Batteries have been essential to the operation of hearing aids since they were first produced in the late 1800s and early 1900s. Vacuum tube hearing aids required two batteries, while carbon hearing aids needed one or more batteries in series, and the user had to wear a separate battery pack. The zinc-oxide batteries used in these devices were expensive and needed to be stored in a cool place to prolong their life. This type of battery discharged at a constant rate, requiring that the user continually turn up the hearing aid's volume as the battery became depleted. Mercury batteries were first produced during World War II. They were smaller and their voltage did not decrease markedly until they were almost depleted. This relieved the user of the need to adjust the volume. In the 1950s, silver-oxide batteries were developed and began to replace the more toxic, environmentally hazardous mercury batteries. However, these batteries were expensive due to the silver content of this type of cell. Rechargeable batteries were tried, but without much commercial success. Today, most hearing aids are powered by zinc-air batteries. This type of battery offers clear advantages over both mercury (less negative impact on the environment) and silver (less expensive to manufacture). Zinc-air batteries require oxygen to produce an electric current. Once purchased, the airtight seal attached to each battery is removed. This allows air (including oxygen) to enter the battery where it stimulates the chemical reaction that produces an electric current. The removable airtight seal allows the battery to be stored for long periods of time with very little loss of power, as long as the seal remains attached. Zinc-air batteries cost an average of $1 apiece, with a useful life that ranges from a few days with a digital CIC to more than a month with a digital BTE instrument. EXTENDING BATTERY LIFE Hearing aid wearers tend to complain about poor battery life if they are required to replace the batteries more than once every week or two. We sought to address this dilemma. Since zinc-air batteries require oxygen to function, we reasoned that depriving the battery of oxygen during periods when it was not in use could extend its useful life. If a zinc-air battery was stored in an air-tight container with minimal dead volume so that the oxygen necessary for the battery to function was kept out, we theorized that the chemical reaction that produces current (and drains the battery) would come to a halt, a halt that could be easily reversed by exposing the battery to air again. To evaluate the effectiveness of this strategy, we used a voltmeter to measure the voltage of two fresh, size 10 zinc-air batteries, labeled “A” and “B.” The two batteries were used in a pair of digital CIC hearing aids during the day. At night or any other time when the hearing aids were not being used, battery A was removed and stored next to the aid while battery B was placed in a small, sealed tube with just enough space to contain the battery (Nalge Nunc CryoTube, part #65234). We repeated this procedure every day for 7 days. After a week, we measured the voltage again. Results showed an increase of approximately 20%-30% in the useful life of battery B. We repeated this experiment for several sets of batteries, and the results consistently showed increased life for batteries stored in the sealed tube. The same procedure was repeated with battery sizes 312, 13, and 675, with similar results. The question of whether a typical hearing aid user would go to the trouble of removing the battery every night and placing it in a cryotube has been considered. One of the authors (SG) is a hearing aid user and has been using this technique for many years. She is currently wearing a pair of CIC style hearing aids. She finds it convenient to remove the batteries at night and seal them in the cryotube, which is then placed in the carrying case along with the hearing aids (whose battery doors are left open so any collected moisture can evaporate). She has been using the technique with the last three models of hearing aids she has owned and finds that the increase in battery life more than compensates for the simple additional step required. In conclusion, the use of small cryogenic tubes, which are inexpensive and readily available, can significantly prolong the life of zinc-air batteries, resulting in reduced expense for hearing aid wearers.