Page 216 - Lindens Handbook of Batteries

9.10 PRIMARY BATTERIES

9.4.4 Special Designs
Designs for special applications are also currently in use. These designs demonstrate the levels of
innovation that can be applied to unusual application and design problems. These are covered in
Sec.9.7.

9.5 CELL COMPONENTS

9.5.1 Zinc

Battery-grade zinc is 99.99% pure. Classical zinc can alloys contained 0.3% cadmium and 0.6%
lead. Modern lubrication and forming techniques have reduced these amounts. Currently, zinc can
alloys with cadmium contain 0.03 to 0.06% cadmium and 0.2 to 0.4% lead. The content of these
metals varies according to the method used in the forming process. Lead, while insoluble in the zinc
alloy, contributes to the forming qualities of the can, although too much lead softens the zinc. Lead
also acts as a corrosion inhibitor by increasing the hydrogen overvoltage of the zinc in much the
same manner as does mercury. Cadmium aids the corrosion-resistance of zinc to ordinary dry-cell
electrolytes and adds stiffening strength to the alloy. For cans made by the drawing process, less
than 0.1% of cadmium is used because more would make the zinc difficult to draw. Zinc cans are
commonly made by three different processes:
1. Zinc is rolled into a sheet, formed into a cylinder, and, with the use of a punched-out zinc disk
for the bottom, soldered together. This method is obsolete except for the most primitive of assem-
blies. Last use of this method in the United States was during the 1980s in No. 6 cells.
2. Zinc is deep-drawn into a can shape. Rolled zinc sheet is shaped into a can by forming through a
number of steps. This method was used primarily in cell manufacturing in the United States prior
to the relocation and consolidation of U.S. zinc-carbon manufacturing overseas.
3. Zinc is impact extruded from a thick, flat disk or calot. This is now the method of choice. Used
globally, this method reshapes the zinc by forcing it to flow under pressure from the calot shape
into the can shape. Calots are either cast from molten zinc alloy or punched from a zinc sheet of
the desired alloy.

Metallic impurities such as copper, nickel, iron, and cobalt cause corrosive reactions with the
zinc in the battery electrolyte and must be avoided particularly in “zero” mercury constructions. In
addition, iron in the alloy makes zinc harder and less workable. Tin, arsenic, antimony, magnesium,
etc., make the zinc brittle and prone to perforation. 4,6
U.S. federal environmental legislation prohibits the land disposal of items containing cadmium
and mercury when these components exceed specified leachable levels. Some states and municipali-
ties have banned land disposal of batteries, require collection programs, and prohibit sale of batteries
containing added cadmium or mercury. Some European countries have similarly prohibited the sale
and disposal of batteries containing these materials. For these reasons, levels of both of these heavy
metals have been reduced to near zero. This impacts directly upon global zinc can manufacture due
to importation of battery products to the United States and Europe. Manganese is a satisfactory sub-
stitute for cadmium and has been included in the alloy at levels similar to that of cadmium to provide
stiffening. The handling properties of zinc alloyed with manganese or cadmium are equivalent; how-
ever, no corrosion resistance is imparted to the alloy with manganese as is the case with cadmium.

9.5.2 Bobbin
The bobbin is the positive electrode and is also called the black mix, depolarizer, or cathode. It is
a wet powder mixture of MnO , powdered carbon black, and electrolyte (NH Cl and/or ZnCl , and
2 4 2