7.
Battery sizing principles in stationary
standby applications
There are a number of methods
which are used to size nickel-
cadmium batteries for standby
floating applications. These include
the “Hoxie” sizing method, the IEEE
1115
.
All these methods must take into
account multiple discharges,
temperature de-rating, performance
after floating and the voltage window
available for the battery. All methods
have to use certain methods of
approximation and each does this
more or less successfully.
A significant advantage of the nickel-
cadmium battery compared to a lead
acid battery, is that it can be fully
discharged without any inconvenience
in terms of life or recharge. Thus, to
obtain the smallest and least costly
battery, it is an advantage to
discharge the battery to the lowest
practical value in order to obtain the
maximum energy from the battery.
The principle sizing parameters which
are of interest are:
7.1
The voltage window
This is the maximum voltage and the
minimum voltage at the battery
terminals acceptable for the system.
In battery terms, the maximum
voltage gives the voltage which is
available to charge the battery, and
the minimum voltage gives the lowest
voltage acceptable to the system to
which the battery can be discharged.
In discharging the nickel-cadmium
battery, the cell voltage should be
taken as low as possible in order to
find the most economic and efficient
battery.
7.2
Discharge profile
This is the electrical performance
required from the battery for the
application. It may be expressed in
terms of amperes for a certain
duration, or it may be expressed in
terms of power, in watts or kW, for a
certain duration. The requirement
may be simply one discharge or many
discharges of a complex nature.
7.3
Temperature
The maximum and minimum
temperatures and the normal ambient
temperature will have an influence on
the sizing of the battery. The
performance of a battery decreases
with decreasing temperature and
sizing at a low temperature increases
the battery size. Temperature de-
rating curves are produced for all cell
types to allow the performance to be
re-calculated.
7.4
State of charge
or recharge time
Some applications may require that
the battery shall give a full-duty cycle
after a certain time after the previous
discharge. The factors used for this
will depend on the depth of
discharge, the rate of discharge, and
the charge voltage and current. A
requirement for a high state of charge
does not justify a high charge voltage
if the result is a high end of discharge
voltage.
16
17
6.9
Water consumption
and gas evolution
During charging, more ampere-hours
are supplied to the battery than the
capacity available for discharge.
These additional ampere-hours must
be provided to return the battery to
the fully charged state and, since they
are not all retained by the cell and
do not all contribute directly to the
chemical changes to the active
materials in the plates, they must be
dissipated in some way. This surplus
charge, or over-charge, breaks down
the water content of the electrolyte
into oxygen and hydrogen; and pure
distilled water has to be added to
replace this loss.
Water loss is associated with the
current used for overcharging. A
battery which is constantly cycled, i.e.
is charged and discharged on a
regular basis, will consume more
water than a battery on standby
operation.
In theory, the quantity of water used
can be found by the faradic equation
that each ampere hour of overcharge
breaks down 0.366 cm 3 of water.
However, in practice, the water usage
will be less than this, as the
overcharge current is also needed to
support self-discharge of the
electrodes.
The overcharge current is a function
of both voltage and temperature, so
both have an influence on the
consumption of water. Figure 5 gives
typical water consumption values
over a range of voltages for different
plate types.
Example: An SBM 161 is floating at
1.43 volts per cell. The electrolyte
reserve for this cell is 500 cm
3
. From
Figure 5, an M type cell at 1.43 volts
per cell will use 0.27 cm
3
/month for
one Ah of capacity. Thus an SBM
161 will use 0.27 x 161 = 43.5 cm
3
per month and the electrolyte reserve
will be used in 500
= 11.5 months.
43.5
The gas evolution is a function of the
amount of water electrolyzed into
hydrogen and oxygen and are
predominantly given off at the end of
the charging period. The battery
gives off no gas during a normal
discharge.
The electrolysis of 1 cm
3
of water
produces 1865 cm
3
of gas mixture
and this gas mixture is in the
proportion of 2/3 hydrogen and 1/3
oxygen. Thus the electrolysis of 1 cm
3
of water produces about 1240 cm
3
of hydrogen.
Figure 5 - Water consumption values for different voltages and plate types
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