Battery Size Calculation Formula

Battery Capacity Equation:

\[ \text{Battery Capacity (Ah)} = \frac{\text{Load Power (W)} \times \text{Required Backup Time (h)}}{\text{Battery Voltage (V)} \times \text{Depth of Discharge} \times \text{Efficiency}} \]

Load Power:

watts (W)

Required Backup Time:

hours (h)

Battery Voltage:

volts (V)

Depth of Discharge:

decimal (0-1)

Efficiency:

decimal (0-1)

Required Battery Capacity:

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1. What is the Battery Capacity Formula?

The battery capacity formula calculates the required amp-hour (Ah) rating of a battery needed to power a specific load for a desired duration, considering voltage, depth of discharge, and system efficiency.

2. How Does the Calculator Work?

The calculator uses the battery capacity equation:

\[ \text{Battery Capacity (Ah)} = \frac{\text{Load Power (W)} \times \text{Required Backup Time (h)}}{\text{Battery Voltage (V)} \times \text{Depth of Discharge} \times \text{Efficiency}} \]

Where:

Load Power — Power consumption of the device(s) to be powered (watts)
Backup Time — Desired runtime during power outage (hours)
Battery Voltage — Nominal voltage of battery system (typically 12V, 24V, or 48V)
Depth of Discharge — Maximum recommended discharge level (0-1, typically 0.5-0.8)
Efficiency — System efficiency accounting for losses (0-1, typically 0.85-0.95)

Explanation: The equation balances energy requirements (power × time) with battery characteristics and practical limitations.

3. Importance of Proper Battery Sizing

Details: Correct battery sizing ensures reliable backup power, prevents premature battery failure from over-discharge, and optimizes system cost and weight.

4. Using the Calculator

Tips: Enter load power in watts, backup time in hours, battery voltage (typically 12V for small systems), depth of discharge (0.8 for lead-acid, 0.9 for lithium), and efficiency (typically 0.9).

5. Frequently Asked Questions (FAQ)

Q1: What's typical depth of discharge for different battery types?
A: Lead-acid: 50-80%, Lithium-ion: 80-90%, NiMH: 80%. Deeper discharge reduces battery lifespan.

Q2: How does temperature affect battery capacity?
A: Capacity decreases in cold temperatures (up to 50% loss at -20°C). Add 10-20% capacity for cold climates.

Q3: Should I add margin to the calculated capacity?
A: Yes, add 10-20% safety margin to account for aging, temperature effects, and unexpected load increases.

Q4: How do I convert between Ah and Wh?
A: Wh = Ah × V. This calculator gives Ah at your specified system voltage.

Q5: What's the difference between CCA and Ah ratings?
A: CCA (Cold Cranking Amps) measures starting power, Ah measures energy capacity. This calculator determines Ah.