A great challenge for hearing instrument manufacturers is to balance the ever-increasing desire for more advanced hearing instrument features with the rather stagnant performance of hearing instrument batteries. In addition, developments geared toward more environmentally friendly batteries, such as mercury-free, or more convenient batteries, such as rechargeable, are also emerging. Although reducing mercury in the environment and making hearing aids easier for consumers to manage are very worthwhile benefits, these technologies represent a step backward in terms of powering hearing instruments. As such, they are quite at odds with increased hearing aid capabilities, and manufacturers are faced with the dilemma of how to do more with less.
The diagram below provides an overview of the factors that determine how long a hearing instrument battery will last. Ultimately, it comes down to the capabilities of the power source and how much power is consumed. The hearing instrument manufacturer can choose what type of power supply to use, but beyond that controls only the factors that add up to power consumption. With each new generation of digital signal processor introduced, power consumption is often reduced. However, additional signal processing for new features will usually increase the power consumption due to greater complexity of the operations to be carried out.
The energy sources that hearing instrument manufacturers have available today include zinc air batteries and rechargeable batteries. Because rechargeable battery technology is available in virtually every consumer small electronic product, one might wonder why so few hearing aids make use of rechargeability. The requirements to a power supply for hearing instruments are that it is small enough to fit inside the device, that it has great enough energy density (energy per volume) to power the device for a defined unit of time (one day of use would be a minimum), and that the voltage remain fairly constant on average and not drop too dramatically as the load changes during use. The graph below shows how today’s rechargeable technologies stack up against zinc air in terms of powering a typical hearing aid for a day. The rechargeable technology batteries fall into the lower left-hand part of the graph, indicating both low energy capacity and low energy density relative to the zinc air technology. The size of the bubbles indicates the cell size, corresponding to 10A, 312 and 13 sizes. Assuming a 16-hour use day, an average current consumption of 2 mA, and a battery voltage of 1.2V, the red dotted lines show what the requirements to the battery would be. Under these conditions, it is clear that even size 13 rechargeable batteries will not do the job. Additional hearing instrument functionality, such as wireless communication, may also cause voltage drops during use that the rechargeable technology cannot handle. For now, rechargeable solutions are suitable for hearing instruments with low requirements to power consumption, which likely translates to more a more simple feature set than virtually all high-end hearing instruments offer.
What about mercury-free zinc air batteries? The purpose of the small amount of mercury that has been used in zinc air batteries was to prevent internal gassing in the cell, which could cause leakage or swelling. Mercury is also a good electrical conductor, which helps to keep voltage high. After many years of research and some less successful iterations, mercury-free zinc air batteries are now available which provide a reliable energy source for hearing aids which for some battery manufacturers seems to be on par with previous zinc air technology. Mercury free batteries with sell-by dates of 2015 or later are likely to yield the most stable performance.
As hearing instrument manufacturers, we continue to direct development efforts to reducing power consumption, but at the same time we are looking forward to the next big breakthrough in energy sources that will open doors for additional functionality and convenience for consumers.