Page 554 - Battery Reference Book

Nickel-cadmium secondary batteries 51/3

51 -1 Nickel-cadmium secondary the same dimensions and capacities as the standard
batteries VB cells and are designed to withstand more severe
operating conditions. They have an improved voltage
51.1.1 Sealed nickel-cadmium batteries characteristic at low temperatures and high discharge
rates and higher mechanical strength and reliability.
SAFT (SociCtC des Accumulateurs Fixes et de Trac-
There are five sizes of VB cell from 40 to 600 mA h
tion), a major inanufacturer of alkaline batteries, pro- available in VB and VBE versions (see Table 51.1).
duce a full range of sealed nickel-cadmium batter- VB button cells have a nominal discharge voltage of
ies, including 'open type (GPX) and semi-open type
(VOK-VOKH-VHP), both of which are higher capa- 1.2Vkell. The rated capacity C5 (Table 51.1) at 20°C
city types. SAFT also produce sealed cells and batter- is defined as the capacity in ampere hours obtained at
ies and button cells (VB, VR and VY types), which the 5 h rate of discharge (0.2C5 A) to an end-voltage of
are discussed below. 1.10 Vkell after a full charge at 0.1 C5 A for 14 h. Rates
Details of the main types of sealed cell and battery of charge and discharge are expressed in multiples of
manufactured by SAFT are given in Table 51.1. the rated capacity C5, thus 0.2C5 A represents 20 mA
The SAFT range consists of button and cylindrical for a cell having a rated capacity, C5, of l00mAh.
types and the VR series. The button cells have very 5C5 A represents 500 mA for the same cell.
thin sintered positive and negative plates with a porous The excellent temperature characteristic of the dis-
separator enclosed by two nickel-plated steel cups. charge of VB cells is illustrated in Figure 5 1.1.
Capacities are up to SOQmAh at 1.2V for a size The life of VB and VBE cells (Table 51.1) can
approximately 35mm diameter x l0mm thick with a extend over 10 years, but this is dependent on oper-
weight of 100 g. Plastics-sleeved packs of button cells ating conditions and is principally influenced by the
are available connected in series to give up to 12V. depth of discharge (discharged capacitykated capacity)
Similar metal-clad packs are also available. in a cycling application and by the number of ampere
The cylindrical batteries are designed for a wide hours overcharged in a continuous charge application.
range of applications requiring high capacity and The great reliability of VB and VBE cells makes their
exceptionally long life. Cells are dimensionally sim- life comparable with that of most electronics com-
ilar to the principai conventional dry batteries. The ponents, During cycling approximately 500 cycles are
sintered plate construction consists of a spirally wound obtained with 100% depth of discharge. After 5 years'
plate assembly, which is tightly fitted into a steel con- continuous charge at 0.005Cs mA, the failure rate for
tainer. All batteries have a special venting system, with VB cells is not greater than 5 per 1000. Under the same
automatic resealing should excess gas pressure occur. conditions, VBE cells offer ten times this reliability: a
The design satisfies the requirements of BS 3456: failure rate of 5 per 10000.
Section 2.36: 1973 on all points, including safety on A typical discharge curve for a VB button cell is
reverse charge. Capacities are up to 1OAh at 1.2V given in Figure 51.2 (continuous discharge).
for a size of 41.7 mm diameter x 90 mm height with VB batteries are also available in standard plastics-
a weight of 400 g. Series connected, plastics-coated
modular packs are available to give up to 12V at the cased button packs made of n identical VB button
cells, where n is between 2 and 24, each of rated
above capacity.
The SAFT VR series of cells have been produced capacity C5 mA h, connected in series and assembled
for applications requiring rapid charge and discharge. in a rigid case, to make packs with discharge voltages
Up to 130A pleak discharge can be tolerated and the between 2.4 V (2 cells) and 28.8 V (24 cells). Packs
rapid charge time on a discharged battery can be as are available made up from VB 4, 10, 22, 30 and 60
low as 15-60niin when using the appropriate charging cells (capacity 40, 100, 220. 300 and 600C5 mAh, see
equipment, which monitors cell temperature, voltage
or time of charge. The maximum capacity available in
this range at the moment is 10 Ah. For uncontrolled L
rapid charge, S,AFT have develloped the VY range of Q: 100 0.1 c, A
E
cells. 80 c5 A
r
SAFT E 60
VB button cells These cells have the form of a button
in various diameters and thicknesses, made up of a 3 20
0
0
.;
stack of disc-shaped sintered plates and separators held 40L -20 -10 0 10 20 30 40 50
in two nickel-plated steel cups, one fitting into the
other and pressed together with an insulating gasket. Discharge temperature ("C)
There is also a high-reliability version of the button Figure 51.1 Influence of temperature on capacity of SAFT sealed
cell with improved performance on high-rate discharge nickel-cadmium sintered plate VB type button cells (Courtesy of
at very low temperatures (VBE cells). These have SAW