How to Size a VRLA Battery for UPS Backup

A VRLA battery bank for UPS backup should be sized from the UPS system voltage, actual protected load, required backup time, and the battery's published discharge data. Start with the UPS manufacturer's required DC configuration, then use the selected model's discharge table at the expected operating conditions. Confirm temperature, end-of-discharge voltage, aging policy, charger capability, rack space, terminals, and cable design before finalizing the quantity.

BATTERY SIZING

Let us have total load of 16 kW with 0.8 pf, which needs to be supported by battery backup for at least 1 hr, when the grid fails. Before choosing the battery with respect to cost or brand, there are many questions to be answered

• Which type of battery?
• C-rating of the battery?
• What is the UPS Voltage window?
• UPS rating?
• No: of Batteries (blocs) required?
• How many cells per bloc?
• How many blocs in series?
• How many blocs in string (parallel)?
• What is the End Cell Voltage (ECV)?
• What is the Temperature correction factor with respect to the ambient
  temperature?
• What is the Discharge power per cell with respect to ECV and backup time?
• What is the Discharge current per cell with respect to ECV and backup time?
• What is the ageing factor considering end of life criterion?
• What is the K factor?
• For space constraints, bloc dimensions?
• No: of blocs per rack?

Battery sizing calculation differs with respect to application as well

• UPS
• Telecommunications
• Solar Photovoltaic

REFERENCE STANDARDS / DOCUMENTS:

• IEEE 1184-2006 – Guide for Batteries for Uninterruptible Power Supply Systems
• Highlighted values are taken from reputed vendor catalogue available on the internet for calculation purpose.

UPS BATTERY SIZING CALCULATION:

Battery load of UPS should be calculated by considering the following 4 margin factors as shown below

Actual load / Inverter Efficiency = Nominal battery load
Temperature correction factor x Design Margin x Aging Margin x Overloading factor = Battery load

Selected Battery type Sealed Maintanence Free VRLA.

UPS Output rating = 20 kVA
Output Power Factor = 0.8
Actual Load = 16 kW
Inverter Efficiency (EFF) = 94%
Nominal Battery load = 17.02 kW

Temperature correction factor for 25°C = 1.00
Design Margin = 1.10
Aging Margin = 1.25
Overloading Factor = 1.10

Calculated Battery load = 25.74 kW

End Cell Voltage (ECV) should be maintained
well within the voltage window of the UPS.

Nominal DC voltage = 400 Vdc
Maximum Rectifier equalizing voltage = 432 Vdc
Inverter low voltage cut-off = 290 Vdc
Voltage drop of DC cables is assumed = 2 Vdc
Minimum required UPS voltage = 292 Vdc

Boost charge = 2.4 Vdc
Equalizing Cell Voltage = 2.25 Vdc
End cell Voltage (ECV) Selected = 1.75 V/Cell
Nominal Cell Voltage = 2 V/Cell

Battery Model Selected = LX-12 150

Total No.of cell x ECV > Cut-off voltage
Let us assume 1 hr be the required backup time

Minimum number of cells required = 166.86 Nos.
Total number of cells selected = 180.00 Nos.
Backup time = 60 mins.
Required cell discharge for 1hrs in Watts = 143.03 W/cell
Available cell discharge for 1hrs in Watts = 203 W/cell
No.of strings = 1 Nos.
Number of cells per bloc = 6 Nos.

As the selected battery bloc is 12V
(Total No.of 2V cells) / 6 = No.of Blocs
180/6 = 30

Number of blocs required = 30 Nos.

UPS datasheet: