Page 390 - Battery Reference Book

Nickel-cadmium secondary batteries 3315

Figure 33.1 Nickel-cadmium battery for starting up diesel generators for emergency lighting (Courtesy of Nife Jungner)

Category ZZ Increasingly, intensive air traffic has cre- For example, the case of an auxiliary supply for
ated a demand to improve regularity (to minimize the direction indicators on a lightweight motor cycle illus-
time when an airport is closed due to bad weather). trates the possibilities of using uncontrolled rapid
The less time an airport is closed, the smaller are the charge with cylindrical sealed nickel-cadmium bat-
losses in landing and starting fees. It also gives nego- teries. When the generator on the motor cycle is a
tiating power to attract flights, since companies will magdyno and when the engine speed is low, an aux-
select airports that are also available in bad weather. iliary supply is necessary to supply the indicators. In
For this reason, there is now a trend to improve inter- this application only the charge presents a problem
national airports to Category 11. This is intended for because the output of the magdyno depends on the
operations down to 30 m decision height and 400 m engine speed and whether lights are on or not. In some
runway visual range. machines the maximum charge current available may
Some airport instruments require completely unin- vary from lOmA to 1 A. The use of a battery with a
terrupted power supply while other instruments and capacity of 0.5 Ah gives a perfect solution because it
runway lights must be able to switch to emergency can withstand overcharge at 2C5 A (1 A).
power in a maximum of 1 s. The power demand of the Another example of using sealed nickel-cadmium
systems requiring completely uninterrupted power is batteries as an auxiliary power source concerns the
not so high, and is normally arranged by direct feeding carrier wave equipment that permits the connection of
from batteries. It is more difficult to arrange an emer- a second telephone subscriber when only one line is
gency supply for the runway lights, which normally available. The signal is modulated at the exchange,
require hundreds of kilowatts. transmitted on the single line and demodulated at the
receiver. The battery is kept on permanent charge for
Auxiliary supply the telephone lines at a current of about 0.02C5A.
When the user is called, the battery supplies the energy
When the power required by a consuming device necessary for the bell and the transmission. The daily
exceeds the output of the source, a local battery is load is small, a few milliampere hours, the source
necessary. Nickel-cadmium sintered plate sealed bat- being a VBE type SAFT battery. A failure rate per
teries, by their performance on discharge at high rates, operational hour of lo-* would be typical for this type
have the perfect attributes for use to provide auxiliary of SAFT battery.
supplies.
The battery plays a similar role to that of a capacitor Vehicle starting and high-power supplies
used to store energy, but with the difference that it
can discharge for considerably longer periods. Except Nickel-cadmium sealed batteries are particularly well
during these periods of discharge, the battery is left on suited to starting small engines and high powers.
permanent charge. Because of their very low internal resistance they can