There are several different sizes of hearing aid batteries. Hearing aid batteries are referred to by a number or a color (see Figure 1). Both the number and color are consistent across manufacturers, so you'll be able to identify a battery using the same naming convention regardless of which company made it.
Hearing aids use a special type of battery called a zinc-air battery. Unlike other batteries that provide less power as they begin to die, hearing aid batteries output a constant power level, but die abruptly. Therefore, a zinc-air battery allows a hearing aid to put out the same level of amplification, instead of becoming quieter and quieter as the battery runs out.
The word "air" in "zinc-air battery" refers to the fact that the batteries do not start to work until they are exposed to air. To let air in, hearing aid batteries have several small holes on the back. In order to prevent the battery from discharging while not in the hearing aid, zinc-air batteries have a sticker tab on the back, covering small holes that allow air in until ready for use. Once you remove the sticker, the battery begins to work, and will continue to discharge even if you put the tab back on. This means hearing aid batteries can be stored for a long time, as long as the sticker is intact.
Just like the batteries you use in your T.V. remote control, hearing aid batteries have a positive and negative end. The positive end is marked with a large plus sign (+), and is also the flat end of the battery covered by the sticker tab. Battery orientation becomes extremely important when changing hearing aid batteries. The internal circuitry of the aid expects electricity to flow in a certain direction, which is provided by the battery. If the battery is inserted incorrectly (with the "+" facing the wrong way) the hearing aid will not receive power, and will not function.