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Battery Life Calculator

How long will it last?
100%
Use Current (Amps)
Use Power (Watts)
Estimated Runtime 0 Hours

Why Your Battery Dies Early

If you have a 2,000 mAh battery and a device that draws 200 mA, simple math says it should last 10 hours ($2000 / 200$).

However, in the real world, you might only get 7 or 8 hours. Why?

Batteries are not 100% efficient. Energy is lost due to:

  • Heat: Internal resistance converts some energy into heat (especially during fast discharge).
  • Voltage Conversion: Power banks must boost their internal 3.7V to 5V (USB standard), losing about 10-15% efficiency in the process.
  • Peukert's Law: For lead-acid batteries, drawing power faster reduces the total available capacity.

The Calculation Formula

To get an accurate estimate, we apply an Efficiency Factor (usually 0.8 or 0.9 for Lithium-Ion).

Time = (Capacity ÷ Load) × Efficiency
  • Capacity: Amp-Hours (Ah) or Milliamp-Hours (mAh).
  • Load: Amps (A) or Milliamps (mA).
  • Efficiency: Typically 0.85 (85%) for real-world scenarios.

mAh vs. Wh: What's the Difference?

mAh (Milliamp-Hour) measures how much current flows over time. It is commonly used for phones and small electronics.

Wh (Watt-Hour) measures total energy work. It accounts for Voltage. This is a better metric for comparing batteries of different voltages (e.g., a 12V car battery vs. a 3.7V phone battery).

The Airline Rule:
Most airlines limit carry-on batteries to 100 Wh. If your power bank says "20,000 mAh," you need to check the voltage to see if it's legal to fly.
Calculation: (20,000 mAh × 3.7 V) / 1000 = 74 Wh. (Safe to fly!).

Battery Types & Lifespans

Type Common Use Efficiency Cycle Life
Alkaline TV Remotes Low (High resistance) Single Use
Li-Ion Phones, Laptops High (~90%) 300 - 500 cycles
LiFePO4 Solar Storage Very High (~95%) 2000+ cycles
Lead Acid Cars Medium (~80%) 3 - 5 years

Energy Toolkit

⚡ Energy Units 🔌 Ohm's Law 💡 Electricity Cost