. The key message is that we need to understand how these hearing aids perform in different listening conditions because it can impact our counseling and the patient’s battery life. 

Our engineers measured battery drain when using binaural wireless streaming hearing aids (2.4 GHz). When the hearing aid was idle, the current drain was at 2.0 mA. When running Facebook, it was at 4.5 mA. When talking on the phone, the current drain jumped up to 5.5 mA. When streaming music by Aerosmith, it was running at close to 6.4 mA. When using the phone with the environmental microphone, the drain was 6.5 mA. It is important to understand that different streaming behaviors and usage habits can cause significant battery drain. 

Summary and Conclusion

With all of the new advancements in technology, it is an exciting time for hearing aid users, as well as hearing care professionals. Understanding how this technology works along with the battery technology is important in ensuring our patients get the most out of their hearing aids.   

In summary: 

• Don't depend on the datasheets for estimating real world battery life for your patients.
• Assess your patient's listening and use habits. Ask them if they stream; if so, ask how much and how often. What type of streaming do they use (e.g., NFMI or 2.4 gHz)?
• Counsel and select a battery that's appropriate for their usage habits. Based on their needs, what kind of battery life do they need? Do they want to go with rechargeable batteries? We're finding that when we offer rechargeables compared to disposables in our clinical trials, 30 to 50% of our patients would prefer them to be rechargeable. We think that number will continue to increase.
• Battery life with NFMI hearing aids will be longer than with 2.4 GHz, which causes more drain. 
• It takes more energy to transmit than to receive.
• When patients are wearing one hearing aid of a pair, battery life could be shortened unless you turn off features like ear-to-ear communication.
• Building patient trust depends on giving patients accurate information. 
• Not all zinc-air batteries are the same.
• Not all rechargeable batteries are the same.
• Know your products and the way they perform. 
• Know your hearing aids.
• Know your batteries.

In conclusion, continue to build patient trust and satisfaction by keeping up on hearing aid and battery technologies.

Questions and Answers

I've never seen battery capacity listed on the packages of batteries.

I have seen them on some packages. Although, recently, when I was at the VA show, I went by a couple of the battery companies' booths, and they did not list the capacity on the package. I had to pick up their spec sheets and look at them online. It is important, if you have a battery rep, or wherever you're getting your batteries, to ask them exactly what is the capacity.

Will silver-zinc be available for 2.4 GHz technologies in both the 312 and a 13 with ZPower?

Right now we're focused on the 312 with the Unitron Moxy and the Starkey Muse. We also do have it in the Beltone Legend in 2.4. We are looking at the 13 batteries. We have a 13 battery. We have 10s, we have 675s. We know that 312 is the most popular battery, so that's why we started there. 

Do you expect to see 2.4 GHz lithium ion batteries? 

I don't know the roadmaps of the companies that are doing 2.4 GHz. I know that Starkey is using the silver-zinc battery. Resound, especially in the Beltone Network, is using silver-zinc. I don't know about other companies that are using 2.4 GHz. That is a good question to ask your reps from the different companies as to what their experience will be, and what they're planning on doing. 

References

Abrams, H.B., & Kihm, J. (2015). An introduction to MarkeTrak IX: A new baseline for the hearing aid market. Hearing Review, 22(6),16.

Joergensen, H.S., Baekgaard, L., and Bendtsen, B. (2013, June). Battery consumption in wireless hearing aid products – Datasheet vs. real-world performance. AudiologyOnline, Article #11899. Retrieved from www.audiologyonline.com

Sharpe, S.J., Rochette, L.M., & Smith, G.A. (2012, May 14). Pediatric battery-related emergency department visits in the United States, 1990–2009. Pediatrics, published online, DOI: 10.1542/peds.2011-0012