IEEE 485 - Battery Design

Recommended Practice for Sizing Large Lead-Acid Storage Batteries

IEEE 485 is a recommended practice published by the Institute of Electrical and Electronics Engineers (IEEE) that provides a method for sizing lead-acid storage batteries used in generating stations and substations. It defines the calculation procedure for determining the required ampere-hour capacity of a battery to supply DC power during specified duty cycles, ensuring that the battery can maintain a minimum terminal voltage throughout the discharge.

Published by: Institute of Electrical and Electronics Engineers (IEEE)

Current edition: IEEE 485-2020 (revision of IEEE 485-2010)

Companion standard: IEEE 1562 (stand-alone PV systems), IEEE 1184 (UPS battery sizing)

Scope

IEEE 485 applies to lead-acid batteries (vented and VRLA) used in DC power systems at generating stations, substations, and industrial facilities where the battery supports protection relays, control systems, emergency lighting, and switchgear operation during AC power outages. The methodology may be adapted for nickel-cadmium batteries using appropriate correction factors.

Sizing Methodology

Duty Cycle Definition

The sizing calculation begins with a defined duty cycle describing the loads connected to the battery over time. Loads are classified as continuous (sustained throughout the discharge), momentary (brief peaks), or intermittent. The duty cycle is typically divided into periods of equal duration with the associated current for each period.

Cell Selection and Correction Factors

Battery cells are selected from manufacturers’ discharge data giving capacity at various rates and temperatures. Correction factors are applied for: operating temperature (capacity decreases below 25 °C), design margin (typically 10–25% added to calculated requirement), and ageing (capacity reduction over service life — typically 80% end-of-life threshold).

Capacity Calculation

The required capacity is calculated period by period through the duty cycle, with each section’s ampere-hour requirement computed based on current draw and duration. The cell type is selected so that the available capacity at the minimum end-of-discharge voltage meets the calculated requirement with all correction factors applied.

Minimum Terminal Voltage

A minimum allowable battery terminal voltage is defined based on the lowest voltage at which connected equipment can operate correctly. The sizing calculation must ensure voltage remains above this threshold throughout the discharge, including the worst-case combination of maximum load and minimum cell voltage at end-of-life.

Disclaimer: This is not a definitive guide, just an indication and it is important that you always go back to the source for legislation.