Page 92 - Battery Reference Book

2/10 Guidelines to battery selection
useful discharge) of about 0.9Vkell is desirable but reliable as well as economical if their size can be
not always possible for many types of cell. accommodated.
Current is directly linked with voltage tolerance, All electrochemical systems are temperature sens-
regulation and load and, by taking into account the itive, some giving better performance than others at
period of time for which the current is required, the the extremes; service temperature affects both capacity
required capacity in ampere hours can be determined. and life. The suggested operations temperature ranges
Ampere hour capacity of the zinc-carbon system can of some battery systems are given in Table 2.5.
be obtained provided that the period of time for which
current is required is known and the end-voltage to
which the equipment can work efficiently is given. Table 2.5
Electrical loading and required service life will deter-
mine the required capacity of the battery and this is Suggested operational Battery system
temperature range("C)
the basic parameter in deciding the type of cell to be
used. Duty cycle may also be a factor in the choice of -7 to +54 Carbon-zinc
battery. Applications requiring heavy pulses of short -29 to +50 Zinc-air
duration will be met by a battery with a different -30 to +50 Alkaline manganese
internal construction from that used for a very low -20 to +50 Mercury-zinc
continuous current duty. -40 to +70 Mercury-cadmium
Battery volume is dependent on the required voltage, -20 to +90 Mercury -indium- bismuth
ampere hour capacity, temperature and termination and -40 to +60 Lithium-based system,
is dictated in many cases by the commercially available -54 to t60 depending on type
types such as the round cell range and layer stack
batteries. Primary batteries are made up of individual
cells; in the case of carbon-zinc batteries these can be The majority of battery manufacturers publish dis-
round or flat cells. A primary battery made from flat charge figures taken at 20°C. By the correct matching
cells, known as a 'layer stack', generally gives a higher of size and current drain, the carbon-zinc and other
voltage and consequently lower capacity than a round primary systems can successfully be operated at lower
cell battery of the same volume. The trend is to fit temperatures than those indicated, but liaison with the
all round cells into equipment, but by careful selection battery manufacturer is essential.
of round cells and a layer stack battery the volume To select a system that can be stored and operated at
required by the batteries can be reduced by more than the extreme temperatures of -40 to +90"C-a require-
40% in some cases. Such combinations have been very ment with which battery manufacturers are sometimes
successful in reducing the size of military portable presented - is difficult and expensive; therefore it is
communication transceivers. In special applications important to specify temperature requirements, partic-
for industrial and military use, shape is generally as ularly the dwell times at the extremes. This is essen-
important as volume. tial information which the designer of the equipment
If a standard commercial battery or cell is chosen, should be able to give to the battery manufacturer.
the size and type of terminals are already specified, but Table 2.6 gives a guide to the period, at 20°C, for
care is none the less required; for example, on round which some battery systems can be stored, at the end
cells the tolerance allowable (BS 397: 1976, Primary of which the cells would have retained between 80 and
Cells and Batteries, covering R20, D size (American 90% of their capacity.
Standards Association) and Vidor SP2) is 2.3mm in
height and 1.6mm in diameter. The difference in size
between cells from different manufacturers is particu- Table 2.6
larly apparent when several cells are connected end to
end in series. The construction of a battery container System Shelf life
at 20°C
to allow for the difference can be difficult and expen- before use*
sive. The use of layer stack batteries having connec- (months)
tors, in which there is no such build-up of tolerances,
can therefore greatly simplify the construction of the Carbon-zinc 18-30
battery container. In applications for industrial and mil- Alkaline manganese 18-30
itary use, termination is of major importance and much Mercury-zinc 24-30
attention is paid to terminal design so that it can meet Mercury-indium-bismuth 36
the environmental testing demanded. Mercury-cadmium 48
Allowable size and weight will sometimes determine Air depolarized 48
Silver oxide-zinc
18
which battery is selected in spite of other requirements. Lithium-sulphur dioxide 60
A premium is usually paid for small size with high
output capacity. Bulky and heavy batteries can be *Varies with cell size