Page 119 - Battery Reference Book

Nickel-cadmium secondary batteries 4/3
4.1 Nickel-cadmium secondary Discharge current
batteries inrensity in % 1,
10h 53
The nickel-cadmium battery is mechanically rugged 5h 100
2.8
and long lived. In addition, it has excellent low- 2.8 r 4h 121 1
temperature characteristics and can be hermetically 2.6 7 7
2.6
5
sealed. Cost, Ihowever, is higher than for either the 2.4 5
2.4
lead-acid or tlhe nickel-zinc battery and, by compar- I -0
-0
> >
I
10 h
ison, its capacity on light drain in terms of watt hours - 10 h
- 2.2
2.2
5h
5h
lu
per kilogram is also poorer than for nickel-zinc. lu 2.0 4h
2.0
4h
Cells using the nickel-cadmium couple can be 3h
3h
divided into two main groups: cells having thick plates 1.8
in which the active material is compressed into finely 1.6 I I I I I I I I I I I
perforated metal sheets in the form of tubes or pock- 0 10 20 30 40 50 60 70 80 90 100 110
% c,
ets, and cells with thin sintered plates in which the
active material: is deposited in porous metal supports. Figure 4.1 Discharge curves showing dependence on capacity
The latter type has a very low internal resistance available at 20°C of a Varta double cell DTN battery (Courtesy of
and a high lo'ad capacity which result in improved Varta)
power-to-volume and power-to-weight ratios. The bat-
teries are manufactured as open or semi-open type self-discharge rate are of advantage. Discharge curves
cells in which the electrolysis gas escapes through a for a DTN battery are shown in Figure 4.1.
vent (which require periodic topping up with water to Chloride Alkad supply their Unibloc range of alkal-
replace the electrolyte) and as fully sealed batteries ine batteries, which are virtually maintenance free but
which are designed in such a way that gas evolu- are not sealed. These range from 7.5 to 27 Ah capacity
tion does not occur and consequently topping up of (Table 4.2). Unlike traditional alkaline batteries, which
the electrolyte, with water is not required. In gen- are built up from individual separately packaged cells,
eral: the open or semi-open types of battery are in Unibloc starts off as a five 1.2 V cell battery in a lidded
the higher capacity range, used for applications such container (Figure 4.2). The Unibloc range is available
as traction and large emergency power installations. in four basic capacities, each providing low-rate dis-
Sealed nickel-cadmium batteries that are usually in charge performance:
the lower capacity range (up to about 30 Ah) are the
7.5Ah
type that find extensive application in the electronics, 12.5Ah Type 5LP7
Type 5LP12
small appliances, domestic products, defence equip- 15Ah Type 5LPl5
ment and space research markets. 27Ah Type 5LP27
The five cells are incorporated into a resilient plastics
4.1.1 Open :and semi-open, low-maintenance, case containing sufficient extra potassium hydroxide
unsealed nickel - cadmium batteries electrolyte to enable periodic topping up of the battery
to be extended up to a maximum of 20 years on a con-
Various manufacturers supply small low-maintenance stant 1.42Vfcell float charge without any boost char-
cells and batteries. Varta supply a non-sealed double ging, although 7- 10 years will probably be the average.
cell (DTN) for portable lamps. This battery is avail- Unibloc batteries are recommended for applications
able in the capacity range 4.5-11 Ah (Table 4.1); it is such as switch tripping, emergency lighting and fire
recommended for applications such as portable lamps alarm operation.
where its light weight, even voltage level, good capa- For most applications, constant-voltage charging is
city, long service iife, lack of maintenance and low recommended to minimize electrolyte loss which is
Table 4.1 Varta DTN range of low-maintenance nickel-cadmium batteries
Charging Type Nominal Nominal Median Nominal Charging Filled Quantity of
Battery No.333 capacity discharge discharge charging voltage weight electrolyte
tYPe in 5h current voltage current (V) (kg) (dm3)
(Ah) in 5h (A) (V) in 7 h (A)
DTN4.5K 2452010 4.5 0.9 2.4 0.9 0.42 0.08
DTN6.5K :!652010 6.5 1.3 2.4 1.3 increasing 0.61 0.11
DTN7K 2702010 7.0 1.4 2.4 1.4 from 2.7 0.75 0.14
DTN12K 3122020 11.0 2.2 2.4 2.2 to 3.6 0.90 0.20