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A Comparison of Calculated Battery Capacity Using the Current IEEE 450 Method and a Proposed Alternate Method for High Rate Discharge Applications

Edward P Rafter, P.E.

There is an on-going debate within the battery industry today regarding the correct method for calculating the capacity of a battery in a high-rate discharge application, such as an Uninterruptible Power Supply (UPS) battery plant. The contention is that the end-of-life requirement is for the battery to provide 80% of the published rate for 100% of the time as suggested through IEEE 485-1997 'Recommended Practice for Sizing Lead-Acid Batteries for Stationary Applications.' However, when the battery is tested in accordance with IEEE 450-1995, 'Recommended Practice for Maintenance, Testing, and Replacement of Vented Lead-Acid Batteries for Stationary Applications', the end-of-life measurement is when the battery provides 100% of the published rate for 80% of the time. It is argued that the existing method in IEEE 450 will suggest that the battery is apparently aging more rapidly than it actually is aging. The capacity calculation method in IEEE 450 is not consistent with the end-of-life requirement set forth in the sizing criteria found in IEEE 485. This paper will show calculations based on the present method in IEEE 450 and a proposed alternative method, including the "resulting difference in the calculated capacity, for flooded lead acid and valve regulated batteries in high-rate discharge applications.

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