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 MIL-HDBK-1012/1
4.2.4.3
UPS Battery Rooms. The most important environmental control system
requirement of the battery room is to keep the battery electrolyte within an
acceptable range of temperature. Normally, this can be done by heating and
ventilation only. Ventilation is also necessary to limit hydrogen
accumulation within the room. Batteries used with UPS equipment give off
negligible heat and low volumes of hydrogen gas when being charged. The high
thermal inertia of the battery electrolyte causes its temperature to float
between the daily ambient temperature extremes; therefore, acceptable
electrolyte temperatures can be maintained by heating and ventilating, even
though the ventilated room temperature may approximate the outside ambient
temperature.
a) Ventilation. The ventilation rate required is dependent upon
the hydrogen evolution rate which, in turn, depends on the type, size,
temperature, charging rate, and number of cells in the battery bank. Hydrogen
gas evolution is highest during the last hours of an equalizing charge when
most of the batteries will be fully charged and the charging energy is simply
hydrolizing the electrolyte. During this period and at 760 millimeters (mm)
of mercury absolute atmospheric pressure, one cell will release hydrogen at
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the rate of 0.016 cubic feet per hour (ft /h) of hydrogen for each ampere of
charging current flowing.
One fully charged lead calcium cell, with 1.215 specific gravity
electrolyte, at 77 degrees F (25 degrees C) temperature, will pass 0.24
amperes of charging current for every 100 ampere-hour cell capacity (measured
at the eight-hour rate), when subject to an equalizing potential of 2.33
volts. This current will double/halve for each 15 degrees F (8 degrees C)
rise/fall in temperature.
The required capacity of a cell (ampere-hours at the eight-hour
rate) and the number of cells for a given UPS application depends on the UPS
size, design and operating time. Where several UPS and battery brands are
available, a variety of combinations are possible.
To calculate the required rate of ventilation for the battery bank
of a 750KVA Emerson UPS (for example), consisting of 182 GNB Inc. cells, type
PDQ-27, each having a nominal 1360 AH capacity at the eight-hour rate and
being equalized at an electrolyte temperature of 92 degrees F (33 degrees C),
proceed as follows:
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