UFC 3 -520-01
June 10, 2002
E-2.1.3 Determine the minimum and maximum system voltage. Select the number of
cells based on the minimum and maximum voltage limits.
E-2.1.4 Calculate the load on each cell and select a cell size capable of supplying the
required load.
E-2.2
As a battery discharges, the battery voltage declines in a predictable manner.
For a constant power discharge, the current will increase in direct proportion to the
voltage decrease in accordance with the following relationship:
Power = Voltage Current
EXAMPLE: The easiest way to discuss battery sizing for a UPS application is
with an example. Assume the UPS specifications applicable to the battery are as
follows:
Size: 7.5 kVA @ 0.8 power factor (6.0 kW)
Inverter efficiency: 0.92 at full load
Maximum input dc voltage: 140 volts
Low voltage cutout: 105 volts
The user specifies that the UPS must power critical loads for a minimum of 30
minutes following a loss of normal power. The user believes that the UPS will be
almost fully loaded for the entire discharge dur ation.
First, calculate the total battery load. Assume the UPS is fully loaded at 7.5 kVA.
The power required from the battery is the real power produced by the UPS
including efficiency losses. The real power produced by the inverter is 6.0 kW
(7.5 kVA x 0.8 PF). Thus, the required battery load is:
Power
6.0 kW
Battery Load (kW ) =
=
= 6.52 kW
Efficiency
0.92
This is the nominal load that the battery must be capable of providing when the
battery has 100 percent capacity at 25 C (77 F). As discussed previously, the
battery will have less than rated capacity if the temperature is less than 25 C
(77 F). Also, a lead-acid battery is normally sized to be capable of fulfilling its
design load requirements at end of battery life (80 percent capacity). Apply the
appropriate correction fa ctors to ensure the battery can meet this load
requirement at end of battery life at the lowest expected temperature. The
correction factors are as follows:
Aging: 1.25 (corresponding to 80 percent capacity)
Temperature: 1.11 (assume lowest exp ected temperature is 15.6 C or 60 F)
The corrected battery load is:
E-5
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