Battery technology has progressed greatly in the last 10 years, adding value and performance to hearing instruments. As digital instrument circuitry has come to dominate the hearing instrument market, the battery industry has moved rapidly to address new power demands and the requirements of these new aids. This article looks at some of the technical issues related to battery technology and testing standards.

Engage someone in the dispensing field in a conversation about high performance hearing aids, and you may attract a cadre of audiologists, hearing instrument specialists, and researchers who will block hallways to expound on the compression ratios, microphone SNRs, kneepoints, and attack/release times of the latest hearing instruments. However, engage a dispensing professional in conversation about battery technology and you’ll often get a blank stare.

It’s simply not fair. While not to take anything away from the brilliant engineers who have exchanged trimpots and transistors for computer controls and digital circuits, battery engineers made very impressive progress in terms of service to the end-user during the last 10 years. Battery prices per milliAmp-hour of service life have actually gone down substantially. “Over the past 10 years, battery capacity has increased in some cases by as much as 50%,” says Kara Salzillo, manager of brand communications for The Gillette Company in Boston, maker of Duracell batteries. “The improvements have been made while battery price to the consumer has remained virtually unchanged, if not decreased slightly. This has created a better value for consumers.”

“In the last few years, the biggest gains we’ve seen in the hearing aid battery industry are in improvements to service life,” says Denis Carpenter, technical manager at Rayovac Corp, Madison, Wis. “When we first made zinc air batteries, they were analogous to how cars used to be made: hard metals with thick construction, larger cathodes, and lots of material that took up precious volume within the battery cell. What we’ve done is slimlined the product and, due to this, we’re now able to get more capacity from the small volume that is available. When Rayovac introduced the size 10 battery in 1986, it was a 50 milliAmp (mA) hour battery. Today, it’s a 90 mA hour battery.”

Carpenter and Rayovac Director of Sales Tom Begley point out that this is a significant number when discussing customer satisfaction with hearing aids. “It shows that those battery manufacturers who have increased their milliAmp hours in their batteries have really made some good progress in their designs,” says Begley. “And these improvements translate into more consumer benefit and better satisfaction with those hearing aids that use these batteries.”

Norm Ryan, director of technology and business development for miniature batteries at Energizer’s Westlake, Ohio, facility agrees. “There will always be development toward two things: power and capacity. Most of the hearing industry is concerned with a normal-performing hearing aid under normal-type conditions, and the big question for battery manufacturers is, ‘How do you extend battery run-time?’ With hearing aid batteries, you’re always constrained by the size of the battery. There is a finite amount of energy that can come from that space. The challenge to the battery manufacturer is what to do with that space to maximize efficiency and get more active ingredients inside that package without risking product failure. Over the last 5-8 years, the hearing aid battery industry, in general, has done an excellent job in maximizing energy in small packages. There certainly are differences that you can find across manufacturers, but in general you can find up to 50% more capacity in the same size package now as opposed to 10 years ago.

“There is a lot of significance to this,” continues Ryan. “The industry has been able to get this kind of energy into the same-size package at the same cost as it was 10 years ago. So consumers’ cents-per-milliAmp hour of battery use are a tremendous bargain.”

Uniqueness of Zinc Air
People who work with zinc air technology often marvel at its eloquence. Zinc air technology is unique because a good deal of what drives the chemical reaction within the battery literally comes from thin air. This is one of the primary reasons that, as long as battery capacity will be defined by the tiny volume that can fit inside a hearing aid, zinc air will likely remain king.

“Zinc air represents the best, most dense power form available because you’re using oxygen as one of the components of the battery,” says Ernie Petrus, director of sales and marketing at Energizer in St. Louis. “The battery is sealed when you first purchase it in order to keep the oxygen out; so it’s actually a very efficient system when you realize that it’s the oxygen outside of the cell that combines with the battery chemistry to create the power. Trying to get another battery chemistry better than that is unlikely in the near future.”

“Zinc air technology is extremely interesting as a power source,” agrees Carpenter. “The first zinc air battery was made around the turn of the century. Therefore, people are always asking about lithium or other potential battery chemistries that they perceive to be new. But the fact is that zinc air is one of the most advanced battery technologies available. As one of the chief engineers at Rayovac has pointed out, if it were not for those pesky little air holes in zinc air batteries [which supply the oxygen], zinc air technology would be the most widely used battery type and would even be more popular than alkaline batteries. Zinc air is almost the perfect battery—except for the holes.”

The holes, while supplying the oxygen that powers the battery cells, also activate the battery once the tab is pulled off. Additionally, zinc air batteries are more susceptible to the climate, incurring a slightly higher risk of drying out or of being affected by moisture. In fact, the latter is coming to the attention of hearing aid battery manufacturers who supply product to the worldwide market, especially to Africa, South America, and Southeast Asia, where the tropical climate can tax zinc air technology, according to Ryan. In extremely humid environments, as well as in third-world applications where a battery might be rationed and used for as long as a month, water can be absorbed into the zinc air cell. This can lead to the battery shutting off, or in worse-case scenarios, corrosion, bulging, or rusting of the cell. While not a problem in the Northern Hemisphere, Ryan says Energizer and other companies are starting to recognize this and tweak their designs to be more tolerant of these environments.

Digital Power Requirements and IEC/ANSI Tests
One of the main challenges for batteries is to maintain a consistent power supply over the service life of the battery. In the days of traditional hearing aid circuits (eg, Class A aids), the drain on a battery could be compared to a spigot attached to a water-bucket: the water (energy) flowed out of the battery cell (bucket) at a fixed rate whenever the spigot was opened. Indeed, older American National Standards Institute (ANSI) and IEC tests for voltage and milliAmp hours of service were based on this principle of continuous energy flow.

In contrast, today’s advanced digital hearing instruments present a far more complex challenge. Power demands can rise and fall depending on what processing functions the digital circuit is performing at the time, what the sound input or listening environment is like, and even what the patient’s hearing test specifies in terms of gain at various input levels. Returning to the water-bucket analogy, this would be like rapidly opening and closing the spigot, making the water run fast or slow. Thus, a digital instrument may operate for the same period of time as an analog circuit, but the rate of energy that has flowed out of the battery during that time can change radically. Analog aids typically start having trouble (eg, cutting out the sound or “motor-boating”) when voltage falls below 0.9V. Digital aids generally start exhibiting similar trouble (motor-boating, changing channels, etc, due to power loss) when voltage falls below 1.1V. However, as technology moves forward, many of the new digital aids may have lower voltage requirements that are comparable to the analog aids.

“Programmable (analog) developments have yielded more flexibility for audiologist in the fitting process,” says Salzillo, “and developments in this area have not had a significant impact on batteries. However, the growth of digital hearing aids in last few years has caused a shift in battery testing practices. Digital hearing instruments tend to have more complex power-demanding features than previous generations, and this requires more precise and complex testing practices for accurate performance measurement.”

Recognizing this, hearing aid manufacturers approached the IEC and asked for a test standard that reflects how a battery is performing within a digital hearing aid. The present standard is a resistive test that has been in existence for many years. Today’s devices are almost all from the Class D-type or digital circuit variety that respond to the various sound inputs or circuit demands. “In a quiet environment,” says Carpenter, “the current runs at a baseline level, but when voices or other noises are present, the circuit responds to the sound environment by requiring more current, thus dragging the voltage down.”

New tests proposed by the IEC for standard hearing aids deal with power requirements similar to the previous analog standards, but then apply a periodic high power load. In this way, the tests gauge whether the battery might drop below a particular voltage level (eg, 1.1V). Most battery manufacturers have agreed to the tests but some have not. “The new IEC tests are still in the proposal state,” says Ryan. “Energizer uses the tests as a standard of reference because we know they’re typical of what the hearing industry wants. I think most of the better battery manufacturers recognize the situation and do this testing.”

The IEC also has a high-power test, which is designed to simulate the high-power BTE instruments used in severe and profoundly deaf applications. Not surprisingly, the most demanding of these tests is for the size 675 and 13 batteries. In contrast, within the the smaller size 312 and size 10 batteries, the high-power tests are less demanding because these aids are not typically used for severe-to-profound losses. “The high drain test is for a minority—probably 2-5% of hearing aid users—who need a high power battery,” says Begley. “High power batteries are a real benefit for these people. We and other manufacturers have found a way to combine the benefits of both high power and capacity. You get the higher operating voltage and the long life all in one battery. Ultimately, that’s what the consumer is looking for—it eliminates all the guesswork.”

A number of hearing aid battery companies introduced high-power batteries when digital aids first came out. However, these batteries tended to have slow sales because they were needed by only a small minority of customers. Additionally, consumers tended to equate high power with high capacity which was incorrect (the batteries, in fact, sacrificed some service-life for the higher power). Most of the people interviewed for this article said that, in general, today’s standard batteries perform well for 70 dB-90 dB applications; it’s the 90 dB-plus range that can be more challenging.

The IEC is continuing to develop tests that will reflect current digital technology, as well as the higher power applications. “It’s possible that some manufacturers could have borderline trouble in achieving new IEC standards on the high power side,” says Carpenter. “This testing was developed by hearing aid manufacturers, and they want a standard test that distinguishes the high power from the non-high-power performance. So, if the battery goes through the high-power test and passes, the manufacturers will know that the battery is applicable for those severe-to-profound losses in the 90 dB range and up.”

In all likelihood, the IEC standard probably will not be adopted until around 2005. “The use of organizations like Catella Generics, near Stockholm, Sweden, an accredited third-party battery testing facility, are often considered critical to establishing standard performance characteristics between battery brands,” says Salzillo. “Such testing should typically use ANSI/IEC standards as basis for competitive testing.”

Future Battery Development
What can be expected in the future for hearing aid batteries? Battery manufacturers will almost certainly continue to focus on service life, power, and reliability—the Holy Grails of the battery industry. “Service life of the hearing aid is still the most important component,” says Carpenter. “However, service life will reach some finite point relative to increases; the days of a 30% improvement in service life are pretty much over. Instead, improvements will be more incremental. I also believe that you may see more power coming into batteries in the future.”

Will batteries get even smaller? Most of the experts interviewed for this article didn’t think so. The small size of certain batteries already makes things difficult for some hearing aid users. In fact, in recent years, the packaging of batteries to address users’ dexterity limitations has become a hot-button for retail and dispensing sales.1 “In addition to the continuous focus placed on improving battery performance and life span, one important area of development has been in the area of product design,” says Salzillo. “Because of their small size, hearing aid batteries have been difficult for the end user to handle, especially since many users have reduced dexterity. Developments in [package design] have created a much more manageable battery replacement process for hearing aid users. Moving forward, the industry will most likely continue to focus on making incremental improvements in battery capacity to create a longer life span in hearing aid devices.”

Ryan suggests that environmental regulations regarding the use of mercury in zinc air may someday become a larger issue. Energizer sells a mercury-free battery in Europe. “In certain regions, like the Pacific Northwest and the Northeast, there has been proposed legislation addressing this issue…Currently, the consumer is not very sensitive to the mercury issue; however, if legislation drives this issue, the situation could change, and the battery manufacturers would have to react quickly.”

1. Strom KE. Charging ahead: Battery marketing and technical matters. Hearing Review. 2003; 10(4):50-55.