UFC 3 -520-01
June 10, 2002
stated, the design margin correction factor is an additional margin to help ensure the
battery has adequate capacity to perform its job. A design margin of 10 percent to 15
percent is typical.
EXAMPLE: Suppose the sizing calculation for a vented lead-acid battery
determined that 5 positive plates per cell were required for the specified duty cycle.
What size cell is needed to account for aging and an expected low operating
temperature of 15.6 C (60 F)? Also, the designer would like to add a 10 percent
The aging correction factor is 1.25 to ensure adequate capacity when the battery is
at end of life. From Table E-1, the temperature correction factor is 1.11 for an
operating temperature of 15.6 C (60 F). The design margin correctio n factor is
1.10. The required cell size is as follows:
Corrected Cell Size = (5 positive plates ) 1.25 1.11 1.10 = 7.63 positive plates
In this case, round up to 8 positive plates. If the application of design margin causes
the calculated cell size to slightly exceed the next size cell, for example, 7.05
positive plates, the designer should, in this case, determine if 7 positive plates are
adequate. Rounding up to the next cell size results in a larger and more expensive
battery. The battery must be big enough to do its job throughout its service life, but
a grossly oversized battery is not desirable either.
In summary, size the battery for the limiting portion of the duty cycle, including
corrections for performance at end of battery life and for the minimum expected
operating temperature. If needed, include an additional design margin.
BATTERY SIZING FOR UPS APPLICATIONS.
Compared to duty cycle sizing, UPS applications often involve a different
approach to battery sizing. If the UPS is the only load placed on the battery (which is
common for many UPS systems), the battery can be sized more easily based on the
UPS constant power requirements. Battery manufacturers also provide sizing charts
based on a constant power discharge. The method of analysis is particularly
straightforward, cons isting of the following steps:
E-2.1.1 Determine total load (and duration) the UPS will place on the battery. The
duration can be the most difficult design factor to specify. If additional backup power
such as a diesel generator is not available, the UP S battery has to be large enough for
the staff to place the system in a safe state in response to a power outage. If diesel
generator backup is available, a 5 -minute backup time might be adequate if the diesel
system operates properly. If the designer allows for diesel starting difficulties, a backup
time of over 30 minutes might be needed.
E-2.1.2 Apply cell sizing correction factors so that the battery can provide the required
load at end of life and the design low temperature.