The options for hearing instrument and implant batteries are wide-ranging and can be overwhelming. Here is an introduction to their technology, care, and use.

every wearable listening or assistive hearing device requires some type of portable power. First invented by the Italian physicist Alessandro Volta in 1800, the battery has remained the mainstay among power options for two centuries.

Named after the inventor, the volt (V) is used to describe battery cell strength. The chemical compounds used in the cell determine whether the battery is a "primary cell" for one-time use, or a "secondary cell" that can be recharged.

The voltage generated by the battery cell is dependent not upon the size but upon the chemicals used to construct it. Multiple batteries used together provide greater power-and a bigger size. While different types of cells made in the same size can be used interchangeably as a power supply, its life and stability may be affected.

It is important to note that using a battery with more voltage than the equipment is designed for can cause permanent damage to the device. In addition, the use of a cell with the right voltage but incorrect chemical characteristics can lead to a shorter battery life or improper functioning of the device. For this reason, only cells of fairly similar voltages are made in any given size.

HEARING AIDS AND IMPLANTS

Battery options for hearing aids and implants vary depending on the device design and manufacturer's recommendations. Some devices use regular zinc air batteries while others require the use of "power" zinc air cells. And some devices require more power than rechargeable cells can provide.

In general, power options for hearing aids and boneanchored implants (BAIs) include disposable zinc air or rechargeable nickel-metal-hydride (NiMH) button cells. BAIs can also use rectangular cells (9V). Cochlear Americas' BAI line, called the Baha, uses size 13 or 675 disposable button cells. Cochlear's Baha Cordelle model uses a 9V disposable or rechargeable battery. The Ponto family of BAIs from Oticon Medical uses disposable or rechargeable button cells, depending upon power needs.

For cochlear implants, the options are a little more diverse, with three general power configurations. The first are disposable or rechargeable button cells. The second are AA alkaline disposable or NiMH rechargeable batteries. The third are a proprietary rechargeable NiMH (from Advanced Bionics, MED-EL) or a lithium-ion (from Cochlear) battery module with a charger. Proprietary rechargeable systems may also have battery module size choices, depending on the device and power needed.

Many hearing aids have an audible low battery indicator while others, specifically those designed for children, may have a visual low battery indicator so adults can be aware when batteries need to be changed.

BUTTON CELLS

Silver oxide batteries were once a popular choice for high-power, behind-the-ear (BTE) hearing aids due to their increased voltage; however, they were expensive and abandoned as mercury batteries gained popularity. The popularity of mercury batteries soared due to the decreased cost, but unfortunately and unknown at the time, they were also toxic.

Have you heard the term "mad as a hatter"? It referred to poisoning from mercury compounds in the felt used to make hats. The toxicity of mercury is widely recognized today, and efforts have been made to reduce the use of mercury in batteries. Prompted by the 1996 Mercury and Rechargeable Battery Management Act, members of the National Electrical Manufacturers Association voluntarily agreed to eliminate mercury in button cells by 2011.

Today, the majority of button cell batteries manufactured for use in hearing aids-such as BTE, ITE (in-the-ear), ITC (in-the-canal), RIC (receiver-in-canal), and CIC (completely-in-canal)-as well as bone-anchored and cochlear implants are mercury-free, zinc air button cells that provide 1.4V. (Rayovac's new mercury-free cochlear implant battery is 1.45V.)

Zinc air button cells last twice as long as their mercury counterparts did. They are environmentally friendly and cost half as much. The cell is activated by oxidizing zinc with oxygen from the air when the colored-tape seal is removed. Zinc air batteries have a long shelf life if adequately sealed to keep air out, and can be stored for up to three years at room temperature with little loss in capacity as long as the seal is not removed.

The size of button cells is universally standardized and coded by color for consumer convenience. These standards are followed whether the batteries are disposable or rechargeable. This color-coding makes it easier to remember what size battery your hearing device uses.

Some battery manufacturers (such as Duracell, Energizer, and Panasonic) offer hearing aid and implant batteries via traditional retail outlets such as chain drugstores. Others-such as Germany's VARTA Microbattery Power One, Switzerland's Renata, and China's Zenipower-offer a professional product line that is only available through a dispenser, distributor, or hearing healthcare professional. Some offer both options, such as Rayovac.

To optimize the life and strength of your zinc air battery, follow these guidelines. Don't remove the tape until you are ready to use the battery. After you remove the tape, let the battery sit for a minute to let air go into the cell to activate it. Store your hearing aid and battery in a "hearing aid dryer" (dehumidifier) when not in use.

At one time, it was common to store mercury hearing aid batteries in the refrigerator to increase shelf life. This is not recommended for zinc air batteries. Cold air can cause water particles to collect, loosening the tape seal and causing oxygen to come into contact with the zinc. This activates the battery before you are ready to use it.

