Page 91 - Battery Reference Book

Primary batteries
therefore a procedure for selecting the most suitable by meeting the critical needs of the application and
system is necessary. The factors involved in the selec- subordinating the others.
tion of primary batteries suitable to meet a particular The selection of a battery is best achieved by setting
requirement are extremely complex. It is essential at out a list of minimum requirements, conditions and
an early stage in the design to liaise with the technical limitations. as follows:
department of the battery manufacturer to ensure that
the characteristics of the battery and the equipment are 1. Maximum permissible voltage at the beginning of
discharge.
matched SO that the user of the equipment obtains the 2. Normal voltage during discharge (voltage stability
best possible performance from both.
There are tkree basic applications for which primary on load).
batteries are used: 3. End-voltage, that is, voltage at which equipment
ceases to function properly.
1. Miniature equipment (worn or carried unobtrusively 4. Current-voltage relationships: constant current
in use). (amps), constant resistance (ohms), constant power
2. Equipment that is portable in use (often moved (watts).
during or between discharge periods). 5. Type of discharge and current drain (duty cycle):
3. Transportable equipment (not frequently carried) continuous, intermittent, continuous with pulses.
and standby systems. 6. Storage and service life.
7. Environmental conditions in storage and in service.
Each of these can be based on one of several electro- 8. Physical restrictions such as dimensions and weight.
chemical systems. Batteries in miniature equipment are 9. Special requirements.
discussed in !<ection 2.1.2, and in portable equipment
in Section 2.1.3. Transportable batteries are usually of It is important that all these points be considered
the rechargeable type and are dealt with in Chapter 2.2. because they are interdependent. This information will
allow the battery manufacture to recommend to the
One of the problems facing a designer is selecting
the correct system for the application from a choice equipment designer a battery suitable for the proposed
of many. Some of the available systems are listed application.
Table 2.4 gives some idea of the relative running
in Table 2.1, which gives open-circuit voltage, aver- costs between three different primary systems: car-
age on-load voltage, energy densities and operating bon- zinc, alkaline manganese and mercury -zinc of
temperatures for various systems, including all those the R20 size cell covering heavy, medium and light
commonly available for commercial use. duties. In this comparison of running costs no account
Although the many types of battery available may is taken of voltage regulations. Although the voltage
seem to make a proper choice difficult, the problem can tolerances differ widely between systems (10% for
be somewhat simplified by first outlining the applica- mercury-zinc, 20% for alkaline manganese, 25% for
tion requirements and then matching a battery to the air depolarized and up to 40% for carbon-zinc), it
job. The preliminary information that must be avail- should be noted that good voltage regulation may be
able before a battery can be specified is set out below. obtained from any battery system using devices such
The discussion of the basic characteristics and features as Zener diodes. Thus, it can be seen that by careful
of various batteries in the following sections will indi- design of a given circuit the lower-cost system could be
cate which one(s) are most suitable for the application. used, or, because of the more stable voltage of another
Unfortunately, the ideal characteristics cannot be found system, the number of cells in a giver, battery could be
in any one battery design; nor can the characteristics reduced, which could reduce the volume and weight.
of one battery always be compared directly with those The operating voltage range of the equipment deter-
of another. Therefore optimum performance of a bat- mines the extent to which the available capacity in a
ter~' in an application can usually be achieved best battery is realized. A cut-off voltage (that IS, complete

Table 2.4 Running cost comparison, relative to Carbon-zinc cell size R20
System Retail price Cost per hour
Heavy-duty* Medium-duty* Light-duty*
(30 mA, (37.5 mA, (10 mA,
30 midday) 4 Wday) 12 Wday)

Carbon-zinc (Vidor SP2) 1 .o 1 .o 1 .o I .o
Alkaline manganese 5.5 2.6 2.8 5.0
Mercury-zinc 26.0 1 .o 10.0 18.0

'End-voltage 0.8 V/cell