Page 389 - Battery Reference Book

33/4 Nickel batteries

of an electronic flashgun, the battery is capable of reliability. The theoretical service life of this unit is
recharging the capacitor in a few seconds at a current 9 years.
of 10-20C5 A. Battery recharge takes 30min (rapid Examples of the application of nickel-cadmium
charge) or 3 h (accelerated charge). batteries in emergency power and lighting applications
SkyLab, the manned space vehicle developed include multistorey buildings, aviation safety, railway
by NASA, was powered by eight secondary vehicles, ships, tunnels, sports areas, shops and offices.
nickel-cadmium batteries (as well as 16 primary Nickel-cadmium batteries are also used for starting
or secondary silver-zinc batteries). Nickel-cadmium up diesel generators to supply electrical power in the
batteries are used to power the NATO111 communica- event of a mains failure (Figure 33.1).
tion satellite. A good example of a situation in which there is a
critical requirement for the very rapid availability of
Emergency supply emergency power is an operating theatre. Here, in the
event of a mains failure, the operating staff must have
In the last few years, the need for emergency sup- a system that restores light and power for instruments
plies during ax. supply failure has been considerably within a fraction of a second. Such systems are usually
extended in electronics and security equipment. Sealed based on the nickel-cadmium battery with associated
nickel-cadmium batteries are kept on constant charge electronic equipment. Another, lesser, example is a
when the a.c. supply is present and incorporated in bell-ringing tower. Losing control of all the bells at
utilization circuits, which must continue to function the same instant of time could have unwanted effects
during a.c. supply failure. Sealed nickel-cadmium bat- both on the music produced and the well-being of the
teries are particularly suited for these applications by ringers.
reason of their ability to accept overcharge, their free- An example in which the proper supply of emer-
dom from maintenance and their reliability. gency lighting and power is mandatory is an airport.
The principal applications in emergency supplies Traffic interruptions due to bad weather at airports is
are in electronics, emergency lighting, fire and burglar an increasing problem in today’s intensive air traffic.
detection, signalling and alarms, medical apparatus and Therefore, airports are equipped with different landing
surveillance systems (including defibrillators, resusci- aids such as runway lights, instrument landing systems
tation units, portable electrocardiographs and blood and radar. An interruption in the supply of electrical
transfusion pumps), switch tripping, logic memories, power to this equipment means that an approaching
control of switchgear, circuit breakers and signalling. aircraft will lose guidance to the runway and a critical
As an example of these applications consider the situation might develop. For this reason, emergency
case of a self-contained emergency lighting unit for a power must be available to use in case of interruption
building. The unit has a charge circuit which applies a of the normal power supply. The International Civil
permanent charge to the battery, at a rate of 0.05C5 A. Aviation Organization (ICAO) has made standards for
Complete charging of the battery is required in 24 h. equipment of airports to allow safe landing at different
When the a.c. supply fails the battery is connected visibilities.
to the lamps by a relay; when the ax. supply is These standards have different categories. The
re-established the lamps go out and the battery is lowest category is ‘Non-Instrument Runway’, where
recharged. 1.5 km visibility is required. The highest category is
Consider as a further example an electrical supply
‘Precision Approach Runway Category 111’ , intended
for the memory in a data system. In order not to lose for operations below 400 m runway visual range
data in case of as. supply failure, certain data sys- (visibility of the lights along the runway). The
tems are fitted with self-contained emergency power standards also determine the airport equipment, which
supplies for relatively low powers. Data systems are
generally supplied from the a.c. supply and are par- landing aids shall have emergency power and the
ticularly sensitive to very short breaks and transient maximum time for switching to emergency power.
voltage variations. To overcome the weaknesses of the The emergency power source can be a power station
completely independent of the airport’s normal power
a.c. supply, there are various types of equipment, in
particular inverters, supplied by battery-charger com- supply, motor generator sets or battery-powered
binations. The inverter derives its supply with a.c. inverters.
supply present from the rectifier and with ax. supply
absent from the battery. Category Z Most airports around the world intended
In these systems the battery serves also as a filter for international traffic are in Category I. This category
for commutating currents injected by the inverter when is intended for operations down to 60 m decision height
switching on or removing the load. For example, a (the height at which the pilot has to abort the approach
300 ms hold for a 5 kV A inverter with 110 V input can if not able to continue visually) and 800m runway
be obtained with a 4Ah battery (1lOVR 4D SAFT). visual range. During a power failure, the instrument
The peak current is 65 A (l6C5 A) and the battery is landing aids must have emergency power within 10 s
charged at a permanent rate of 0.02C5 A with excellent and the runway light within 15 s.