8.1
Charging generalities
The block battery can be charged by
all normal methods. Generally,
batteries in parallel operation with
charger and load are charged with
constant voltage. In operations where
the battery is charged separately from
the load, charging with constant
current or declining current is
possible. High-rate charging or
overcharging will not damage the
battery, but excessive charging will
increase water consumption to some
degree.
8.2 Constant voltage charging
methods
Batteries in stationary applications are
normally charged by a constant
voltage float system and this can be
of two types: the two-rate type, where
there is an initial constant voltage
charge followed by a lower voltage
floating voltage; or a single rate
floating voltage.
The single voltage charger is
necessarily a compromise between a
voltage high enough to give an
acceptable charge time and low
enough to give a low water usage.
However it does give a simpler
charging system and accepts a
smaller voltage window than the two-
rate charger.
The two-rate charger has an initial
high voltage stage to charge the
battery followed by a lower voltage
maintenance charge. This allows the
battery to be charged quickly, and
yet, have a low water consumption
due to the low voltage maintenance
level.
The values used for the block battery
ranges for single and two-rate charge
systems are as shown in Table 5
below.
To minimize the water usage, it is
important to use a low charge
voltage, and so the minimum voltage
for the single level and the two level
charge voltage is the normally
recommended value. This also helps
within a voltage window to obtain the
lowest, and most effective, end of
discharge voltage (see Battery sizing
chapter 7).
The values given as maximum are
those which are acceptable to the
battery, but would not normally be
used in practice, particularly for the
single level, because of high water
usage.
18
19
7.5
Aging
Some customers require a value to be
added to allow for the aging of the
battery over its lifetime. This may be a
value required by the customer, for
example 10 %, or it may be a
requirement from the customer that a
value is used which will ensure the
service of the battery during its
lifetime. The value to be used will
depend on the discharge rate of the
battery and the conditions under
which the discharge is carried out.
7.6
Floating effect
When a nickel-cadmium cell is
maintained at a fixed floating
voltageover a period of time, there is
a decrease in the voltage level of the
discharge curve. This effect begins
after one week and reaches its
maximum in about 3 months. It can
only be eliminated by a full
discharge/charge cycle, and it cannot
be eliminated by a boost charge. It is
therefore necessary to take this into
account in any calculations
concerning batteries in float
applications. This is used in the sizing
program, the IEEE sizing method and
the published data.
8.
Battery charging
single level (V)
two level (V)
min max min max
floating
SBH
1.43 1.50 1.45 1.70 1.40
SBM
1.43 1.50 1.45 1.70 1.40
SBL
1.43 1.50 1.47 1.70 1.42
Table 5 - Charge and float voltages for the block battery ranges
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