Page 195 - Uninterruptible Power Supplies

Batteries
Batteries 193

was to reduce size, minimize gas evolution, and reduce water usage.
The use of such cells for UPS applications came after this event, and
the desire for very low maintenance was not really considered as a high
priority.
The end result is a cell enclosed with a pressure valve, ensuring that
there is a slight gas pressure. The issue of recombination of such evolved
gases when the electrodes are surrounded with electrolyte was addressed
by using a special very fine fiberglass mat [Note: This is often referred to
as absorbent glass mat (AGM).] into which the electrolyte was soaked.
The amount of electrolyte allowed in the cell is strictly limited, that is, no
residual electrolyte can be seen or heard. During cell formation paths are
created through the electrolyte held in the AGM to allow gas (oxygen) to
migrate from the positive electrode to the negative electrode, where it
recombines with hydrogen to form water.
The VRLA cell is attractive due to its size, some 50 percent smaller
than Plante; it requires no special environment, that is, a battery room
with room surfaces constructed to be protected against acid attack and
also special ventilation to ensure gases evolved are evacuated; and its
capital cost is some 50 percent less than Plante. In addition, since the
cells are sealed there is little maintenance, that is, no checking of elec-
trolyte levels is required. It does, however, have a shorter design life.
This design life is usually 5, 10, or 15 years depending on construction
and, of course, operating conditions including temperature and cycling.
The cells are usually mounted vertically although there are some
designs now available which can be operated horizontally, with front-
mounted terminals, which can simplify stacking and, hence, save
space.
VRLA clearly has the same chemical reactions as mentioned above
and its construction is somewhat different than the flooded cells. A typ-
ical example is shown in Fig. 7.8. In fact the series of plates (see Fig. 7.9)
are pasted with lead oxide (PbO)/dilute sulphuric acid which is then
converted to lead dioxide as the cell is formed. As stated previously,
there is no excess electrolyte held within the AGM (i.e., no residual liq-
uid can be visually seen). For this reason the VRLA cell is often referred
to as working on the starved electrolyte principle.The whole cell is
sealed and a small valve is placed at the top of the cell to allow for any
gas buildup to be expelled, the venting of gas under certain conditions
occurring at above 1 atmosphere pressure. Note that this is not likely
to occur except under fault conditions. Under normal operation the
pressure in the cell is likely to be 3 lb per square inch (20684 Pa).
As previously mentioned there has been, for many years, a simple
method to recombine the gases evolved in the chemical reaction, it
involves the use of a catalyst to recombine the hydrogen and oxygen.
Until recently this had only been employed on flooded-type cells, this

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