Page 467 - Handbook of Electrical Engineering

456 HANDBOOK OF ELECTRICAL ENGINEERING

If there is a total failure from the main supply then it will usually be necessary to maintain the
continuous current for 4 hours so that the state of the plant will be known during the failure. During
this time it would be expected that the main supply would be restored. Hence the 4 hours can be
used as the ‘operating cycle’ of the battery in the event that the charger is unable to supply current.
Batteries may be installed in several ways, e.g. integral with the charger, in a separate cubicle
or on open racks. The choice usually depends upon the physical size of the complete battery. Large
batteries are more suited to an open rack installation.

17.2.2.1 Worked example
Consider the following situation as an example.

A switchboard consists of 20 circuit breakers. Each circuit breaker has, two indicator lamps
each taking 1 amp continuously, a tripping solenoid taking 5 amps for one second, and a spring
charging motor for reclosing which takes 3 amps for 30 seconds. The battery needs to supply current
for 4 hours when a mains failure occurs. The ampere-hour (AH) duty is:-

• Lamps 20 × 2 × 1 × 4 = 160 AH
20 × 5 × 1
• Tripping = 0.03 AH
3600
20 × 3 × 30
• Spring charging = 0.5AH
3600
• Contingency typically 15% = 24.08 AH
• Total capacity = 184.61 AH rounded up to 185 AH

The contingency allows for the battery being in a partial state of charge before the loss of
supply. The rated AH capacity and voltage are now known. Reference 3 gives other examples plus
a general description of battery charging principles.
In recent years there has been a tendency to prefer Pb cells instead of NiCd cells. This has been
due to the development of what has become known as ‘maintenance free’ or ‘sealed type’ lead-acid
batteries. The basic concept is one of retaining the gases evolved during the charging process and to
allow the oxygen to recombine as float charging takes place, see Reference 4. If the operating and
ambient conditions are not subject to excessive variation then the concept is satisfactory in practice
and the life expectancy of the battery can be as much as 10 years.
If too much gas is evolved and is released through a special safety valve than the life expectancy
will be reduced. The amount of gas evolved is a function of the float charging current level and the
ambient temperature. The temperature of the electrolyte will be a function of the ambient tempera-
ture of the air surrounding the battery. Therefore a high float charging current and a high ambient
temperature will cause the life expectancy to fall. If the ambient temperature has an average value
◦
◦
of 30 C then the life expectancy will be halved, and at 40 C reduced to a quarter, i.e. 2 to 3 years
instead of 10.
In practice it is therefore essential to ensure that the temperature within the battery room or
◦
cabinet remains reasonably constant and as close to 25 C as possible, the lower the temperature