Page 208 - Uninterruptible Power Supplies

Batteries
206 Chapter Seven

Characteristic Hard SAN Polycar- PVC ABS Polypro-
Rubber bonate Plyene
Durability Excellent Excellent Excellent Excellent Excellent
Rigidity Excellent Excellent Excellent
Impact Excellent Excellent
Resistance
Typical Case to Epoxy Epoxy Epoxy Epoxy Thermal Weld
Cover Bond
Typical
Terminal Seal
Transparent Yes Yes Yes No No
Flame No Yes Yes Optional Optional
Retarding
Gasses
Evolved when
Burned
Vented Cells X X X X
VRLA Cells X X
Typical 20 yr. Stat. 20 yr. Stat. 20 yr. Stat. 20 yr. Stat. SLI
Applications 5-10 Yr. Traction
Stat. 5-10 Yr. Stat.
Figure 7.22

facture the finishing charge evolves excess water, and due to this
process fissures are established in the gel between the plates. It is
through these fissures that the oxygen percolates during charging,
traveling from the positive to negative plate. Since silica is added to the
electrolyte it is clear there is slightly less acid available, thus lower
capacity. However, the gelled electrolyte has nearly 100 percent contact
with the plates and the walls of the cell, thus good thermal conductiv-
ity is a result. Thermal run-away susceptibility is reduced and such
cells are better suited to high ambient temperature areas. The addition
of phosphoric acid to the electrolyte will double the cycle life capabili-
ties but give a reduced float service life.
Development of the VRLA type of cell continues. For example, new
methods are being used to create the plate: Instead of being cast,
manufacturers are attempting to form plates from a strip of lead,
punching out sections, thus providing a continuous strip which may
be cut to the required length. The result appears similar to Fig. 7.9.
The resultant plate is then stretched slightly and the lead-oxide paste
mixture added. The aim is to produce a 5-min autonomy period cell,
and this probably will be created by making plates thinner and
increasing the porosity of the active plate material. Porosity increase
will allow a better availability of the material to the electrolyte. There
is also a demand for cells with increased life, thus reducing mainte-
nance costs. The aim here is for a cell with a good 10 year life at 27
to 29°C (80 to 83°F).

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