UFC 3 -520-01
June 10, 2002
Evaluate the duty cycle, section by section, to determine which section of the
duty cycle is limiting in terms of battery size. The cell size is selected based on the
most limiting portion of the duty cycle. A generalized representation of a duty cycle is
shown in Figure E -1 .
Figure E -1. Generalized Duty Cycle
The battery sizing analysis of a duty cycle determines the required cell size
for each section. Depending on the load profile, it is not guaranteed that the last section
containing all periods will be limiting. For example, the cell size might be established by
the first minute of the duty cycle if many loads are energized at once. IEEE 485
provides worksheets to assist with the calculation process. Battery manufacturers
provide similar worksheets.
The battery sizing methodology determines the cell size for the defined duty
cycle when the battery capacity is 100 percent and at the reference temperature of
25 C (77 F). For most batteries, end-of-life occurs when capacity falls to 80 percent of
the rated capacity. Also, depending on the installation, the actual battery temperature
might be well below 25 C (77 F), and battery capacity decreases as temperature
decreases. Apply correction factors to the calculated cell size to account for these
effects. The net result is that the selected cell size must be larger so that it can meet its
design requirements at end -o f-life at the design low temperature.
Under ideal conditions, a battery can have 90 percent to over 100 percent
capacity when new. As the battery ages, its capacity will eventually fall to 80 percent,
which is the commonly accepted point at which the battery should be replaced. Below
this capacity, the rate of degradation can increase rapidly. As part of the battery sizing
process, size the batter y so that it can fulfill the duty cycle requirements at its end of life.