Page 211 - Uninterruptible Power Supplies

Batteries
Batteries 209

devices are now available but at some cost, and there has now devel-
oped two schools of thought:

1. Use the existing simple built-in battery monitoring system in the
UPS and replace the cells say every 5 to 6 years for a VRLA battery
with a design life of 10 years.
2. Install a complete monitoring system, maintain the system replac-
ing cells as and, when required, obtain a longer battery life.
The disadvantage of the former is that rogue cells may not be noticed,
and such cells will induce weaknesses in their immediate neighbors.
The advantage of course is cost!
Thus, a range of battery monitors is now available which measure
continuously individual cell/blocks for voltage, impedance, current, and
temperature variations. Such systems are sophisticated and require
consideration. It should be noted that IEC is preparing a specification
for such devices under the title IEC 62060.
Monitors usually take the form of a wall-mounted receiving station
to which are connected small signal wires from each cell block. These
can take the form of a fuse-protected wire or a glass-fiber cable. There
is an alternative available which runs the test wire in series through
the entire battery, thus eliminating multicabling. The system mea-
sures current, voltage, impedance, temperature, and variations during
normal float and discharge and compares such results with known
results for a healthy battery irrespective of battery age. The figures are
computed to form a trend of cell performance enabling action to be
taken to forestall any failures. The system may also be interrogated by
a normal office PC or via modem telephone link to a central mainte-
nance area. (See Figs. 7.23 to 7.27.)

Nickel-Cadmium Cells
Chemical Reactions
Nickel-cadmium batteries work under the chemical equation as shown
below:
2NiOOH 2H 2 O Cd discharge → charge 2Ni (OH) Cd (OH)
←
The positive plate uses nickel hydroxide as the active material, and
the negative plate uses cadmium hydroxide.
The cell utilizes as an electrolyte a solution of potassium hydroxide
with a small amount of lithium hydroxide to improve cyclic and high-
temperature performance. The solution is used purely as a method of
ion transfer and no chemical change to the solution occurs. The support
structure for the active material is steel which is unaffected by the
chemical activity within the cell.

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