June 10, 2002
STATIONARY BATTERY AND CHARGER SIZING
BATTERY SIZING FOR APPLICATIONS WITH A DUTY CYCLE.
The designer of a backup power system has to determine the battery size.
The battery can carry more load or perform longer as it is made larger, but a larger
battery is also more expensive, requires more floor space, and increases the life cycle
cost. For these reasons, provide a technical basis for the battery size.
The classic method of sizing a battery is based on determining the specific
load requirements and selecting a battery size capable of supplying that load for the
specified time. IEEE 485 is the best industry reference for this type of cell sizing and
should be reviewed as part of a battery sizing evaluation. IEEE 1115, IEEE
Recommended Practice for Sizing Nickel-Cadmium Batteries for Stationary
Applications, provides equivalent sizing information for nickel-cadmium batteries.
The battery duty cycle is the load that the battery is expected to supply for a
specified period of time. Generally, the duty cycle is described in terms of the worst-
case load that the battery is expected to supply. The battery would have to carry all or
part of the connected load under any of the following conditions:
System load exceeds the battery charger capability.
Battery charger output is lost (could be by charger failure or loss of ac input).
All ac power is lost in the facility.
The worst case load usually occurs when all ac power is lost because other
emergency loads might be energized in addition to the normally-energized loads. For
example, loss of all ac power might require the additional energization of emergency
lighting, circuit breaker components such as trip coils or spring charging motors, and
emergency diesel engine cranking power. The duty cycle must consider all of these
E-1.5. The following design inputs are needed to determine a battery size:
Discharge capability of selected battery type.
Load requirements, including duration.
Minimum and maximum system voltage limits.
Temperature, aging, and design margin allowances.