Page 240 - Battery Reference Book

19/6 Nickel batteries

modular packs (12 and 24V, Table 51.1), both in a
range of sizes. Standard packs provide for applications
requiring high performance on discharge at large cur-
rents either continuously or in short-duration peaks.
The extended range modular packs are supplied in
a plastics case in the range 0.5-10Ah at 12V and
24V. (Rated capacity is for discharge at the 5 h
rate at 20°C after nominal charge and for a termi-
nal voltage of l.OV/cell.) The packs can be dis-
charged steadily at up to the rated capacity and can
be given a nominal charge at a constant current of
0.1Cj for 14h irrespective of their initial state of
Bottom insert ' charge. Alternatively, the packs can be given a fast
charge. They will withstand a continuous charge of up
Figure 19.5 Ever Ready (Berec) sealed nickel-cadmium button to 0.05C5 and will operate in the temperature range
cell construction (Courtesy of Ever Ready (Berec)) -40 to +50"C. They may be stored indefinitely in any
state of charge without deterioration in the above tem-
Button-type cells consist of a cup (positive pole) perature range.
and a battery cover (negative pole); the electrodes are The design of Union Carbide Eveready high-rate
tablets wrapped in a microporous nickel or nickel- cylindrical nickel-cadmium cells features a pressed
plated wire gauze and separated by a fine gauze powder negative electrode, which consists of dry
separator, which is frequently made of non-woven blended active materials pressed into an expanded
polyamide. The capacity of this type is up to about metal carrier. This is claimed to give the cell outstand-
3 Ah at a voltage of 1.24 V. Typically the size of the ing cycle life and long-term overcharge capability with
battery for this capacity is 50 mm diameter x 25 mm essentially no fade and with little or no memory effect.
thick, with a weight of 135g, and permissible oper- A number of the Eveready cylindrical cells feature the
ating temperatures of -20 to +45"C. These cells are 'jelly roll' construction illustrated in Figure 19.7.
also available in welded and plastics-covered packs of
up to ten cells. SAFT, for example, supply five sizes of
button cell from 40 to 600 mA h in their standard (VB)
and improved performance (VBE) series. Button cells
can be assembled in a rigid case to make packs with
discharge voltages between 2.4V (2 cells) and 28.8V
(24 cells). SAFT supply packs made up from VM 4,
10, 22, 30 and 60 cells (capacity 40, 100, 220, 300 and
600Cj mA, Table 51.1). The final assemblies have, of
course, the same rated C5 capacity as the individual
cells but vary in discharge voltage between 2.4V (2
cells) and 28.8V (24 cells).
Varta and other suppliers produce assemblies of two
to ten button cells. Assemblies are available in stock
formats, including insulating sleeve, plastic cassettes,
plastic boxes and sheet steel cases. Also available are
environment-proof assemblies in which the intercon-
nected cells are resin potted in a plastics container.
An advantage of button cell packs is that these can
operate as very low compact low-voltage capacitors
giving much greater energy storage in a much smaller
space. VB button cell packs can deliver very high cur-
rents, the maximum discharge rate being l0Cs mA. It
should be ensured that the packs are not discharged to
below 1 V/cell. Short-duration peak discharges may be
as high as 50-100Cs mA at 0.8 V/cell.

Cylindrical cells
A cutaway diagram of a typical cylindrical cell is
shown in Figure 19.6. Cylindrical cells can be obtained Figure 19.6 Ever Ready (Berec) sealed cylindrical nickel-cad-
in standard battery packs (6-12 V) and extended range mium cell construction (Courtesy of Ever Ready (Berec))