Page 216 - Uninterruptible Power Supplies

Batteries
214 Chapter Seven

Since the electrolyte is acting only as an ion exchange medium, it is
possible to increase the reservoir of electrolyte and thus reduce main-
tenance.
The plate separators consist of a microporous polymer/nonwoven
felt sandwich, designed to ensure complete plate separation and good
design life.

Sealed Type
Nickel-cadmium cells are available in both open-type and sealed ver-
sions. Their advantages are long life and ability to withstand repeated
cycling and also to work with a wider ambient temperature range ( 40
to 60°C). See Fig. 7.30. The main disadvantages are its high initial cost
and, compared to VRLA cells, size, and for the open-type nickel-cad-
mium, the need for a well-designed battery room. Advantages for the
open type are long life, which can be 20 years at normal ambient tem-
peratures (25°C), and its ability to withstand higher temperatures than
lead acid (see Fig. 7.31). Also, they will stand a fair amount of rough
treatment: Cells may be left discharged with no ill effect. Overcharging
or undercharging causes no effect whereas with lead acid cells there is
the danger of plates deforming. Short-circuiting of cells can be tolerated.

Charging of Cells
Charging of cells under most UPS conditions will occur at 1.41 to 1.45
VPC using a voltage-controlled current-limited circuit (see Fig. 7.32).
End of discharge occurs for most UPS applications at about 1.06 VPC.

Available Capacity
120%
110%
Sintered/plastic bonded Ni-Cd
100%
90%

80%
VRLA
70%
60%
Pocket plate Ni-Cd
50%
–40 –30 –20 –10 0 10 20 30 40 50 60
Temperature °C
Effect of temperature on performance
Figure 7.30 Effect of temperature on performance.

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