Page 296 - Battery Reference Book

Manganese dioxide-magnesium perchlorate primaty batteries 25/3
The magnesium dry cell consists of a magnesium The construction of a typical manganese dioxide-
anode, a manganese dioxide-carbon cathode and an magnesium perchlorate non-reserve button cell is
electrolyte, which is usually magnesium perchlorate shown in Figure 25.2. Constructed in a manner similar
or magnesium bromide. The basic cell reaction is: to that of the paper-lined cylindrical carbon-zinc
cell, the can, extruded from a special magnesium
Mg + 2Mn02 + H2O + Mg(OH), + Mn203
alloy, serves as the anode. The cathode consists of
A Kraft paper separator keeps the anode and cathode manganese dioxide and carbon black, with magnesium
apart but permits ionic conduction. The magnesium perchlorate or bromide in water as the electrolyte. An
anode is made up of AZ21A alloy. The construction of absorbent paper separator electrically insulates the two
the cell is similar to that of the conventional cylindrical electrodes while permitting ionic flow (current flow)
cell, with the magnesium anode also serving as the when a load is placed across the cell on discharge.
cell container. The cross-section of one such cell is The open end of the can is sealed with a plastics cover,
shown in Figure 25.1. Resent cells have a mechanical which is constrained by means of a plated steel ring.
seal with a vent to permit the escape of hydrogen gas The cover uses a safety vent to guard against abuse
formed by the following parasite reaction: such as accidental shorting of the battery, which might
cause gassing and excessive internal pressure.
Mg + 2H20 + Mg(OH), + H2
Seal ring - /ZiC
Metal terminal cap
\Positive
Seal washer
Air
space
Carbon electrode-

Magnesium can

‘MnO, /electrolyte
rn ix
‘Paper liner
-Paow SeDarator

/ Negative

Magnesium Figure 25.2 Marathon CD-size magnesium cell (Courtesy of
Marathon)
can Cathc : Carbon ‘Bottom
mix rod washer
Figure 25.1 Cross-section of magnesium perchlorate-rnan-
ganese dioxide dry cell (Courtesy of Eagle Picher)