Page 479 - Battery Reference Book

47/12 Constant-current charging

Capacity chargeable before safety valve open 47.3.6 Low-temperature constant-current
62 5 charging
Charging should preferably be done at normal tem-
500
perature, but can be done between 0 and 40°C. Typ-
ical charging voltages are lower at a temperature
375
higher than room temperature, and higher at tempera-
tures lower than room temperature. Typical charging
2 50 curves for nickel-cadmium cells at 0°C are shown
in Figure 47.11. Manufacturers claim that occasional
125 overcharging during a few weeks of cell life with nor-
mal recommended charging current will not damage
0 the cells.
0.5 1 2.5 5 10 15 25 50 Table 47.3 compares the maximum charge rates (in
Charging rate (A) milliamperes) for stated charge times (in hours) recom-
mended for various charging temperatures between
-30 and +lO"C in the normal and the continu-
ous (float) charging of a series of SAFT VB series
Charge time (Table 51.1) sealed nickel-cadmium button cells.
Figure 47.12 presents a series of typical voltage
Figure 47.10 Duration of ultra-rapid charge for SAFT VR 0.5AA
sealed cylindrical nickel-cadmium cells: charge duration versus versus capacity charged curves for various discharge
charging rate (Courtesy of SAm rates for a series of button nickel-cadmium batteries
(SAFT VB series, Table 51.1).
at very high rates if the capacity charged is limited to When the temperature of batteries is less than YC,
a few per cent of the rated capacity. This technique is it is necessary to limit the rates of charge and over-
applicable with complete safety over the temperature charge. The charge rate must be chosen so that the
range -40 to +60°C. In the case of one-shot char- internal pressure on overcharge is less than the oper-
ging, the available capacity is small, but the power ating pressure of the valves in the case of cylindrical
available for short periods remains high. Thus, fully cells or the deformation pressure in the case of but-
discharged button cells will accept between -40 and ton and sealed rectangular cells. Figure 47.13 shows,
+60T charge rates of up to 2OCsmA for a limited for cylindrical and button cells, the maximum permis-
time. For example, §AFT VB 10 (0.1 Ah) button cells sible rates on overcharge for short periods (a few days)
(Table 51.1) charged for 5 s at 1 A have an available
capacity of 2400mA.
47.3.5 Uncontrolled rapid constant-current
charging
This method is applicable to nickel-cadmium cylin-
drical cells and batteries such as the SAFT VO series
(Table 51.1), which can withstand higher rates of
charge and overcharge than standard cylindrical cells
such as the SAFT VR range. This extra performance 0 2 4 6 8 101214
is made possible by the use in battery design of highly Time (h)
porous plates which improve oxygen consumption at (a) Type KR15/51, 0.4 A h, current 0.05 A
the negative plate. A further consequence of this is
that the available capacity per unit volume of the for-
mer type of cell is reduced to about 40-50% of that
of the latter type. However, the power-to-volume ratio
remains high. Over-charge at high rate may continue
for several days without affecting the battery life.
The ability to overcharge at high rates permits the
uncontrolled rapid charge of cells of unknown state of 0 2 4 6 8 101214
charge using chargers of simple design and relatively Time (h)
low cost. The times for uncontrolled rapid charge
for cells that can stand higher rates of charge and (b) Type KR35/92,5.6 A h, current 0.6 A
overcharge (e.g. the SAFT VX range (Table 51.1)) Figure 47.11 Charge voltage versus charge time of Nife Jungner
are between 30 and 60 min in the temperature range sealed nickel-cadmium cells at constant current and 0°C
0-30°C. (Courtesy of Nife Jungner)