Page 459 - Battery Reference Book

4514 Constant-potential charging

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130

110 -
- -
100 0.20
90 -

I I I I
0 1 2 3 4 5 6 7 8 9 I0203040 5060 7080
Charging time (h)
Figure 45.2 Charging characteristics of a Yuasa sealed lead-acid battery at 25°C. (C is the 20 h rated capacity. For further details, see
text) (Courtesy of Yuasa)

Table 45.3 Reduction in charge time with lor
increasing charging potential

Constant charging Hours to achieve a
potential (V) charging volume of
100% 120%
6.9 20 50
I
1
1
I
I
I1 I
I
1
1
1
20 5 20 14 15 16 17 18 19 20 21 22 23 24 25
50 4 1
Charging time for fully discharged cell (h)
Figure45.3 Sample discharge data for Eveready OB90 and
45.1.2 Nickel-cadmium batteries OB90T sealed nickel-cadmium cells, 1.2V, 90A h capacity.
Charge voltage 1.35-1.5OV (Courtesy of Union Carbide)
Figure 45.3 shows a constant-potential charging curve
for a 1.2V, 90pAh capacity nickel-cadmium cell. by limiting the charge or overcharge current to the
This curve demonstrates the relationship between maximum permissible. It is sufficient to connect a
charging rate and the time required to fully charge simple resistance in the charge circuit.
a fully discharged battery when a charge voltage of
1.35-1.5V is used.
Constant-potential charge has always been used as a 45.1.3 CP chargers for calcium-lead alloy
rapid-charge system for open and semi-open types of batteries
nickel-cadmium cell. This type of charge applied to
sealed nickel-cadmium cells without precautions has Figure 45.4 shows the charging circuits recommended
almost always led to the destruction of the batteries. by Yuasa for constant-potential charging of NP bat-
In fact, the voltage of sealed cells diminishes after the teries. The constant-potential current-limited charger
end of charge and the effect of this is to increase the (Figure 45.4(b)) is available from National Semi-
charge current to proportions that are not acceptable to conductors (IC LM723C Voltage Regulator). The
the batteries. It is possible, however, to use a simple Motorola MC 1723CL and the Fairchild MA 723DC
means to charge sealed cells at constant potential equipment have equivalent performances.