Battery Life Calculator

Estimate battery runtime based on capacity, load current, and discharge efficiency

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Technical Engineering Guide
1. What This Calculator Determines

This tool calculates battery runtime (in hours) given a battery's capacity and the connected load's current draw. Runtime estimation is critical for:

  • Electronic System Design: Sizing batteries for IoT devices, portable electronics, and embedded systems
  • Backup Power Planning: Determining UPS runtime for critical systems
  • Product Development: Validating battery life claims for consumer electronics
  • Renewable Energy Systems: Sizing battery banks for solar/wind installations
  • Experimental Setup: Planning duration for data logging or field measurements
2. Core Electrical Formula
t = (C × η) / (I × D)
t
Runtime (hours)
C
Battery capacity (mAh or Ah)
η
Discharge efficiency (decimal, e.g., 0.9 for 90%)
I
Load current (mA or A)
D
Discharge rate multiplier (C-rate)
Unit Consistency: The calculation preserves SI unit relationships: 1 A = 1000 mA, 1 Ah = 1000 mAh. Ensure capacity and current units match (mAh with mA, Ah with A) for accurate results.
3. C-Rate Fundamentals

The C-rate expresses discharge current relative to battery capacity:

  • 1C: Discharge current equal to capacity (e.g., 2A from 2Ah battery)
  • 0.5C: Half the capacity current (1A from 2Ah battery)
  • 2C: Double the capacity current (4A from 2Ah battery)

Higher C-rates reduce effective capacity due to internal resistance and polarization effects.

4. Peukert Effect (Lead-Acid Batteries)

The Peukert equation accounts for capacity reduction at high discharge rates:

Ceffective = Crated / (I / Crated)n-1
Ceffective
Available capacity at discharge current I
Crated
Rated capacity at 1C rate
n
Peukert exponent (typically 1.15-1.3 for lead-acid)

This calculator uses n=1.15 for lead-acid when the Peukert option is enabled. Lithium batteries exhibit minimal Peukert effect.

5. Energy Calculations (When Voltage Provided)

With battery voltage V, the tool computes:

Etotal = C × V
P = I × V
Eusable = Etotal × η
Etotal
Total energy storage (Wh)
P
Load power (W)
Eusable
Available energy considering efficiency (Wh)
6. Engineering Considerations & Limitations
Calculation Assumptions:
  • Constant current discharge (not power)
  • Linear voltage discharge profile
  • Constant efficiency throughout discharge
  • Room temperature operation (25°C)
  • Battery at full charge initially
Real-World Factors Not Modeled:
  • Temperature effects on capacity (reduced at low temperatures)
  • Voltage drop under load
  • Capacity fade with cycle life
  • Self-discharge during storage
  • Cut-off voltage considerations
7. Common Calculation Mistakes
  • Unit Mismatch: Using mAh with A (off by factor of 1000)
  • Ignoring Efficiency: Assuming 100% of rated capacity is usable
  • Peukert Neglect: Using rated capacity for high-rate lead-acid discharge
  • Average vs. Peak Current: Using peak current instead of average drain
  • Voltage Confusion: Multiplying mAh by nominal voltage for Wh without conversion
8. Practical Example

Scenario: IoT sensor with 2000mAh Li-ion battery (3.7V), drawing 10mA average current with 90% discharge efficiency.

t = (2000mAh × 0.9) / 10mA = 180 hours = 7.5 days
Etotal = 2.0Ah × 3.7V = 7.4 Wh
P = 0.01A × 3.7V = 0.037 W
9. Frequently Asked Questions
Q: Why is my actual battery life shorter than calculated?

A: Real batteries have voltage sag, temperature sensitivity, aging effects, and protection circuits that reduce available capacity. Add 20-30% margin for critical applications.

Q: When should I use C-rate vs. direct current input?

A: Use C-rate when you know the battery's maximum continuous discharge rating. Use direct current when you've measured your load's actual current draw.

Q: How does battery chemistry affect the calculation?

A: Chemistry determines nominal voltage, discharge efficiency, Peukert exponent, and safe operating C-rates. Li-ion maintains higher voltage during discharge than NiMH.

Q: What's the difference between energy (Wh) and capacity (Ah)?

A: Capacity (Ah) measures charge storage. Energy (Wh) = Capacity × Voltage, representing work capability. For different voltage batteries, compare Wh, not Ah.

10. Safety & Usage Disclaimer
Important Safety Notice:

This tool provides theoretical estimates only for educational and planning purposes.

  • Do not use for safety-critical system design without professional review
  • Actual battery performance varies by manufacturer, age, and conditions
  • Lithium batteries require proper protection circuits to prevent fire hazards
  • Always consult battery datasheets for maximum discharge rates and safety limits
  • This calculator assumes ideal conditions; real-world results will differ
11. Tool Specifications
  • Calculation Method: Client-side JavaScript with no data transmission
  • Accuracy: Theoretical calculation with 2 decimal places for hours
  • Rounding: Intermediate values use floating-point precision
  • Applicable Range: DC systems only; not for AC backup time calculations
  • Last Formula Review: September 2025
Battery Types
Typical Characteristics
  • Li-ion 3.7V
  • LiPo 3.7V
  • NiMH 1.2V
  • Lead-Acid 2V/cell
  • Alkaline 1.5V
Select battery type above to auto-fill typical voltage and efficiency values.
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Help Guide

Battery capacity is typically measured in mAh (milliampere-hours) or Ah (ampere-hours). This represents how much current the battery can supply over time.

Real-world batteries don't deliver 100% of their rated capacity. Li-ion batteries typically achieve 80-90% efficiency, while lead-acid may be only 70-80% efficient.

The C-rate is the rate at which a battery is discharged relative to its maximum capacity. A 1C rate means the discharge current will discharge the entire battery in 1 hour.