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Types of constant-current charging 47/13

Table 47.3 Comparison of maximum charge rates in normal and continuous (float) charging at various temperatures recommended for
SAFT VB series sealed nickel-cadmium button cells
Cell type VB 4 VB 10 VB 22 VB 30 VB 40

Rated capacity (Ah) 40 100 220 300 600
Nominal discharge voltage per cell (V) 1.20 1.20 1.20 1.20 1.20
Normal charge from -30 to +lO"C, -30 (100) 0.4 1 .o 2.2 3.0 6.0
max. charge rates (mA) for charge time -20 (55) 0.8 2.0 4.0 6.0 12.0
(h) in brackets -10 (30) 1.5 4.0 8.0 11.0 21 .0
0 (20) 2.5 6.0 13.0 18.0 36.0
+10 (14) 4.0 10.0 22.0 30.0 60.0
Continuous charge from -30 to +lO"C, -30 (100) 0.2 0.5 1.0 1.5 3 .0
ma. charge rates (mA) for charge time -20 (55) 0.4 1 .0 2.0 3.0 6.0
(h) in brackets -10 (30) 0.8 2.0 4.0 5.0 11.0
0 (20) 1.2 3.0 6.5 9.0 18.0
+10 (14) 2.0 5.0 11.0 15.0 30.0

Occasional overcharge at these rates beyond the prescribed times is not detrimental to the cell. Rapid and ultra-rapid charge systems can be
obtained for charging in times from a few seconds to 1 h

1.6 and on permanent overcharge. A number of methods
- 1.5 2.5 mA at 0 OC are used to vary the charge current as a function of
temperature:
2 1.4
m ooc Fit into the battery a temperature sensor that oper-
z i.3
> 1.2 t ates on the charger to adjust the charge current to
suit the battery temperature.
1.1 1 I I I 1 I I i Charge at constant current and voltage limited to
0 8 16 24 32 40 48 56 1.55 Vkell, a voltage setting that will avoid all risk
Capacity charged EmA h) of internal overpressure at low temperature.
(a)
Use the charge current corresponding to the lowest
r 13 rnA at 0 "C operating temperature.
1.5 Also, in certain cases, for example when charging
u&e= 0 OC
2 1.4 at low temperatures, constant-current charging can be
0)
1.:
2 1.3 I combined with voltage limitation. With a normal char-
1.2 i- state is reached after 14-16h at room temperature.
5
5 ging current of approximately 0.1C A a fully charged
1.1 I L I I I I I I A lower current, 0.04-0.05C A, is often used as con-
Cl 44 88 132 176 220 264 308 tinuous charging current in different applications. The
Capacity charged (mA h) charging efficiency is then lower, because proportion-
(b) ally more energy is used to produce oxygen. The char-
1.6 ging efficiency is also dependent on the temperature,
- 1.5 and decreases with rising temperature.
>
- 1.4
m
z 1.3 47.3.7 Permanent or floating
> constant-current charging
1.2
In applications with permanent charge, the object of
0 120 240 360 480 600 720 840 the charge current is to maintain an already fully
Capacity charged IinA h)
charged nickel-cadmium battery in this state over a
(C)
period of time and when necessary to recharge any
Figure 47.12 Voltage versus capacity for 'normal' charging of capacity discharged at regular intervals or in a random
SAFT VB series sealed nickel-cadmium button cells. (a) UB4 manner. The first charge may be carried out either at
40Cs mh h; (b) UBZ 220C5 rnA h; (c) VB60 600C5 mA h (Courtesy the permanent rate if this is sufficient (>O.O2C5 A), or
of SAFT)
at a higher rate where the permanent charge value is