Page 412 - Battery Reference Book

Mercury-indium-bismuth primary batteries 3713

37.1 Mercury-zinc primary batteries The mercuric oxide-zinc (mercury) cell for minia-
ture applications is usually based on the familiar button
Mercuric oxide. batteries have been widely used as construction using a compressed cathode of mercuric
secondary standards of voltage because of the higher oxide and graphite (added for conductivity) in a plated
order of voltage maintenance and ability to with- steel can. The cell seal is supported by a cathode
stand mechanical and electrical abuses (Figure 37.1). sleeve. on top of which is placed a synthetic separ-
For use as reference sources in regulated power ator and an electrolyte absorbing pad. The electrolyte
supplies, radiation detection meters, portable poten- is a solution of potassium hydroxide. The amalgamated
tiometers, electronic computers, voltage recorders, sci- zinc anode is added and the cell sealed with a poly-
entific and miiitary equipment, hearing-aids, electronic meric gasket and a metal top cap.
wrist watches and cameras, and similar equipment, The mercury cell has a low internal resistance
desirable features are as follows: and high cathode efficiency. Discharge characteristics
are substantially flat, which is an obvious advantage
Voltage stabiZi9 versus time The uniform voltage for hearing-aid use. Capacity retention of the system
of the mercury cell is due to the efficient nature of on storage is good, with typically 95% of capacity
the cathode. Over long periods of time, regulation retained for 10 months storage at 45°C and for 2 years
within 0.5-1.0% is maintained; for shorter periods, at 20°C. Multi-cell batteries using the mercury system
regulation of 0.1% may be realized. are available for applications requiring higher voltages,
Short-circuil currents Momentary short-circuits and some cylindrical sizes are produced. In general, the
will cause no permanent damage with almost high cost of the system restricts it to those uses where
complete recovery of full open-circuit e.m.f. within space is at a premium or where voltage regulation is
minutes. critical.
Heavy load currents Depending on the type of In hearing-aids, the current drain may be of the order
cell used, high drains without damage can be of I d for a total discharge of several days. The
obtained; recovery to full open-circuit e.m.f. is low-resistivity separator is able to retain the droplets
rapid. of mercury which form as the cathode is discharged.

Miniature applications have become more important In a watch, the batteries must perform adequately
in recent years with the general acceptance of behind- over a period of months or yeas, and the discharge
the-ear hearing-aids and the advent of the electronic pattern may be one of short periods of a drain of
watch. High energy density per unit volume is the tens of milliamperes superimposed on a microampere
prime requirement for a battery in these products. The continuous drain. Under these conditions a separator
mercuric oxide-zinc, silver oxide-zinc, zinc-air and with carefully controlled properties is required to avoid
lithium-based systems appear to be likely contenders possible mercury penetration and short-circuiting of
for this market. Although the last two types of battery the cell.
have been produced in sizes suitable for miniature A general review of the types of application found
applications, they are not widely available in this for mercury-zinc batteries available from one par-
format ticular supplier (Crompton-Parkinson) is found in
Table 37.1, together with information on capacities
available and battery weights. Voltages vary between
1.35 and 97.2V and capacities vary up to 28Ah.
These batteries are of the button cell or cylindrical
design.
1.4 r
----.-e--.--.--.-
l mA drain 37.2 Mercury-cadmium primary
batteries

Applications include mines, munitions, buoys, bea-
cons, pyrotechnic igniters, missiles, sealed metering
equipment, indicator lights and civil military electronic
devices.
L A 37.3 Mercury-indium-bismuth
0.3 primary batteries
0 1'50 300 450 600 750 900 1050
Life to 0.9 V (h)
These have found applications in location beacons and
Figure 37.1 Merclury-zinc El cell: cell load voltage versus time at memory core standby supply batteries.
1350Q at constant load at 21°C