Page 93 - Battery Reference Book

Primary batteries 2/7 1
Shock or libration may indicate the need for a of mercury that form as the cathode is discharged.
rugged battery construction. Unusual rates of acceler- In a watch, the batteries must perform adequately
ation or high-altitude operation are also vital environ- over a period of months or years, and the discharge
mental considerations. Storage time and temperature pattern may be one of short periods of a drain of
under any of these conditions should be noted. tens of milliamperes superimposed on a microampere
It should be noted from Table 2.3 that batteries with continuous drain. Under these conditions a separator
different type:; of electrochemical system are chosen with carefully controlled properties is required to avoid
for different types of duty application. Obviously, possible mercury penetration and short-circuiting of
some batteries based on a particular type of electro- the cell.
chemical system are more suited to particular applica- Silver oxide-zinc cells are often specified for elec-
tions than are others, and here one is confronted with tronic watch applications. Sodium hydroxide elec-
the dilemma that any particular primary battery manu- trolyte, which has a lower conductivity than potassium
facturer will have only some of these electrochemical hydroxide, is often used because it has a lower ten-
systems included in their range and that the one recom- dency to ‘creep’ at the seal. The separator in the silver
mended may not be absolutely the best from the total oxide system must retain soluble silver species pro-
range available. It is advisable, therefore, to discuss duced by chemical dissolution of the oxide, and a mul-
battery requirements at an early stage of design with tiple layer separator of low-porosity film achieves this.
several battery suppliers covering the whole range of There are two types of silver oxide cell: one has
types of battery. a cathode of monovalent silver oxide (Ag20) and the
other type uses divalent silver oxide (Ago). The latter
2.1.2 Batteries in miniature equipment type has a higher theoretical potential (1.8V) and,
because there is an additional chemical reduction from
Miniature applications have become more important in Ago to AgzO it has a higher capacity. (The theoretical
recent years with the general acceptance of the behind- energy density is 424 W hkg.) The two-stage reduction
the-ear hearing-aid and the advent of the electronic process would normally result in a discharge curve
watch. High energy density per unit volume is the with two plateaux at 1.7 and 1.5 Q and the voltage drop
prime requirement for a battery in these products. The in the middle of discharge may necessitate a voltage
mercuric oxide-zinc, silver oxide-zinc, zinc-air and regulator in the equipment. If the surface layer of the
lithium-based systems appear to be likely contenders electrode is of Ag20, however, discharge takes place
for this market. Although the last two types of battery at the lower potential throughout. In order to achieve
have been produced in sizes suitable for miniature voltage stability, the surface may be treated to reduce
applications, they are not widely available in this Ago to AgnO or, in various patented arrangements,
format. These systems will therefore be discussed later a ‘dual-oxide’ system may be adopted. Nigher raw
in their usual cylindrical form, and the conclusions material costs mean that silver oxide cells are more
drawn then may explain the difficulties that have expensive than their mercury equivalents.
prevented their wide acceptance.
The mercuric oxide-zinc cell for miniature applica- 2.1.3 Portable-in-use batteries
tions is usually based on the familiar ‘button’ construc-
tion using a compressed cathode of mercuric oxide and This category includes test eqnipment, portable radio
graphite (added for conductivity) in a plated steel can. apparatus, lighting and calculators, and is a much
The cell seal is supported by a cathode sleeve on top larger market than the miniature or transportable
of which is placed a synthetic separator and an elec- sectors.
trolyte absorbing pad; the electrolyte is a solution of Lithium has several advantages as a possible anode
potassium hydroxide. The amalgamated zinc anode is material for an electrochemical power source. It has
added and the cell sealed with a polymeric gasket and a low equivalent weight and density and is the most
a metal top cap. electronegative element that is solid at normal tempera-
The mercury cell has a low internal resistance and tures. However, lithium reacts with water, the common
high cathode efficiency. Discharge characteristics are electrolyte solvent, and with most non-metallic ele-
substantially flat, an obvious advantage for hearing- ments and compounds; it is therefore normally essen-
aid use. Capacity retention of the system on storage tial to use a non-aqueous electrolyte.
is good. Multi-cell batteries using the mercury system Solid electrolyte lithium batteries have been
are available for applications requiring higher voltages, produced for low-drain applications. The Catalyst
and some cylindrical sizes are produced. In general, the Research Corporation produce a lithium-iodine
high cost of the system restricts it to those uses where solid electrolyte system, rated at 20pA for heart
space is at a premium or where voltage regulation is pacemaker use.
critical. For a battery that is to sustain high current densities
In hearing-aids, the current drain may be of the order at ambient temperatures, a solid electrolyte is unlikely
of 1mA for a total discharge of several days. The to be acceptable. A suitable electrolyte solvent must
low-resistivity separator is able to retain the droplets provide stable solutions of the electrolyte over a wide