🔋 ELECTRICAL

Free Online Battery Bank / DC UPS Sizing Calculator

Size a lead-acid or Li-ion battery bank (Ah and cell count) for a DC or UPS system from load, backup time and voltage. IEEE 485 / IS 16242 basis. Free online tool, no sign-up.

📐 Standard: IS 16242 / IEEE 485
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Battery Bank / DC UPS Calculator
Reference: IS 16242 / IEEE 485
🔋 ELECTRICAL
Size a lead-acid or Li-ion battery bank (Ah and cell count) for a DC or UPS system from load, backup time and voltage. IEEE 485 / IS 16242 basis. Free online tool, no sign-up.
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ℹ️ About This Calculator

A battery bank must deliver the DC load for the required backup time at an acceptable end-of-discharge voltage. This calculator sizes the bank - the amp-hour capacity and the number of cells - from the load, backup duration, system voltage and design margins, following IEEE 485 / IS 16242.

Capacity is the load current times the backup hours, then adjusted by three factors: an ageing factor (1.25, so the bank still works at end of life), a temperature factor (capacity falls below 25 °C), and a design margin. The cell count comes from the system voltage divided by the per-cell end-of-discharge voltage. High-rate/short-duration duties (like UPS) use a discharge-rate correction because batteries deliver less at high current.

📐 Battery Bank Sizing

IS 16242 / IEEE 485

C = (I × t × K_age × K_temp × K_margin) / K_rate
Cells = system voltage / end-of-discharge voltage

C = required capacity, Ah
I = load current, A;  t = backup time, h
K_age ≈ 1.25;  K_temp per temperature;  K_rate per discharge rate

🧮 Worked Example

Example: A 10 A DC load for 2 hours needs 20 Ah base, ×1.25 ageing ×1.1 temperature ≈ 27.5 Ah (before high-rate correction) - so a 30 Ah bank. For a 110 V system with lead-acid cells discharged to 1.75 V, the cell count is 110 / (2 × 1.75) ≈ 31 cells, rounded to a standard string.

📊 Battery Sizing Factors (IEEE 485)

Correction factors applied to the base amp-hours:

FactorTypical valueReason
Ageing1.25Capacity drops to ~80% at end of life
Temperature1.0 – 1.2Capacity falls below 25 °C
Design margin1.1 – 1.15Future load growth, uncertainty
Discharge rate>1 for high rateLess usable Ah at high current

Lead-Acid vs Lithium

VRLA lead-acid is cheaper with a 3-5 year design life; Li-ion (LFP) costs more but lasts 8-10+ years, is lighter, tolerates deeper discharge and higher temperatures, and needs less space - increasingly used for UPS and DC systems. Match the end-of-discharge voltage and float/boost regime to the chemistry.

Indicative factors - use the manufacturer's discharge tables and IEEE 485 for a full calculation.

❓ Frequently Asked Questions

How do I size a battery bank? +
Multiply the load current by the backup hours for the base amp-hours, then apply an ageing factor (~1.25), a temperature factor and a design margin, and correct for the discharge rate. The cell count is the system voltage divided by the per-cell end-of-discharge voltage.
What is the ageing factor for batteries? +
About 1.25 - a battery is considered end-of-life when its capacity falls to roughly 80% of rated, so sizing at 1.25× the requirement ensures it still meets the load near the end of its service life. IEEE 485 uses this factor.
How many cells does a battery bank need? +
Divide the system DC voltage by the per-cell end-of-discharge voltage. For a 110 V system with lead-acid cells discharged to 1.75 V per cell, that is about 62 cells for 2 V cells - the exact count depends on the cell voltage and chemistry.
Lead-acid vs lithium battery - which for a UPS? +
VRLA lead-acid is cheaper but has a 3-5 year life and is heavy; lithium (LFP) costs more but lasts 8-10+ years, is lighter and more compact, tolerates higher temperatures and deeper discharge. Lithium is increasingly chosen for UPS and DC systems despite the higher initial cost.
How does temperature affect battery capacity? +
Capacity falls at lower temperatures - a battery below 25 °C delivers less than its rated amp-hours, so a temperature correction factor (often 1.0-1.2) increases the required size. High temperatures give more capacity but shorten life.
Why derate for high discharge rate? +
Batteries deliver less usable capacity when discharged quickly (the Peukert effect). A short-duration, high-current UPS duty gets fewer amp-hours than the slow-rate rating, so a discharge-rate correction increases the required capacity.
What is end-of-discharge voltage? +
The minimum cell voltage the battery is allowed to reach at the end of the backup period - about 1.75 V for a 2 V lead-acid cell. It sets how deeply the battery is used and, with the system voltage, the number of cells in the string.
What standard covers battery sizing? +
IEEE 485 is the standard method for sizing lead-acid batteries for stationary applications, and IS 16242 covers stationary batteries in India. Manufacturers provide discharge tables that these methods apply for the required duty.

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⚠️ Disclaimer: For preliminary engineering design only. Verify all results with a licensed engineer before use. Full disclaimer →