NorthStar NSB12-310 Applications Manual page 11

• The size cable selected should consider the NEC
code, allowable voltage drop and maximum load
current expected per string.
• All parallel strings should be joined in parallel at a
separate busbar, "J" box or the UPS.
• Applications with more than five parallel strings
should contact NorthStar Battery.
10.14 Commissioning the Batteries
Batteries lose some initial charge during shipment and
storage, therefore it is highly recommended to give the
batteries a refreshing charge prior to commissioning. A
constant voltage charge is the ONLY charging method
allowed. A refreshing charge shall be performed with
at least 2.35 V/cell at 77°F (25°C) for at least 24 hours.
Failure to observe these conditions may result in greatly
reduced capacity and service life.
10.15 Battery Sizing Calculator
The UPS Battery Sizing Calculator is designed to help
determine the number of VRLA battery strings required
to support a UPS load.
Step 1: Calculating the power required from the battery
system, KWb.
The power required from the battery (KWb) to support
the UPS load can be calculated manually. To calculate
the KWb, you need the following information, Size of
UPS in KVA, Power Factor and Inverter Efficiency.
The KWb can be calculated using the formula below.
KWb = ((Size of UPS) x (Power Factor))/(UPS Efficiency)
For example,
KWb= ((800KVA) x (1.00)) / 0.975 = 820.5 KWb
Step 2: Calculate the watts/2V cell required to support
KWb load
The DC Link Voltage is required in order to determine the
number of 12V batteries required in the string, as well as
the power required from the battery in watts/cell. Each
lead acid VRLA battery has six (6), 2V cells connected in
series to make a 12V battery.
The watts/cell can be calculated using the formula below.
Watts/cell = ((KWb) x (1000 watts/KW) / (DC Link
Voltage/2V per cell)
Form: SES-541-01711
For example on a 480V UPS,
Watts/cell = ((820.5KWb) x (1000 Watts/KW) / (480V/2v/
cell) = 3419 watts/cell
Step 3: Determine the watts/2V cell available from
battery.
To determine the number of parallel battery strings
required to support the load, the watts/2V cell for
specific batteries, runtimes and end voltages needs to
be determined. In order to determine the following
information is required:
Required Run Time - How long does the load need to be
supported?
End Cell Voltage - Voltage at which the UPS stops the
discharge.
Temperature Compensation - Adjustment to compensate
for power loss or gain due to changes in temperature
from 25°C.
Safety or Aging Factor - A Safety or Aging Factor can
be used to adjust the power required. NorthStar
recommends that at a minimum a 5% safety factor is
used to compensate for cable losses within a cabinet.
For our example:
Runtime = 10 Min
End Voltage = 1.67VPC
Temperature = 25°C
Safety Factor =5%
The watts/2V cell can be found in the NorthStar's
Constant Power Discharge Ratings Tables. For a 10min
discharge and an end voltage of 1.67 vpc the NSB12-
730RT provides 899 watts/2V cell.
To apply the Safety or Aging Factor multiple the watts/2V
cell by (100%-Safety Factor) 95% = 899 watts /2V cell x
(100% – 5%) = 854.3 watts/ 2V cell.
To calculate the number of strings required, divide the
system watts/2V cell by the battery watts/2V cell.
For example:
System Requirement is 3419 watts/2V cell
Adjusted Constant Power for battery is 854.3
Issued: 12-08-16 ECO-101191