VRLA Battery Sizing for Reliable UPS Backup

Power Projects Author

Calculate VRLA UPS battery load using inverter efficiency, temperature, design, aging & overload margins ensure accurate sizing.

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 are in a 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 aging factor considering the 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 the 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 a reputed vendor catalog available on the internet for calculation purposes.

UPS BATTERY SIZING CALCULATION:

The 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 Maintenance 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

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Categories: : Battery