🔋 Battery Life Calculator
Enter your battery capacity and the device's current draw to estimate how long it will run.
Estimated life (real)
16 hours
Theoretical maximum
20 hours
Life ≈ capacity (mAh) ÷ current draw (mA), then multiplied by an efficiency factor (~70–85%) for real-world losses. It's a rough estimate: actual runtime also depends on discharge rate (the Peukert effect), temperature, the cutoff voltage and how the load varies over time. 🔒 In your browser.
How the battery life calculator works
A battery's capacity in milliamp-hours divided by the load's current in milliamps gives the theoretical runtime in hours. Real batteries deliver less than the label, so the tool multiplies by an efficiency factor (about 70–85%) to give a more realistic estimate alongside the theoretical maximum.
This is a rule-of-thumb, not a guarantee: actual runtime also depends on the discharge rate (batteries deliver less capacity at high current — the Peukert effect), temperature, the device's cutoff voltage, and how the load varies over time. Use it to compare options and size a battery, then test.
Frequently asked questions
How do I calculate battery life?
Battery life (hours) ≈ capacity (mAh) ÷ current draw (mA), then multiplied by an efficiency factor (~0.8) for real-world losses. A 2,000 mAh battery powering a 100 mA device lasts roughly 16 hours in practice.
What does mAh mean?
Milliamp-hours — a measure of charge capacity. A 2,000 mAh battery can (ideally) supply 2,000 mA for one hour, 1,000 mA for two hours, or 100 mA for twenty. Divide by your device's current draw for the runtime.
Why does my battery last less than the calculation?
Real capacity is lower than the rating, especially at higher currents (the Peukert effect), in the cold, or once voltage sags below the device's cutoff. The efficiency factor accounts for some of this, but treat the result as an estimate.
How do I find my device's current draw?
Check its specifications, measure it with a multimeter or USB power meter, or estimate from its power (current = power ÷ voltage). Enter that milliamp figure as the load.
What efficiency factor should I use?
Around 0.8 (80%) is a reasonable default for many batteries and loads; use lower (0.7) for high-drain or cold conditions and higher (0.85–0.9) for gentle, steady loads. Adjust it to match your situation.