Page 207 - Uninterruptible Power Supplies

Batteries
Batteries 205

Calcium is added to the plate in small quantities, about 0.8 percent
for the same reason, that is, to strengthen the plate during its manu-
facturing process. Higher percentages of calcium have undesirable
effects, such as corrosion in the form of grid growth. And in many cases
this has itself been superseded by an alloy incorporating the use of tin,
again in small quantities (0.6 percent). This alloy increases the grain
size of the metal, the larger the grain size the lower the corrosion at the
positive plate.
Very small amounts of silver added to the alloy have the effect of
reducing resistance thus increasing the cell’s ability to give a high rate
discharge. Clearly, such amounts are very small due to cost.
The charging of cells has, until recently, been based on a float charge
system, usually based on 2.27 V per cell (VPC) with current limit. By
and large, UPS systems employ constant voltage with current limit cir-
cuits. But it has now been proved that short periods of charge followed
by longer periods of open circuit with no charge are beneficial to cell
life. The advantages are a reduction in corrosion of the plates, which
occurs during charging, and loss of electrolyte. Claims of 30 to 50 per-
cent increase in life are made.
Plastic containers vary in material used depending on application
(Fig. 7.22), ABS (acrylonitrile-butadiene-styrene) being the favored
material to meet the various flameproof standards such as BS 6290 and
UL94V-0. There are, however, other flameproof materials available such
as some grades of polypropylene which are approaching the same flame-
proof standards but at a lower cost.
In the construction of the battery block, it should be noted that most
applications require the battery to provide power for a relatively short
time (e.g., 15-min autonomy period). Clearly, the plates and, indeed,
the collection of power from the plates through the post thus have to
provide a low resistance path. In practice the post, on a full power dis-
charge, can generate heat and in some circumstances it may be prefer-
able to fit a brass insert within the post to reduce resistivity, to provide
a robust connection point, and to lower temperature. See Fig. 7.23a.
In addition to the above AGM variety of VRLA, a variation employ-
ing pure lead plates is available. This cell operates at a slightly higher
pressure, and since pure lead is employed corrosion is reduced, charg-
ing rate can be dramatically improved, and its temperature tolerance
is improved (e.g., at 30°C design life is approximately 7 years). The
cycling, that is, the number of charging/discharging events, is also
improved, especially for the periodic, small discharges incurred on
most UPS applications.
In addition to the VRLA cells described above there is the gel-type
VRLA cell which differs in construction. The glass mat is replaced by a
fumed silica thickening agent in the electrolyte. The mix, in liquid
form, is poured into the cell. This substance then gels. During manu-

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