How to Size a Switchgear Battery System
Sizing a switchgear battery system is typically done with automated computer software, but it's possible to calculate it by hand using the peak discharge rate and the total amp-hour rate. Battery manufacturers provide tables describing the performance of battery models under various scenarios. The momentary peak load requirement of the switchgear battery bank is the primary factor affecting which battery cell type you'll need.
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Instructions
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Open your battery manufacturer's substation loads table and select the peak discharge rate applicable to your circumstances. For example, suppose a 75kV substation loads table lists a peak discharge rate of 90 amps for a lead acid battery operating a load for 10 hours at 125 Volts Direct Current, stored in a building with a constant temperature of 77 Fahrenheit.
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Calculate the 1-minute discharge rate by dividing the peak discharge rate by the ambient temperature factor and dividing again by the design margin. For example, a peak discharge rate of 90 amps, in a 77 Fahrenheit room with an ambient factor of 1, with a 0.75 battery-aging factor and a 0.9 design margin, will have a 1-minute discharge rate of:
90 Amperes / 1 / 0.75 / 0.9 = 133.33 Amperes
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3
Open your battery manufacturer's table and select the total Ampere-hour rate for your scenario. For example, suppose the total Ampere-hour rate for the above example is 30 Ampere-hours.
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Calculate the actual total Ampere-hour rate by dividing the table value by your ambient temperature factor and dividing again by the battery-aging factor. For example, the above example has an actual total Ampere-hour rate of:
30 Ampere-hours / 1 / 0.75 = 40 Ampere-hours
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Open the battery manufacturer's table and select the battery cell type which meets the worst-case condition between the 1-minute discharge rate and the actual total Ampere-hour rate. For example, suppose a Model C battery has a 1-minute discharge rate of 150 Amperes and an 8-hour Ampere-hour rate of 200 Ampere-hours for lead acid cells at 1.75 volts per cell. These specifications exceed the previous example's 1-minute discharge rate of 133.33 Amperes and 8-hour Ampere-hour rate of 40 Ampere-hour.
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Tips & Warnings
Nickel cadmium cells use a final voltage of 1.14 volts per cell and lead acid cells use 1.75 volts per cell.
