My Hearing Aid Battery Doesn’t Last Long, So What Can be Happening?
My post last week suggested that when advanced hearing aid features are activated (adaptive noise cancellation, adaptive directionality, etc.), while they seemed to have essentially no significant impact on hearing aid delay performance, these feature activations do have an impact on the current drain. And, current drain determines hearing aid battery life – the higher the current drain, the shorter the battery life.
Last week’s post related to measuring the impact of advanced hearing aid features on delay, the time involved from the initial stimulus reception by the hearing aid until it is processed and delivered to the hearing aid speaker/receiver. Delay is characteristic of digital hearing aids, with the suggestion that long delay times could be audible, objectionable, or even result in auditory confusion. However, results of measurements made during the past seven years suggests that delay times appear to be sufficiently short (less than 10 msec.) to have any significant impact, regardless of whether advanced hearing aid performance features were activated or not.
However, last week’s post suggested that when advanced hearing aid features are activated, measurement results show that such activation has an impact on current drain, and this could be significant.
The Relationship Between Current Drain and Battery Life
Generally, battery life is calculated based on the current rating in milli Amperes per Hour and it is abbreviated as mAh. Ampere is an electrical unit used to measure the current flow towards the load – in this case, the hearing aid amplifier. The battery life can be calculated from the input rating of the battery and the load current of the circuit. Battery life will be high when the load current is less and vice versa. The calculation to find out the capacity of a battery can be mathematically derived from the formula on the right.
Hearing Aid Battery Capacity in Milli Amps per Hour
Many companies manufacture hearing aid batteries. However, they all use the same color coding to differentiate the four main sizes of batteries.
A hearing aid using a size 312 and having a current drain of 1 mAmp would have a battery life of 126 hours as calculated from the formula (Figure 2). If hearing aids had a constant current drain, battery life could be predicted reasonably. Unfortunately, they do not have constant current drain, meaning that when more is asked of the hearing aid circuitry (adaptive features, etc.), current drain increases and battery life shortens.
Hearing aid manufacturers generally publish expected battery life based on the hearing aid being measured according to ANSI (American National Standards Institute) S3.22, 2003, meaning that the hearing aid is set without its adaptive features operating. For example, the published specifications may record the current drain at 1.0 mA (mAmp), and as in Figure 2 for a size 312 battery, would provide a calculated 126 hours of battery life. However, when the hearing aid is operating with its adaptive features activated (adaptive noise cancellation, adaptive feedback cancellation, adaptive directionality, etc.), it can be expected to pull a higher current drain. If it pulled 1.8 mA, the expected battery life would then be 74 hours – a reduction of not quite half! Of course, any audio streaming associated with hearing aids would pull an even higher current drain and shorten the battery life even more.
A Layman’s Simplified Guide to Current Drain
Think of a hearing aid battery as a water tank where a certain amount of water (energy) is stored until it is to be released. The larger the tank, the greater the storage (Figure 3).
When water is released from the tank through a hose, think of this as current (like in a stream). The volume of water flowing through the hose over a certain period of time can be measured. The larger the hose, the greater the volume to be measured and vice versa. The difference being that it is the amount of current flowing, rather than water.
Current is measured in Amperes (Amps). Using the water tank as an example, if we have two tanks, each with the same amount of water, but with hoses emerging of two different sizes, when water begins to flow, the rate (drain) will be less in the tank with the narrow hose (Figure 4). In electrical terms, we would say that the flow rate (drain) from the tank with the narrower hose would have less current (and hence, less current drain), than that of the wider hose. (Battery voltage and resistance come into play also, but are not germane to this discussion).
Do Hearing Aids Have the Same Current Drain?
Figure 5, taken from a student project at Purdue University (Alexander, 2016), provides the current drain for hearing aids they tested. These values were with adaptive features turned “ON.” These hearing aids did not all have the same battery, and hence the current drains measured cannot be compared directly. The real purpose of providing this graph is to show that not all hearing aids have the same current drain. Still, the four hearing aids using the 312 cell produced current drains from 0.7 to 1.4 mA. Check Figure 2 to determine how much difference in battery life these hearing aids allowed.
Next week’s post will look at this topic again in a way that is more meaningful when it compares the same hearing aid’s current drain against itself with adaptive features turned “Off” and then “On.”
Alexander, J. (2016). Personal communication. Purdue University.