Battery Rating Calculation

🔋 Battery Rating Calculation for UPS Systems

1. Why Series Connection?

• Series Connection → Voltage adds up, current remains same.

• Example: 18 batteries × 12V each → 216V DC bus.

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2. Formula for Battery Capacity (AH)

$$\text{Battery Capacity (AH)} = \frac{DC\ Current \times Duration\ (Hrs)}{\%\ Capacity\ Utilization}$$

Where:

$$DC\ Current = \frac{UPS\ (KVA) \times 1000 \times Load\ PF}{\eta_{inverter} \times End\ Voltage}$$

• $VAH = AH \times V_{nominal}$

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3. Worked Example (UPS = 5 KVA, 1 Hr Backup, 18 Batteries)

Step 1 – Find DC Current:

$$DC\ Current = \frac{5 \times 1000 \times 1}{0.95 \times (10.5 \times 18)} = 27.85\ A$$

Step 2 – Find Battery AH:

$$Battery\ AH = \frac{27.85 \times 1}{0.62} = 44.91 \approx 42 AH$$

Step 3 – Total VAH:

$$VAH = 18 \times 12 \times 42 = 9072\ VAH$$

✅ Hence, 18 Nos. of 12V / 42AH batteries will suffice.

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4. Assumptions Used

1. DC Voltage = 216V for 5 KVA UPS

2. End cell voltage = 10.5V / battery

3. Load Power Factor = 1.0

4. Inverter Efficiency = 95%

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5. Capacity Utilization (%)

Duration	% Capacity Utilization
0.5 Hr	52%
1 Hr	62%
2 Hr	74%
3 Hr	83%
4 Hr	85%

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6. Key Notes for Engineers

• Amp-Hour (AH): Approximate measure of energy capacity.

• Limitations: Accurate only at rated discharge current and duration.

• Battery Aging: With discharge, voltage falls and resistance rises.

• Testing: Best check for dead battery → load voltage test.

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