Understanding Silver-Zinc Rechargeable Hearing Aids

Battery Performance, Charging, and Best Practices

Presented by ZPower Battery


Barry Freeman, PhD

Vice President of Business Development, ZPower

19 February 2019

Rechargeable vs disposable batteries

Primary batteries, also known as disposable batteries, are designed to be used once and discarded. Disposable hearing aid batteries typically last anywhere from 3-14 days depending on three factors: the capacity or amount of energy stored in the battery, the programmed settings and features of the hearing aid, and the listening habits of the user. Disposable hearing aid batteries rely on zinc-air chemistry, which provides high energy density, but lacks rechargeability.

Secondary, or rechargeable hearing aid batteries, may be used over and over again by the hearing aid wearer. Rechargeable batteries are typically charged every night to provide continual daily use. Like primary disposable batteries, the daily battery life varies by battery chemistry, the amount of energy stored in the battery, and the product programming and the listening habits of the user. The batteries themselves will typically last as long as one year (longer in some cases).

Rechargeable battery types

There are three types of rechargeable hearing aid batteries:

  1. Nickel metal hydride (NiMH)
  2. Silver-zinc
  3. Lithium-ion

Nickel metal hydride (NiMH) hearing aid batteries have been around for more than a decade. However, due to the limited capacity of the battery, and the ever-increasing energy demands of advanced hearing aids, these batteries do not have sufficient energy to last a full day. In addition, they cannot be charged more than 300 times (i.e., cycle life) and, therefore, do not last a full year.

Silver-zinc and lithium-ion hearing aid batteries have ushered in a new era of rechargeability for hearing aid users, with both types of batteries offering greater daily and overall battery life than NiMH batteries. While both types of batteries offer higher energy density than NiMH batteries, silver-zinc chemistry offers slightly higher energy density when compared to similar sized lithium-ion microbatteries. Silver-zinc also offers a size advantage in that the batteries can be made small enough to be used with hearing aids that use size 312 batteries whereas lithium-ion batteries cannot.

Safety differences

Silver-zinc batteries operate on a lower voltage (1.6-1.7V) and are non-toxic and non-flammable, meaning they are safe to handle, and there are fewer risks if swallowed or ingested (or their friends, family, or pets). Lithium-ion batteries, on the other hand, operate at a higher voltage (3.6-3.7V) and do carry considerable risks related to flammability and toxicity. Due to these risks, lithium-ion batteries are sealed into the hearing aid unit.

Disposable batteries as a backup

Thanks to the safety of silver-zinc technology, silver-zinc hearing aid batteries are often removable from the hearing aid case, and interchangeable with disposable zinc-air batteries. This provides flexibility to hearing aid users when they go “off the grid” and need to use disposable batteries for a period of time. Just ask the hearing aid wearers in Texas, Florida, or Puerto Rico who were unable to charge their hearing aids due to power outages and needed to use zinc-air batteries in order to hear. Forgetting to charge your hearing aid batteries is also less of a risk when disposables are available as a backup.

How long do rechargeable hearing aids last on a single charge?

ZPower recently completed performance testing on four popular hearing aid models to determine how long silver-zinc rechargeable hearing aid batteries last on a single charge. Depending on the hearing aid model, and the number of hours of wireless streaming, the rechargeable batteries provided anywhere from 7.7 (with full time streaming) to 24.7 (without streaming) hours of use time.

Differences between hearing aid models

ZPower’s testing revealed significant differences in use time from one hearing aid model to another. As you can see in the chart below, products B and C which used new low power blue tooth circuitry had longer daily battery life than competitive products, regardless of how much time was spent wirelessly streaming.

Differences in battery life for four models

Rechargeable Battery Life By Model

What factors impact performance

As you can see in the chart above, wireless streaming has a significant impact on how much life you can expect out of your hearing aids. But there is more to the story. Why do some hearing aids perform consistently better than others in terms of battery life?

There are four primary factors that influence battery performance:

  1. Advanced features
  2. Wireless streaming
  3. Amount of amplification
  4. Battery current drain

Advanced features

Modern hearing aids employ a number of advanced features, which provide louder, cleaner, more comfortable sound. Technologies like noise suppression and feedback cancellation continuously monitor and process sound on an onboard computer. These active processes increase power consumption leading to higher battery drain. These features not only differ among the hearing aids but also by the hearing aid user.  For example, a person with a more pronounced hearing loss may require more feedback cancelation than someone with less hearing loss and may have more demands on the battery and shorter battery life.

Wireless streaming

Wireless streaming is another technology that leads to increased power consumption. A significant amount of energy is used every time you stream audio from your smartphone, TV, iPad, or other wireless accessories. Wireless streaming also happens when your left and right hearing aids talk to each other to exchange audio or data. Many hearing aid pairs maintain a constant wireless connection to provide more comfort and benefit in noisy environments. The more you stream, the more the battery will be drained.

Amount of amplification

Battery performance is also impacted by the amount of amplification being provided by the hearing aids. When you turn up the volume of your hearing aids, the hearing aids will drain the battery at a slightly faster rate. For those with more severe hearing loss, more amplification is required to provide adequate volume. Unfortunately, this also means more power consumption.  Also, if one ear has more hearing loss than the other ear, the battery life on the more impaired ear could be shorter.

Battery current drain

Battery current drain refers to how fast the hearing aid drains power from the battery. Hearing aid manufacturers typically publish current drain figures for every new hearing aid they produce. When current drain is measured by the manufacturer, hearing aids are put into a “test mode,” which disables advanced features and wireless connectivity. As a result, manufacturer specifications of current drain typically underestimate true battery drain in the real world.

Findings from ZPower’s lab indicate that current drain in the real world almost always exceeds what is stated by the manufacturer, especially when advanced features and wireless streaming are active.  Think of current drain like the gas consumption of your car.  Some cars get better MPG than others due to the size of the engine or the features in the automobile and the way they are driven (e.g., highway or city).  This is the same with hearing aids.  They all are not the same and not all hearing aid wearers use their hearing aids in the same manner.

Manufacturer specs vs reality

Current Drain

How to use ZPower hearing aids

When using hearing aids powered by ZPower silver-zinc rechargeable batteries, there are a few important things to remember. These tips will help you get the most out of your batteries, and ensure you maximize both the daily and overall life of the batteries.

Charging your hearing aids

  • Rechargeable hearing aids should be charged every night to maximize battery life. If you charge the battery every other night, this will not help extend the battery life, and could lead to issues down the road. Silver-zinc chemistry must have the opportunity to recharge to full capacity every night to get the best performance from the battery.
  • Leave your hearing aids in the charger when they are not being worn. The charger will not overcharge the hearing aids and will ensure the batteries stay at full charge. Silver-zinc batteries do NOT have a memory like older rechargeable batteries.  To assure maximum performance, they need to be placed in the charger when not in use.
  • Do not unplug the charger while the hearing aids are inserted. The ZPower charger has intelligence and is programmed to turn on the hearing aids when not charging.  If the charger is not receiving power, then the hearing aids will automatically be turned on. This will cause the hearing aids to turn on, and the batteries to begin discharging.
  • Remember, your hearing aids may take up to 7 hours to recharge fully, and the charging lights will turn from blinking to solid green when charging is complete.
Zpower Charger

Maintaining your charger

  • To ensure your hearing aids charge properly, it is important that the hearing aids and charger are dry and clean before charging.
  • Use a soft tissue to remove moisture or debris on the hearing aids and charger contacts.
  • The soft brush that is included with hearing aids may also be used to remove debris from the charging wells.

Storing your hearing aids

  • Short term storage - If you don’t plan to wear your hearing aids for a few days, simply leave the hearing aids in the charger with the charger plugged in.
  • Long term storage - If you plan to take a longer break from your hearing aids, it’s best to remove the rechargeable batteries from your hearing aids and store them separately.

Check out this list of best practices for using your ZPower hearing aids!

ZPower Battery Best Practices