Question
I would like to find out the effects of the following on the performance or operation of zinc air cells: the effect of ambient conditions as the result of air pollution, and the effect of temperature and relative humidity (when it is below or above the specified operating temperature), which is normally 0 to 40 degrees. I would also like to find out, what equipment do I need to determine and plot the discharge curve for the zinc/air cell.
Answer
I will answer the question on the effects of temperature and humidity based on what happens when the battery is being used, not on the effects during storage. A hearing aid battery has a given operating voltage (e.g. 1.25 volts) and it is made to function until the voltage drops below its endpoint (e.g. 1.1 volts). The operating voltage of the battery changes with temperature. As the temperature gets lower the battery voltage gets lower. As the temperature gets higher the operating voltage gets higher. Therefore service life will suffer as the temperature approaches 0 and actually improve as the temperature approaches 40. This is because in the colder environment the operating voltage is closer to its endpoint and so it will run for a shorter period of time before it reaches the endpoint. The warmer environment has the opposite effect. Because the hearing aid is worn close to the body, the actual operating temperature is different from that of the ambient environment thus moderating the effects of temperature.
The zinc air hearing aid battery air holes allow the battery to interact with the environment. If the humidity is low the battery loses moisture. If the humidity stays low for a prolonged period of time it will lower the typical service life of the battery because of dry out. If the humidity is high the battery takes up moisture which fills up the internal space allotted for cell discharge, which results in a shortened service life. The overall effect of service life depends on many factors such as time and how high or low humidity is.
I cannot answer the question concerning pollution.
The simplest way to create a discharge curve is to attach a resister in series with the battery. For instance, we would take a 13 size battery and attach a 1500 ohm resistor in series to it. We then read the voltage on the battery every hour until it reaches 0.9 volts (over 300 hours later).
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Denis Carpenter, Rayovac Corporation, Zinc Aid Technical Manager, BS-Chemistry, UW-Lacrosse, 25 years experience in zinc air research and development.