User manual

Discharge Constant Power Watts/Cell
DISCHARGE CURRENT
MINUTES
HOURS
AT 25ºC (77ºF)
www.enersys.com
Publication No: US-NP-AM-003 • January 2011 7
Figure 1. 20-Hour Rate Capacity Selection Chart
Battery Capacity Selection
Discharge
Figure 3. Discharge Characteristic Curves: Genesis
®
NP Batteries
Figure 2. High Rate Discharge Capacity Selection Chart
Figure 1 may be used to determine battery size (expressed in
Ampere Hours of capacity), for a specific application. To
determine the capacity of the battery, establish the discharge
current for the length of discharge time required. The point
where the current and time lines intersect is the minimum
capacity battery needed for the application. It is recommended
you also refer to Figures 3 before making your final decision.
Figure 2 is the high rate capacity selection chart for Genesis
®
NPX series batteries. These batteries are typically used for UPS
constant power applications under 30 minutes.
 Discharge Characteristics
The curves shown in Figure 1, and the discharge rates shown
in Table 3 illustrate the typical discharge characteristics of
Genesis NP batteries at an ambient temperature of 25˚C
(77˚F). The symbol “C” expresses the nominal capacity of the
Genesis NP battery, measured at a 20 hour discharge rate.
Please refer to General Specifications to determine the
nominal capacity rating of the specific model.
The industry standard for designating the nominal capacity of
a sealed lead acid battery involves a discharge test for a given
number of hours to a final pre-set end voltage. The average
current value multiplied by the hours of discharge time
determines the capacity rating of that particular battery.
Since manufacturers vary in their rating standards, it is
always a good practice to question the rating standard.
Table 1 shows how the rated nominal capacity
decreases when the discharge load is higher than the 20
hour rate. This table should be consulted when selecting
a battery for a high discharge application.
The discharge rates depicted in Figure 2 and Table 2
reference watts per cell of the DataSafe
®
NPX series of
batteries. These batteries are designed for Uninterruptable
Power Supply (UPS) applications where high rate discharge
performance (under 30 minutes) is typical. To determine the
battery kilowatt rating required for a UPS system, refer to the
following formula: KVA rating of UPS x Power Factor (Pf)
÷ inverter efficientcy = Total Battery Kilowatts (KWB)
 Temperature characteristics
At higher temperatures, the electrical capacity that can be
taken out of a battery increases. At lower temperatures, the
electrical capacity that can be taken out of a battery
decreases. Figure 4 shows the temperature effects in relation
to battery capacity.
Figure 4. Temperature Effects in Relation to Battery Capacity
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