Battery Life Calculator
Enter the battery capacity (mAh) and your device's average current draw (mA); the tool factors in efficiency losses to show a realistic runtime in days, hours and minutes.
Defaults to 80%. Approximates voltage regulator losses, self-discharge and the battery's real discharge curve.
Enter the battery capacity and current draw to see the runtime (current cannot be 0).
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How Is Battery Life Calculated?
A battery's theoretical runtime is simply capacity (mAh) / current draw (mA). That is an ideal figure that assumes 100% of the battery's energy is usable. In reality, voltage regulator losses on the circuit, the battery's own self-discharge, temperature effects, and the drop in usable capacity as the battery's voltage sags all make the real runtime shorter. This tool multiplies the ideal runtime by an efficiency/derating factor (80% by default) to give a more realistic estimate.
Example Device Current Draws (2000 mAh battery, 80% efficiency)
| Device | Typical current | Estimated runtime |
|---|---|---|
| ESP32 (Wi-Fi active) | ~150 mA | 10 hours 40 minutes |
| Arduino Uno (active) | ~45 mA | 1 day 11 hours 33 minutes |
| Raspberry Pi Zero W (idle) | ~120 mA | 13 hours 20 minutes |
| Bluetooth beacon (deep sleep avg.) | ~1 mA | 66 days 16 hours |
Frequently Asked Questions
Why is real battery life lower than the capacity/current calculation?
A simple capacity divided by current gives an ideal (100% efficient) runtime. In practice, voltage regulator losses, the battery's self-discharge, temperature effects and the drop in usable capacity as voltage sags all reduce the usable energy. That is why this tool multiplies the ideal runtime by an efficiency percentage (80% by default) to give a more realistic estimate.
What does mAh (milliamp-hour) mean?
mAh describes the electrical charge a battery can store. For example, a 2000 mAh battery can theoretically supply 2000 mA for 1 hour, or 200 mA for 10 hours. The real runtime is lower than this due to the efficiency losses described above.
How do I find a device's average current draw?
The most accurate method is measuring it with a multimeter or a USB power meter. If the device has variable loads, such as Wi-Fi, a display or a motor, take a time-weighted average of the active and sleep-mode currents. Datasheets also list typical and maximum current figures, but a real measurement is more reliable.