6.5
Short-circuit values
The typical short-circuit value in
amperes for a block battery cell is
approximately 9 times the ampere-
hour capacity for an L type block,
16 times the ampere-hour capacity
for an M type block and 28 times
the ampere-hour capacity for an
H type block.
The block battery with conventional
bolted assembly connections will
withstand a short circuit current of this
magnitude for many minutes without
damage.
6.6
Open circuit loss
The state of charge of the block cell
on open circuit slowly decreases
with time due to self-discharge. In
practice this decrease is relatively
rapid during the first two weeks, but
then stabilizes to about 2 % per month
at 20°C.
The self-discharge characteristics of
a nickel-cadmium cell are affected by
the temperature. At low temperatures,
the charge retention is better than at
normal temperature, and so the open
circuit loss is reduced. However, the
self-discharge is significantly
increased at higher temperatures.
The typical open circuit loss for
the block battery for a range
of temperatures which may be
experienced in a stationary
application is shown in Figure 2.
6.7
Cycling
The block battery is designed to
withstand the wide range of cycling
behavior encountered in stationary
applications. This can vary from low
depth of discharges to discharges of
up to 100 % and the number of cycles
that the product will be able to
provide will depend on the depth
of discharge required.
The less deeply a battery is cycled,
the greater the number of cycles it is
capable of performing before it is
unable to achieve the minimum
design limit. A shallow cycle will give
many thousands of operations,
whereas a deep cycle will give only
hundreds of operations.
Figure 3 gives typical values for the
effect of depth of discharge on the
available cycle life, and it is clear
that when sizing the battery for a
cycling application, the number and
depth of cycles have an important
consequence on the predicted life
of the system.
6.8
Effect of temperature
on lifetime
The block battery is designed as
a twenty year life product, but as
with every battery system, increasing
temperature reduces the expected life.
However, the reduction in lifetime
with increasing temperature is very
much lower for the nickel-cadmium
battery than the lead-acid battery.
The reduction in lifetime for the nickel-
cadmium battery, and for comparison,
a high quality lead acid battery is
shown graphically in Figure 4. The
values for the lead acid battery are
as supplied by the industry and found
in Eurobat and IEEE documentation.
14
BlockBat 3/11/98 10:12 Page 17
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