RECHARGEABLE BUTTON CELLS

Power One's ACCU Plus is one rechargeable hearing aid and implant button cell series available. It uses NiMH as the chemical compound housed in stainless steel. Its voltage is compatible with zinc air batteries, and it is best suited for hearing aids that can operate for 10 days or more without needing a change of battery. Some hearing aids and implants require more power than that provided by this rechargeable battery, so check the manufacturer recommendations for rechargeable options.

The manufacturer says each ACCU rechargeable battery replaces up to 57 conventional batteries. Although varying with hearing instrument and power drainage, the life span is between 12 and 18 months. ACCU is designed to save the user money over time, after an up-front investment in the charger as well as the cost of each battery.

Keep in mind that rechargeable batteries used in hearing aids with a higher power drain or with advanced features may not last throughout the day. They may require changing and recharging, which some consumers may find inconvenient or cumbersome.

Like zinc air batteries, rechargeable button cells should be stored at room temperature. There are no tabs to remove for activation of the cell; it is a self-contained, sealed system. Because they are environmentally friendly and do not contain mercury, lead, or cadmium, they can be disposed of in normal household waste. Power One offers a complete line of rechargeable options for all four button cell battery sizes, along with charger solutions that include a pocket charger and a pen charger.

Siemens' Motion, Pure, or Pure Carat series hearing instruments operate using either disposable or rechargeable button cells. The Siemens eCharger recharges batteries and also dries out instruments overnight.

Solar Ear, produced by a Brazilian nonprofit, is a hearing aid whose batteries are rechargeable using solar energy. Its solar charger powers three NiMH battery sizes as well as NiMH AA batteries that can be used in other assistive listening devices. (See "Global Solutions," page 22.)

ROUND AND RECTANGULAR BATTERIES

AA and AAA are the most common round batteries and 9V the most common rectangular battery used by consumers today. While these batteries are too large for today's miniature hearing aids, they are options for cochlear implants, FM systems, assistive listening devices, and other portable electronic equipment.

The majority of AA, AAA, and 9V alkaline batteries have been mercury-free since the early 1990s and can be disposed of with normal household waste. However, do not throw the batteries into a fire due to risk of explosion, and do not throw away a large amount of them at one time. Because batteries are not 100 percent discharged when disposed of, disposing multiple batteries at one time can cause them to feed off the charge, creating a safety risk. Currently there are no universal, data-based, cost-effective, and safe recycling programs for alkaline batteries, although local programs may exist.

Alkaline batteries should be stored at room temperature. They have a shelf life of seven to 10 years. Battery contact surfaces and battery contacts within the device can be kept clean by rubbing with cloth or a pencil eraser. While disposable and rechargeable batteries can often be used in the same device, never use a disposable battery in any device stored in a battery charging station. The chemicals are not compatible with the charger and will cause prolific leaking and corrosion.

Rechargeable AA, AAA, and 9V batteries are available in a number of metals and from various manufacturers. These include nickel-cadmium (NiCad 1.5V), NiMH 1.5V, and lithium-ion 3V. NiMH rechargeable batteries are the most commonly manufactured and used in assistive listening devices.

Keep in mind that although lithium-ion batteries in AA and 9V sizes have been introduced, they are not compatible with devices that use traditional disposable or comNiMH rechargeable cells, due to their increased voltage. They could cause damage to the device.

PROPRIETARY RECHARGEABLE BATTERIES

Two hearing aid manufacturers offer rechargeable hearing instruments. Solar Ear offers two digital BTEs and one digital RIC that are rechargeable using its solar charger. Last year, Hansaton introduced the AQ X-Mini, a rechargeable RIC. This hearing instrument system has an inductive charging station and sealed battery compartment that prevents insertion of the wrong battery. The digital charging system allows 20 hours of operation per charge, with an expected battery life of five years.

Cochlear implant manufacturers Advanced Bionics, Cochlear, and MED-EL each use proprietary rechargeable systems for their implants. Advanced Bionics' rechargeable NiMH power system is called the Auria PowerCel, and is available in two sizes. Cochlear offers a lithium-ion rechargeable system in two sizes as well. MED-EL's NiMH rechargeable system is called DaCapo.

Consider the battery options appropriate for your device, and check with your hearing healthcare professional if you are unsure. Keep in mind that new options become available as technology changes and improves. Then weigh the benefits and limitations for each and decide which best suits your needs.

Author affiliation:

By Nannette Nicholson, Ph.D., and Joshua Spann

Author affiliation:

Nannette Nicholson, Ph.D., is an associate professor and director of audiology in the Department of Audiology and Speech Pathology at the University of Arkansas for Medical Sciences in Little Rock. She has a joint faculty appointment at the University of Arkansas at Little Rock and a clinical staff appointment at Arkansas Children's Hospital.

Joshua Spann is a second-year doctor of audiology student at the University of Arkansas for Medical Sciences and a graduate assistant in the Department of Audiology and Speech Pathology.