Page 581 - Battery Reference Book

Nickel-cadmium secondary batteries 51/21
capacity ranges; are 20 rnA h to 1 A h for the button cell The heat problems vary somewhat from one cell
and 15OmAh to 4A h for the cylindrical. type to another, but in most cases internal metal
Electrical arid physical characteristics of some of strip tab connectors overheat andor the electrolyte
the button and cylindrical nickel-cadmium cells and boils. General overheating is normally easy to pre-
batteries that comprise the basic Eveready range are vent because the outside temperature of the battery
listed in Table 51.24. All of these cells may, of course, can be used to indicate when rest, for coding, is
be assembled in series to make up batteries of various required. In terms of cut-off temperature during dis-
voltages. Table 51.24 shows the electrical and physical charge, it is acceptable to keep the battery always
characteristics of 159 cells and batteries in the Union below 65°C. The overheated internal connectors are
Carbide range. The items are arranged in ascend- difficult to detect. This form of overheating takes place
ing voltage categories varying from 1.2 to 14.4V. in a few seconds or less, and overall cell tempera-
Within any voltage category, batteries are arranged in ture may hardly be affected. It is thus advisable to
ascending order of capacity. Union Carbide also sup- withdraw no more ampere seconds per pulse, and to
ply assemblies of stacks of two to ten button cells withdraw it at no greater average current per com-
(Table 51.25). plete discharge, than recommended for the particular
All Evereadly high-rate cylindrical cells have a cell in question. In special cases, where cooling of
resealing pressare vent, except CH1.8 and CH2.2 the cell or battery is likely to be poor, or unusu-
(Table 51.24)$ which have a puncture-type failsafe ally good, special tests should be run to check the
venting mechamism. This vent permits the cell to important temperatures before any duty cycle adjust-
release excess gas evolved if the cell is abused, for ment is made.
example. When the internal pressure has dropped to Output capacity in any discharge composed of
an acceptable level, the vent will reseal, permitting the pulses is difficult to predict accurately because there
cell to be recycled in the normal manner with little or are an infinite number of combinations of current, 'on'
no further loss of electrolyte or capacity. time, rest time and end-voltage. Testing on a spe-
Eveready sealed nickel-cadmium cells and batter- cific cycle is the simplest way to obtain a positive
ies exhibit relatively constant discharge voltages. They answer.
can be recharged many times for long-lasting economi- Self-discharge characteristics of various types of
cal power. They are small convenient packages of high Eveready cell are illustrated in Figure 51.10, which
energy output, hermetically sealed in lead-resistant shows a decline in percentage of rated capacity avail-
steel cases, and will operate in any position. The cells able over a 20-week period. (For identification of cells
have very low internal resistance and impedance, and see Table 51.24.)
are rugged and highly resistant to shock and vibration. At elevated storage temperatures, self-discharge will
The temperature range under which these cells may be considerably higher than at room temperature.
be operated is wide. Use at high temperatures, how- Union Carbide recommend that batteries be stored at
ever, or charging at higher than recommended rates,
or repeated discharge beyond the normal cut-offs, may
be harmful. In the case of button cells, which do not 100
contain a safety vent, charging at temperatures lower 8,90
80
than those recommended may cause swelling or cell r-
'E 70
rupture. g,2 60
Sealed nickel-cadmium cells should not be charged 2 50
E.:
in parallel unless each cell or series string of the $ 40
30
parallel circuit has its own current-limiting resistor. a 20
Minor differences in internal resistance of the cells 10
12 14 16 18 20
8 10
may result, after cycling, in extreme variation in their 0 2 4 6 Time (weeks)
states of charge. This may lead to overcharge at exces-
sive currents in! some cells and undercharge in others. (a) CF and CH cells
Except in the case of complete discharge, neither cell
condition nor state of charge can be determined by
open-circuit voltage. Within a short time after charging
it may be above 1.4 V; it will fail shortly thereafter to
1.35 V and continue to drop as he cell loses charge.
High-rate nickel-cadmium cells will deliver exceed-
ingly high currents. If they are discharged continu- 0 2 4 6 8 10 12 14 16 18 20
ously under sh'ort-circuit conditions, self-heating may Time (weeks)
cause irreparab~le damage. If the output is withdrawn
in pulses spaced to limit the temperatures of a few (b) B and BH cells
critical areas in[ the cell to a safe figure, high currents Figure 51 .IO Charge retention at 21 "C of Eveready sealed nickel-
can be utilized. cadmium cells (Courtesy of Union Carbide)