Constant Current Charging
Two Stage Constant Voltage Charging
This charging method is not often utilised for valve
regulated lead acid batteries, but is an effective
method for charging a number of series connected
batteries at the same time, and/or as an equalising
charge to correct variances in capacity between
batteries in a series group.
Extreme care is required when charging Powerline
SC with a constant current. If, after the battery has
reached a fully charged state, the charge is
continued at the same rate, for an extended period
of time, severe overcharge may occur, resulting in
damage to the battery. Figure 14 shows a typical
constant current charging circuit; Figure 15 shows
the characteristics of two Powerline SC12-6 under
continuous overcharge conditions.
Two stage constant voltage charging is a
recommended method for charging valve regulated
lead acid batteries in a short period of time and then
maintaining them in a fully charged float or standby
condition. Figure 16 illustrates the characteristics of a
two stage constant voltage charger.
The characteristics shown in Fig.16 are those of a
constant voltage, current limited charger. In the
initial charging stage, the current flowing into the
battery is limited to a value of 0.30C Amps. The
charging voltage across the battery terminals rises,
during the charging process, to a value equal to the
constant voltage output of the charger, which is set to
2.45 volts per cell. Whilst continuing to charge, in
stage 1 (A-B), at 2.45 volts per cell, the current will
eventually decrease to point "Y", where the value of
this decreasing current is"sensed" causing the circuit
to switch into the second stage (B-C), reducing the
charging voltage from 2.45 volts per cell to a constant
voltage, float/standby, level of 2.3 volts per cell. The
switch to stage two, where the constant voltage level
of 2.3 volts per cell is applied, occurs after the battery
has recovered about 80% of its rated capacity. This is
one of the most efficient charging methods available
as the recharge time is minimized during the initial
stage whilst the battery is protected from overcharge
by the system switching to stage 2 (float/standby)
charge at the switching point"Y".
POWERLINE
SC
SERIES
07
Charge
Figure 15. Characteristics of two SC12-6 under
conditions of continuous overcharge
OVERCHARGING CURRENT:0.6A(0.1CA)
AMBIENT TEMPERATURE:20 C(68 F)
CAPACITY TEST:1.5A to 10.2V EVERY 100 HOURS
n=2
o
o
CHARGING TIME (HOURS)
CAP
ACIT
Y
A
T
3HR
D
ISCHARGE
0
100
200
300
400
500
600
700
800
900
1HR
2HR
3HR
4HR
AC
Figure 14. Constant current charging
Figure 16. Charging characteristics of a two
stage constant voltage charger
CHARGE
VOL
T
AGE
CHARGEING
C
URRENT
CHARGE CURRENT
CHARGE VOLTAGE
"Y" SWITCHING POINT
NOTE:Current can drop
to as low as 0.002C Amps
A
B
C
CHARGING TIME
AC
T
D
1
C
1
11
12
6
5
R
3
7
IC
10
2
13
C
2
R
4
R
5
R
1
TR
1
R
2
VR
Batt.
D
2
Figure17. Example of a two stage constant
voltage,Current limited charging circuit
4
3
VD
0
1
2
3
4
5
6
7
8
9
10
CHARGE VOLTAGE
CHARGED
VOLUME
AFTER 50% DISCHARGE
AFTER 100% DISCHARGE
CHARGING TIME (HOURS)
7.50
7.00
6.50
6.00
5.50
CHARGING CURRENT
140
120
100
80
60
40
20
0
0.25
0.20
0.15
0.10
0.05
0
5.00
4.50
4.00
3.50
11.0
12.0
13.0
14.0
15.0
(V)
0.25CA - 7.50V(15.0V,5.0V) CONSTANT VOLTAGE
CHARGING AT 20 C (68 F)
o
o
Figure13.charging characteristics
CHARGED
VOLUME
CHARGING
CURRENT
CHARGE
VOL
T
AGE
FOR
6V
BA
T
T
ER
Y
(%)
(xCA)
(V)
CHARGE
VOL
T
AGE
FOR
12V
B
A
T
TER
Y
CHARGE
VOL
T
AGE
FOR
4V
BA
T
T
ER
Y
Figure 12. Charging characteristics
(%)
140
120
100
80
60
40
20
0
0.05
0.10
0.15
0.20
0.25
7.50
7.00
6.50
6.00
5.50
(xCA) (V)
0.25CA-7.20V(14.4V,4.8V)CONSTANT VOLTAGE
CHARGING AT20 C(68 F)
o
o
CHARGED
VOLUME
CHARGING
CURRENT
CHARGE
VOL
T
AGE
FOR
6V
BA
T
T
ER
Y
CHARGED
VOLUME
CHARGE VOLTAGE
CHARGING CURRENT
AFTER 100% DISCHARGE
AFTER 50% DISCHARGE
0
2
4
6
8
10
12
14
16
18
20
11.0
12.0
13.0
14.0
15.0
5.00
4.50
4.00
3.50
CHARGE
VOL
T
AGE
FOR
12V
B
A
T
TER
Y
CHARGE
VOL
T
AGE
FOR
4V
BA
T
T
ER
Y
CHARGING TIME (HOUSE)
0
(V)
R
6
