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:

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:

6.0 *kW*

=

= 6.52 *kW*

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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