What information is missing to perform the battery calculation?

Management has requested a 15-minute battery bank assuming full load on the UPS.

The UPS vendor has provided the following specifications of the UPS:

• Rated power: 30 kVA

• Rectifier input voltage: 400 V/3 phase

• Rectifier input power factor: 0.8

• Battery rated voltage: 384 V

• Number of cells: 192

• End of discharge voltage: 308 V

• Inverter output voltage: 400 V/3 phase

• Inverter output power factor: 0.8

What information is missing to perform the battery calculation?
A . UPS efficiency
B . Inverter efficiency
C . Load imbalance on the phases
D . Available battery charging current

View Answer

Answer: A

Explanation:

To determine the required capacity of the battery bank for the 15-minute runtime at full load, one must know the total power requirement that the battery bank must supply. The specifications provided include most of the necessary details, such as rated power, input voltage, battery voltage, and discharge voltage. However, one critical piece of information is missing: the UPS efficiency.

Detailed Explanation

In a data center UPS system, the battery bank is designed to supply power for a set duration when there is an input power failure. The UPS efficiency affects the actual power the UPS can deliver to the load compared to the power it draws from the batteries. The efficiency factor is necessary to accurately calculate the required capacity of the battery bank since it determines how much input power is needed from the batteries to supply the load at full capacity. The formula typically used to determine battery capacity involves factoring in UPS efficiency, as it allows you to understand the losses within the UPS system.

If UPS efficiency is not considered, there would be an inaccurate estimation of the actual power needed from the batteries. For instance, if a UPS has 90% efficiency, only 90% of the power drawn from the batteries reaches the load. Without knowing this efficiency, it is not possible to calculate the battery bank size accurately, as you cannot accurately estimate the losses within the UPS itself.

EPI Data Center Specialist References:

According to EPI Data Center Specialist training, understanding the UPS efficiency is essential for battery sizing. Without it, the calculations could lead to either under sizing or oversizing the battery bank, which affects both reliability and cost-effectiveness of the UPS system. The EPI Data Center Specialist course emphasizes that battery sizing must account for all losses within the UPS system, with efficiency being a primary factor in these calculations.