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Sizing battery banks for switchgear and control applications is commonly performed using software designed specifically for that purpose. Just input the required load profile, and the program selects the optimum battery configuration. Although this is quite simple, an engineer should be capable of performing a straightforward hand calculation - either to confirm the results of a software-generated solution or to serve as an accurate design for a simple battery system.
Do you know how to perform these calculations by hand? If not, then read on. This article will help you understand the basic premise of sizing switchgear battery systems and provide an example calculation for clarification of the concept. IEEE Standard 485-1997, "Recommended Practice for Sizing Lead-Acid Batteries for Stationary Applications," also provides detailed guidelines for battery sizing.
Number of cells
Battery banks for switchgear and control applications are made up of many cells. These cells are typically wired in series to achieve a desired voltage and may also be wired in parallel to achieve additional ampere capacity. Sizing of these battery banks, therefore, includes selecting the number and type of cells to be used.
The terminal voltage per cell varies with the battery's chemical composition. The required number of series wired cells to achieve the more common DC control voltages for switchgear control is shown in Table 1. Selection of the type of cell is based on the required peak ampere output and total Ampere-hour (Ah) output capacity for the load and duration.
Load type
IEEE Standard 485-1997 classifies individual DC loads as continuous, non-continuous, and momentary. Typical continuous loads include lighting, continuously energized coils, and power to protective relay and communications systems. Non-continuous loads are less common and include critical ventilation system motors and valve actuators with operating times exceeding 1 minute. Momentary loads do not exceed 1 minute in duration and include inrush currents and circuit breaker operations.
The duty cycle imposed on switchgear batteries...