Battery Life Calculator

Estimate how long your battery will last.

✅ Formula verified against standard references • Last Updated: Jan 2026

Calculator

Standard is 0.8 (80%)

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Estimated Runtime

0 Hours

0 Minutes

Understanding Battery Life

Calculating battery life isn't just about dividing capacity by consumption. In the real world, batteries are never 100% efficient. Factors like heat, voltage sag, and the discharge curve (Peukert's Law) mean you rarely get the full theoretical capacity.

The standard formula we use is: Runtime = (Capacity × Efficiency) / Consumption. We default to 0.8 (80%) efficiency, which is a safe bet for most Li-Ion and Li-Po batteries used in electronics.

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Expert Tip: The 80% Rule
Never plan for 100% discharge. Deeply discharging a battery (to 0%) significantly shortens its lifespan. For critical applications (like drones or medical devices), plan your battery capacity so you only use 80% of it before recharging. This can double or triple your battery's cycle life.

Common Mistakes

  • Mixing Units: Ensure capacity is in mAh and consumption is in mA. If you have Amps (A), multiply by 1000 to get mA.
  • Ignoring Voltage: This calculator assumes the battery voltage matches the device voltage. If you are using a step-up or step-down converter, you must calculate in Watt-hours (Wh), not mAh.
  • Overlooking Self-Discharge: Batteries lose charge over time even when not in use. For long-term projects (months/years), factor in self-discharge rates.
TC
✅ Alex Chen, Calcs.top
Updated November 26, 2025

Frequently Asked Questions

How do I calculate battery life?

Battery Life = (Battery Capacity in mAh × 0.8) ÷ Device Consumption in mA. The 0.8 factor accounts for external factors and efficiency losses. For example, a 2000mAh battery powering a 200mA device lasts about 8 hours.

Why is actual battery life shorter than calculated?

Calculations are theoretical. Real life is affected by temperature, battery age, discharge rate (Peukert's Law), and voltage drops. We use a 0.8 (80%) efficiency factor to give a more realistic estimate.

What is mAh?

mAh stands for milliampere-hour. It measures the energy capacity of a battery. A higher mAh rating means the battery can store more energy and run a device for longer.

How do I convert Watts to Amps?

If your device lists power in Watts (W), use the formula: Amps = Watts ÷ Volts. Then multiply by 1000 to get mA. For example, a 5W device running on 5V draws 1 Amp (1000 mA).