Page 316 - Lindens Handbook of Batteries

13.22 PrImArY BATTErIES

Zinc/air batteries for hearing aid applications have been improved and refined over the years to
meet stringent device and user requirements. With the advent of digital aids in the late 1990’s, the
battery design has had to adapt to higher current and pulse requirements. Today’s zinc/air hearing aid
batteries offer up to twice the capacity of the original designs of the late 1970s.
These improvements have been achieved by optimizing the internal anode volume without
exceeding the standard external cell dimensions. 33,34 The zinc content is maximized without com-
promising the internal free volume needed for expansion as zinc metal is converted to zinc oxide.
If the free internal volume is not adequate, the cell could have excessive expansion, leakage, or a
premature end of life failure.
recently, zinc/air prismatic batteries have been offered for OEm applications. The prismatic
35
battery offers a 5 mm thin, prismatic construction designed to work with a variety of applications.
These batteries can last up to 3 times longer than alkaline batteries of a similar volume. As with all
zinc/air batteries, the application needs to consume the energy quickly to achieve the full capacity
advantage. Table 13.6 summarizes the characteristics of these prismatic batteries.

taBle 13.6 Characteristics of Zinc/Air Prismatic Batteries

Size PP425 PP355 PP255
Length (mm) 36.0 32.2 22.6
Width (mm) 22.0 14.7 10.3
Thickness (mm) 5.0 5.0 5.0
3
Volume (cm ) 3.96 2.37 1.16
Cell weight (g) 11.7 6.8 3.4
Continuous rate capability < 200 mW < 100 mW < 50 mW
Capacity rated (mAh) 3600 1800 720
Source: Energizer Zinc/Air Prismatic Handbook.

13.9.2 voltage
The nominal open-circuit voltage for a zinc/air battery is 1.4 V. This value can vary from manufac-
turer to manufacturer because of differences in anode and cathode chemistry. Typically, the open-
circuit value can range from 1.4 to 1.5 V. The initial closed-circuit voltage at 20°C ranges from 1.15
to 1.35 V depending on discharge load. The discharge is relatively flat, with the typical end voltages
falling between 0.9 and 1.1 V.
In order to ensure freshness and long-term shelf life, the zinc/air battery’s air holes are covered with
a tape tab. The tab is designed to mute air ingress to the point that it lowers the open-circuit voltage
(OCV). This lower tabbed voltage helps to determine if the tape tab is properly attached to the battery.
If the battery tape tab does not properly adhere, allowing excess air access, the OCV will be
above 1.40 V. This would be the same OCV as if the zinc/air battery had been left untabbed for a
couple of hours. If this condition occurs when the battery is in storage, the battery may dry out and
not function when used.
It is important that the tape tab does not lower the voltage of the battery too much. A tabbed cell
OCV of less than 1.0 V may not rise fast enough when untabbed to properly start the device the
battery is powering.
Figure 13.19 illustrates the time it takes to achieve a functional voltage based on the initial tabbed
voltage. The lower the tabbed voltage, the longer is takes to reach the functional voltage. rise time
can be influenced by changes in the air cathode chemistry, cell limiting current, or air hole design.

13.9.3 energy density
Zinc/air batteries have the highest volumetric energy density of any other primary button or coin cell
chemistry system. The common hearing aid batteries range from 1300 Wh/L in the Pr70 (size 10)
to 1400 Wh/L in the Pr44 (size 675).