Page 274 - Battery Reference Book

23/4 Mercury-zinc batteries
or nickel) in contact with both the zinc electrode and upwards against the external crimped edge of the
the electrolyte. It is for this reason that the cell has a outer jacket, tightening that portion of the seal and
centre case (nickel-plated steel) to provide mechanical relieving the portion between the top and the inner
strength and an inner case of suitable high hydrogen steel cell case. Venting will then occur in the space
overpotential to avoid corrosion of the anode. The between the internal cell container and the outer
inside of the cell top is also electrochemically com- steel jacket. Should any cell electrolyte be carried
patible with the zinc anode. The cell components used into this space, it will be retained by the safety
provide precise mechanical assemblies having maxi- absorbent ring. Corrosive materials are therefore not
mum dimensional stability and a marked improvement carried with the escaping gas through the vent hole
in performance over batteries of the carbon-zinc type. at the bottom of the outer steel jacket. After venting
Most major producers of mercury-zinc cells and excess gas and reducing the internal pressure, the
batteries use a vented construction, which renders their cell stabilizes and reseals the top seal, continuing
products leakproof and free from bulging under all normal operation in the circuit.
normal working conditions. Self-venting occurs auto-
matically if, for example, excessive gas is produced
within the cell under sustained short-circuit conditions. 23.2 Mercury-zinc cardiac pacemaker
This type of cell is manufactured in a wide variety of batteries
structures to provide batteries of varying size, voltage
and capacity. A wide range of purpose-built batteries The first pacemaker batteries were produced in about
can also be obtained. For maximum reliability intercell 1973, and since then many improvements have been
connection is by spot welded strips of nickel-plated made in their design. The layout of a Mallory mer-
steel. Outer containers are made from materials rang- cury-zinc pacemaker battery is shown in Figure 23.2.
ing from cardboard to metal. The anode consists of amalgamated zinc, which is in
Some design features used by various manufactur- contact with the welded top assembly, the inner face of
ers in the construction of mercury-zinc cells are as which is also amalgamated to achieve electrochemical
follows: compatibility with the anode. It is surrounded by the
absorbent barrier, which is impregnated with a solution
1. Depolarizing cathodes of mercuric oxide, to which of sodium hydroxide saturated with zinc oxide. This
a small percentage of graphite is added, are shaped type of electrolyte minimizes hydrolysis of the zinc,
as shown in Figure 23.1 and either consolidated to which causes production of hydrogen and results in
the cell case (for flat electrode types) or pressed capacity loss. The annular cathode consists of mercuric
into the cases (of the cylindrical types).
2. Anodes are formed of amalgamated zinc powder of oxide mixed with silver powder, which forms a solid
amalgam with the mercury appearing during discharge.
high purity, in either flat or cylindrical shapes.
3. A permeable barrier of specially selected material It is in contact with the welded can assembly, which
forms the positive terminal of the battery. The seal-
prevents migration of any solid particles in the cell,
thereby contributing effectively to long shelf and ing ring and insulators, which divide the battery into
service life and also inhibiting the diffusion and two compartments, reduce the risk of short-circuit and
migration of soluble mercuric hydroxide species electrical leakage. The gasket is made of Neoprene;
from the cathode, which causes self-discharge of this material keeps its elasticity for a long period of
the zinc anode. time even under mechanical stress, thus ensuring the
4. Insulating and sealing gaskets are moulded of best possible seal.
nylon, polyethylene or Neoprene, depending on The battery shown in Figure 23.2 has a nominal
the application for which the cell or battery will voltage of 1.35 V and a nominal capacity of 1000 mA h
be used. defined for a load of 1250 Q, with a cut-off voltage
5. Inner cell tops are plated with materials that provide of 0.9V. For lighter loads, tests on early production
an internal surface to which zinc will form a zinc batteries have given the results shown in Table 23.1,
amalgam bond.
6. Cell cases and outer tops of nickel- or gold-plated Table 23.1 Mallory cell 317653 discharge at 73°C
steel are used to resist corrosion, to provide greatest
passivity to internal cell materials and to ensure Load Minimum Average Maximum
good electrical contact. (W
7. An outer nickel-plated steel jacket is generally used ~ ~~
for single cells. This outer jacket is a necessary 11 mAh 954 1066 1080
component for the ‘self-venting construction’ used months 10.5 12.0 12.25
on some cells, which provides a means of releasing 22 mAh 690 94 1 1035
excess gas in the cell. Venting occurs if operating months 15.2 20.7 23.2
abnormalities such as reverse currents or short-
circuits produce excessive gas in the cell. At mod- 33 mAh 622 791 1000
erately high pressures, the cell top is displaced months 20.8 26.0 33.0