Page 587 - Battery Reference Book

Nickel-cadmium secondary batteries 51/25
21°C or lower for this reason. When button cells (B and 14,
BH cells) have been stored for a long period (over 6
months), regardless of storage temperature, they should - 1.0 1:3P==ki
- 1.2
not immediately be charged, but should first be fully
rn
discharged and then charged once at half the normal > 1.1
%
rate; that is, 28 h at the 20 h rate. This procedure is not 5 0.9
required for the CF and CH cells. -o.*~ I I , I I I , ,
The overcharge capability of Eveready cylindrical 0.7
nickel-cadmium cells is outstanding. The first dis- 0.6
charge after the 2-year charge period yields a slightly 0 10 20 30 40 50 60 70 80
reduced voltage curve and 65% capacity. The sec- Time (minl
ond cycle after 2 years’ continuous overcharge pro- Figure 51.12 Initial and subsequent cycles of Eveready sealed
vides essentially the same discharge curve as the initial nickel-cadmium cylindrical cells after shallow discharges: 1, initial
one. discharge; 2, second full-depth discharge after 100 cycles at 40%
Figure 5 1.1 1 shows capacity maintenance versus depth discharge; 3, first full-depth discharge after 100 cycles at
40% (C/5 (240mA) for 2 h) depth discharge (Courtesy of Union
months of continuous overcharge at the 20 h rate with Carbide)
periodic discharges every 3 months at the 1 h rate. The
cells maintain 90% of their initial capacity after 2 years Table 51.26 Union Carbide sealed nickel-cadmium batteries:
of this overcharge regimen. This pattern of use would operating temperature limits
occur where batteries are left on charge continuously
and used one cycle only on an occasional basis. (These Cell Charge Charge Discharge Storage
comments do not apply to the CH1.8 and CH2.2 types; type rate temperature temperature temperature
see Table 5 1.2 1 .) (h) (“C) (“ C) (“C)
Memory effect is that characteristic attributed to
nickel-cadmium cells wherein the cell retains the char- B 10 0 to 45 -20 to +45 -40 to +60
0 to 45
-20 to +45
-40 to +60
BH
10
acteristics of previous cycling. That is, after repeated CF 1 to 3 15.6 to 45 -20 to +45 -40 tQ +60
shallow-depth discharges the cell will fail to provide CF 10 0 to 45 -20 to +45 -40 to f60
a satisfactory full-depth discharge. Eveready cylin- CH 10 0 to 45 -20 to +45 -40 tQ +60
drical nickel-cadmium cells are particularly noted for
their lack of memory effect. Figure 51.12 shows ini-
tial and subsequent cycles after repeated shallow dis- Union Carbide have summarized the effects of low
charges. The graph shows the initial discharge curve and high temperature on the storage, discharging and
and the first and second discharge curves after 100 charging of Eveready cells and batteries as shown in
40% depth discharge cycles. Subsequent full-depth dis- Table 51.27.
charges yield nearly equal capacity to the initial curve Sealed nickel-cadmium cells have a high effective
at slightly reduced voltage levels. (These comments do capacitance. Their impedance is so low that cells
not apply to the CH1.8 and CH2.2 types. being, in effect, continuously overcharged make
Eveready sealed nickel-cadmium cells and batteries excellent ripple filters. Cell impedance is dependent
experience a relatively small charge of output capacity on frequency and state of charge of the cell. It is
over a wide range of operating temperature. Charging, lower for a charged cell than it is for a discharged
however, must be done in a much narrower tempera- cell. Values of impedance and resistance of Eveready
ture range. Temperature limits applicable to operation cells are shown on the individual specification sheets
of the batteries are listed in Table 51.26. for each cell (available from the manufacturer).
Cycle life of the nickel-cadmium sealed cell
depends both on cell design and on the type of use to
0, IlOr which it is subjected. Apart from violent abuse, the use
factors that most seriously influence life expectancy
550 --
are:
1. Amount of overcharge (excessive overcharge is
undesirable).
i,c 70 0 L 2. Temperature of charge and overcharge (elevated or
6
lowered temperature is undesirable).
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 3. End-point requirements regarding rate and capacity
Time {months) (increased cycle life will ordinarily be the result of
a shallow discharge regimen).
Figure 51.1 1 Capacity maintenance versus continuous over-
charge of an Eveready sealed nickel-cadmium CHI .2 cylindrical Any treatment that causes a cell to vent itself
cell discharged at IC (1.2A). First discharge every 3 months after
continuous overcharge (Courtesy of Union Carbide) is harmful. Frequent or extended venting of even