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8.4
Charge efficiency
The charge efficiency of the battery is
dependent on the state of charge
of the battery and the temperature.
For much of its charge profile, it
is recharged at a high level of
efficiency.
In general, at states of charge less
than 80 % the charge efficiency
remains high, but as the battery
approaches a fully charged condition,
the charging efficiency falls off. This is
illustrated graphically in Figure 7.
8.5
Temperature effects
As the temperature increases,
the electrochemical behavior becomes
more active, and so, for the same
floating voltage, the current increases.
As the temperature is reduced then
the reverse occurs. Increasing the
current increases the water loss, and
reducing the current creates the risk
that the cell will not be sufficiently
charged. Thus, as it is clearly
advantageous to maintain the same
current through the cell, it is necessary
to modify the floating voltage as the
temperature changes. The
recommended change in voltage
required, or “temperature
compensation”, is -3 mV/ °C, starting
from an ambient temperature of
+ 20°C to + 25°C.
8.6
Commissioning charge
It is recommended that a good first
charge should be given to the battery.
This is a once, only operation, and is
essential to prepare the battery for its
long service life. It is also important
for discharged and empty cells which
have been filled, as they will be in
a totally discharged state.
A constant current first charge is
preferable and this should be such
as to supply 300 % of the rated
capacity of the cell. Thus, a 250 Ah
cell will require 750 ampere hours
input, e.g. 50 amperes for 15 hours.
Cells which have been stored for less
than one year should be charged for
15 hours at the recommended charge
current* before being placed in
service.
Cells that have been stored for more
than one year, or have been supplied
empty and have been filled, should
be charged for 15 hours at the
recommended charge current *
discharged to 1.0 volts per cell and
then charged for 10 hours at the
recommended charge current*.
In cases where it is not possible to
provide constant current charging,
it is possible to achieve this with a
constant voltage by using a high
voltage level, e.g. 1.65 voltage limit
may be used for 20 to 30 hours, if
the current limit is approximately
equivalent to the 5 hour charge
current*. If the current rating is lower,
then the charge time should be
increased accordingly.
When the charger maximum voltage
setting is too low to supply constant
current charging, divide the battery
into two parts to be charged
individually at a high voltage.
The battery can now be put into
service.
*Please refer to the installation and
operation instruction sheet.
Figure 7 - Charge efficiency as a function of state of charge
110 %
100 %
90 %
80 %
70 %
60 %
50 %
40 %
30 %
20 %
10 %
0 %
0 20 40 60 80 100 120 140
Percentage
charge efficiency
Charge input (% Rated Capacity)
capacity charged
Charge at Constant Current
(current 0.2 C
5
A)
BlockBat 3/11/98 10:12 Page 27
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