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How this is calculated#

A stationary battery carries a site's d.c. loads when the charger loses its supply: protection relays, breaker trip and close coils, emergency lights, radios, controls. Those loads change through the outage. The duty cycle lists them in order, as a current for a number of minutes.

The tool adds the ampere-hours of each period: the current times the minutes over sixty. That is the charge the duty takes out of the battery.

A battery gives less than its rated capacity when it discharges faster than its rated hours, or to a higher end voltage. So the tool multiplies the duty by a rate factor you read from the maker's table. It then applies a temperature factor for a cold battery room, a design margin for growth and unknowns, and an ageing factor so the battery still does the duty near the end of its life. The product is the capacity by ampere-hours. When a later period draws more than the one before it, that sum is too small, and the tool warns.

The end voltage per cell is the lowest voltage your loads accept, at the battery terminals, over the number of cells in series. Read the maker's capacity at that end voltage.

The charger must carry the standing load and put back what the duty took, with a margin for charge losses, in the recharge time you set. The tool adds the continuous load to the duty's ampere-hours times the recharge factor over the hours.

AmpSize does not hold IEEE 485 or any cell's capacity factors. With the maker's Kt on every period, the tool works the last section, each change in current times its Kt, and that figure governs. Every factor is an assumption you set, and the result says so.

Worked examples#

• A 110 V substation battery for a trip, eight hours of standing load and a close needs 112.1 Ah. The duty takes 81.5 Ah, with a charger of 17.5 A to recharge in 12 hours.
• A 125 V battery for a trip and three hours of lights and controls needs 159.2 Ah. The duty takes 91.5 Ah, with a charger of 25.1 A.
• A 48 V radio site with a transmit burst needs 49.7 Ah. The duty takes 36.17 Ah, with a charger of 12 A. The burst draws more than the radio load, so the tool warns that the sum understates the battery.
• A 24 V control panel with a valve close at the end needs 11.7 Ah. The duty takes 8.5 Ah, with a charger of 5.2 A. The valve close draws more than the load before it, so the tool warns that the sum understates the battery.

Limits#

Without Kt values, the tool adds ampere-hours. A battery's capacity falls at high rates, so a heavy load of a minute or two can need far more than its ampere-hours show. The rate factor carries that, and only the maker's table gives it.

It does not pick a cell, a chemistry or a float voltage, and it does not check ventilation or the battery room. Follow the maker's data for those.

See also: Battery, Fire alarm battery and DC cable size.