Page 123 - Battery Reference Book

Nickel-cadmium secondary batteries 4/7

nominal capacity. Partial discharging will significantly
increase the number of cycles achieved. Similarly,
discharging at high rates will reduce the expected life
of the battery.
Sintered electrode batteries supplied by Varua. when
continuously trickle charged at rates between one-
fifth and one-half of the 10h rate, will give, typi-
cally, a life of approximately 5 years to 60-70% of
nominal capacity.
When a fast charge is applied to a sintered cell
battery at a maximum of 20 x 110, provided the charge
is terminated at 80% capacity. a life can be achieved
similar to that of a sintered cell battery at normal rates
of charge.

Energy density
The energy contents of the various types of sealed
nickel-cadmium battery supplied by one particular
manufacturer are as shown in Table 4.4.
Number of cycles (X IO3)
Figure 4.5 Estimated life cycle at 20°C as a function of discharge Table 4.4 Energy contents of various sealed nickel-cadmium
(SAFT sealed nickel-cadmium batteries) (Courtesy of SAW batteries
Volumetric Gravimetric
over several hours; failure rate also of the order of energy energy
10-' per cycle with permanent maintenance charge at densig density
rates below 0.05C5 A. (WWdm3) (Wh/kg)
During the service life of sealed nickel-cadmium
batteries obtained from a reputable manufacturer, there Button cell (mass plate) 61-62 21-22
are practically no failures through loss of performance, Round cell (mass plate) 69 22
and the mortality law is very pirobably of an expo- Round cell (sintered plate) 78 21
nential nature, under the following types of service Rectangular (mass plate) 37 18
Rectangular (sintered plate)
52
21
condition:
1. 500 cycles with charge for 14h at O.1CsA and
complete discharge over several hours. Cylindrical, rectangular and button designs of sealed
2. 1000 cycles with the same type of charge but nickel-cadmium battery are available from most sup-
discharge 50% of capacity over several hours. pliers. These three types are available with compressed
3. At least 1500 cycles with rapid charge at 2CsA and with sintered electrodes. Batteries with sintered
and discharge over several hours; the failure rate in electrodes have a very low internal resistance and
this case is tgpically of the order of failures a high load capacity which results in an improved
per cycle. power to size and weight ratio. These three types of
4. At 2000CsAh with overcharge at a rate below nickel-cadmium cell are discussed in detail in Part 2.
0.05C5 A, the maximum service life is 8 years (at Generally speaking, nickel-cadmium button cells
a rate of 0.02Cs A): 200OCs Ah corresponds to are available in a lower capacity range than cylindrical
100 000 h or a theoretical service life of 11 years. cells. Thus one manufacturer supplies button cells in
the 20-l000mAh range and cylindrical cells in the
As a further example of the life expectancy of 150-4000mAh range, Voltages of between 1.2V and
sealed nickel-cadmium batteries Varta claim that 14.4V (12 cell) are available.
their products are not affected by prolonged storage The characteristics (including discharge characteris-
provided the recommended storage temperatures (-40 tics, storage, battery life, safety and battery testing) of
to +50"C) are not exceeded. Thus less than 20% sealed nickel-cadmium batteries have been discussed
capacity loss occurs in 2 years storage at -2O"C, further in a book published by Gates Energy Products
approximately 40% in 10 or 12 months at 20"C, and (see Bibliography).
proportionally higher rated capacity loss at higher
temperatures. In applications of these cells where 4.1.3 Nickel-cadmium button cells
they are operated in a purely cyclic mode (that is,
chargeldischarge cycles), mass plate batteries will give Nickel-cadmium button cells have a nominal
300 cycles at the 10h rate to approximately 80% of discharge voltage of 1.2 V per cell. The rated capacity