User manual

6.8 Constant Current (CC) Charging
Constant current (CC) charging is another efficient method
of charging CYCLON
®
battery single cells and monoblocs.
CC charging of a cell or battery is accomplished by the
application of a nonvarying current source. This charge
method is especially effective when several cells are charged
in series since it tends to eliminate any charge imbalance in a
battery. CC charging charges all cells in the battery.
Figure 6-2: Constant Current Charging at 0°C
Figure 6-3: Constant Current Charging at 25°C
Figure 6-4: Constant Current Charging at 45°C
Figure 6-2 is a family of curves depicting cell voltage vs. time
at different CC charging rates at an ambient temperature of
0°C (32°F). Figures 6-3 and 6-4 show the same curves bit at
two different temperatures - 25°C (77°F) and 45°C (113°F),
respectively. Table 6-4 summarises these three graphs.
Table 6-4
Parameter Temperature C/5 C/10 C/15 C/20
0°C (32°F) 2.91 2.87 2.84 2.82
25°C (77°F) 2.83 2.79 2.76 2.73
45°C (113°F) 2.78 2.71 2.67 2.64
0°C (32°F) 2.91 2.87 2.84 2.82
25°C (77°F) 2.83 2.79 2.76 2.73
45°C (113°F) 2.78 2.71 2.67 2.64
As shown by these curves, the voltage of the cell rises
sharply as the full charge state is approached. This increase
in voltage is caused by the plates going into overcharge when
the majority of the active material on the plates has been
converted from lead sulphate to spongy lead on the negative
plate and lead dioxide on the positive plate.
The voltage increase will occur at lower states of charge when
the cell is being charged at higher rates. This is because
at the higher CC charge rates, the charging efficiency is
reduced. The voltage curves in Figures 6-2 to 6-4 are different
from those of a conventional lead-acid cell due to the effect
of the recombination of gases on overcharge within the pure
lead-tin system.
While CC charging is an efficient method of charging,
it requires a greater degree of control to prevent serious
overcharge. Continued application at rates above C
10/500,
after the cell is fully charged, will be detrimental to the life
of the cell.
At overnight charge rates (C
10/10 to C10/20), the large
increase in voltage at the nearly fully-charged state is a
useful indicator for terminating or reducing the rates for a
CC charger. If the rate is reduced to between C
10/1000 and
C
10/500, the cell can be left connected continuously and
yield a float life up to 8 to 10 years at room temperature
(25°C/77°F).
These graphs and charts reflect data obtained from CYCLON
cells that had been cycled three times at the
C
10/5 rate. Thus, these numbers should not be treated
as specification values but rather as guidelines to follow when
developing or using a CC charger.
1.8
2
2.2
2.4
2.6
2.8
3
0 5 10 15 20 25 30
C/5 C/10 C/15 C/20
Voltage Profiles at 0°C
Constant Current Charging
Time (Hours)
Voltage
C/5 C/10
C/15 C/20
Voltage Profiles at 25°C
Constant Current Charging
1.8
2
2.2
2.4
2.6
2.8
3
0 5 10 15 20 25 30
Time (Hours)
Voltage
2.8
2.6
2.4
2.2
2
1.8
Voltage Profiles at 45°C
Constant Current Charging
0 5 10 15 20 25 30
Time (Hours)
Voltage
C/5
C/10
C/15 C/20
Peak Voltage,
volts per cell
Time to reach
peak voltage,
hours
14
Publication No: EN-CYC-AM-007 - December 2008
www.enersys-emea.com