Page 106 - Battery Reference Book

2/22 Guidelines to battery selection

As is seen in Table 2.12, the energy density data 1. Determine the battery requirements, including:
quoted by different battery manufacturers for any par- physical - size and weight limitations, shape,
ticular type of battery vary over a wide range - more shock and vibration resistance, operating posi-
so in the case of the newer types of battery such as tion, acceleration, high-altitude use;
nickel-zinc, silver-zinc and silver-cadmium where electrical - voltage, current drain (initial and
battery designs have not yet been fully optimized. operating), constant or interrupted demand, dis-
In addition to battery design, several factors such charge schedule;
as battery size and whether the data quoted refer environmental - storage and operating temper-
to a cell or a battery (where the additional weight atures, moisture and humidity factors;
or volume of the outer case affects the calculated special considerations - cost, replace or re-
energy density) have to be taken into account. For sil- charge, service life, shelf life, operating
ver-zinc cells Yardney quote 70-120 W hlkg depend- schedule, activation, type of terminals, end-
ing on type, while Eagle Picher quote 55-209 W hlkg point voltage (if equipment will not operate
for cells and, as would be expected, a lower value below a certain critical voltage).
of 37-114Whlkg for batteries (where extra non- 2. Establish relative importance of requirements-
capacity-producing weight is required). The theoretical determine those that are mandatory and those that
energy density of the silver-zinc cell is 440 W h/kg are desirable. List the requirements in order of
(that is, only weight of active material is taken importance.
into account, while weight of battery case, separa- 3. Compare the characteristics of each battery system
tors and terminals is ignored). On a volumetric basis with the battery requirements. For each requirement
Yardney quote 150-250 W h/dm3 and Eagle Picher list those systems that can meet the requirement.
80-415 W Wdm3 as cells and 55-262 W h/dm3 as bat- 4. Determine necessary compromises. The selected
teries. These data highlight the differences between the system must meet the mandatory requirements.
products and the effect on energy density of whether Trade off on the desirable requirements, beginning
the data are calculated for a cell or for a battery. Com- with those of least importance.
parable data for the silver-cadmium system are as Through proper battery system design following the
above. demands of the user, one of the following battery
It is thus seen that considerable caution must be
exercised when interpreting claims for energy density systems can be established:
in manufacturers’ literature and, indeed, this comment 1. An automatically activated primary.
would apply to many of the quoted performance data 2. A manually activated primary.
of cells and batteries. 3. A rechargeable secondary.

When designing for any one of the above batteries,
2.3 Conclusion the following special considerations for each of the
systems should be kept in mind:
In conclusion, it can be said regarding the battery
selection process, whether primary or secondary bat- 1. Automatically activated primary:
teries are being considered, that the design engineer (a) Method of activation - mechanical or electrical.
is faced with many commercial portable power sys- (b) Activation time required to distribute elec-
tems in a wide variety of models. The battery selection trolyte.
process cannot therefore be reduced to an exact sci- (c) Wet stand time after activation prior to applica-
ence. Seldom does any one battery system meet all tion of load.
the requirements for a given application. The selec- (d) Orientation during activation - upright, upside-
tion of a battery is further complicated by the fact that down, etc.
the performance characteristics of battery systems vary (e) Orientation during operation.
with temperature, current drain, service schedules, etc. (f) Temperature in storage prior to use.
Consequently the selection process usually involves a 2. Manually activated primary:
trade-off or compromise between battery requirements (a) Soak time after activation prior to use.
and battery system characteristics. (b) Cycle life (if any) - number of charge/discharge
The battery or portable power source is an integral cycles.
part of the electrical system and should be considered (c) Wet shelf life (if any) - wet stand time after
as early as possible in the design process. In selecting activation.
a battery the following four basic steps should be (d) Temperature in storage prior to use.
followed: