Page 222 - Uninterruptible Power Supplies

Batteries
220 Chapter Seven

The negative plate is better able to accept charge and hydrogen pro-
duction only occurs when the plate is fully charged. So the cell is con-
structed with a larger than normal negative plate. Thus, oxygen is
produced prior to the production of hydrogen. This oxygen is then trapped
in porous separators and eventually displaces some electrolyte in the sep-
arators. This eventually reaches the negative plate where it reacts chem-
ically as follows:

2Cd O 2 2H 2 O 2Cd (OH) 2
or electrochemically

O 2 2H 2 O 4E 4OH
Thus, some of the cadmium on the plate is discharged by the reaction
to cadmium hydroxide. This is reduced to cadmium by the recharging
activity within the cell. The production of gas is low and the small amount
of gas evolved (mostly hydrogen) is vented via a low-pressure reclosing
flame-arresting vent, operating pressure 0.2 bar. Gas evolution is limited,
recombination levels being between 85 and 95 percent. Typically, 3.5 cc of
gas per A/h per cell per day of which 2.5 cc are hydrogen.

Comparison of Various Types of Cells
A simple comparison of battery types used for a UPS system is detailed
below.
Prices are rated against Plante at 100 percent (which should give a
20-year life). All VRLA cells are rated at 10-year design life, the excep-
tion being the nickel-cadmium sealed cell, which should give a 20-year
life. Rating is 26.6 kW, autonomy period 15 min, float voltage is 409 V,
end of discharge is 306 V, or 180 lead acid cells float 2.27 VPC to end of
discharge 1.7 VPC. Ambient temperature 20°C.

Total Total Total
cell Number of cells Dimension (mm) weight volume,
Cell type price or blocks L D H kg m 3

Plante 100% 180 cells each 190 133 212 2061 0.96
VRLA
(AGM type) 23.8 30 12 V each 350 166 174 690 0.30
VRLA
(AGM pure lead) 29.0 30 12 V each 197 169 173 477 0.17
VRLA
(gel type) 27.4 30 12 V each 234 169 190 660 0.23
Nickel-cadmium,
sintered plate 319.0 282 cells each 86 86 276 761 0.58
Nickel-cadmium,
sealed 348.0 141 3.6 V each 145 195 406 2608 1.62

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