Battery Life Calculator
Estimate a battery's runtime from its capacity, the load current, and a real-world efficiency factor.
How it works
A battery's capacity in milliamp-hours tells you roughly how much current it can supply over time. Divide that capacity by how many milliamps your circuit draws and you get a first estimate of runtime in hours.
Real batteries never deliver their full rated capacity, so the tool folds in an efficiency factor. A 2000 mAh cell powering a 100 mA load at 85% efficiency lasts about 2000 × 0.85 ÷ 100 = 17 hours.
The result is shown in both hours and days, which is handy for anything meant to run unattended — a sensor node, a data logger, or a battery-backed clock you'd rather not recharge every afternoon.
Frequently asked questions
Why include an efficiency factor?
Datasheets rate capacity under ideal conditions. In practice, high drain, cold temperatures, and the battery's own internal resistance mean you get less, so an efficiency below 100% gives a more honest estimate.
What efficiency should I use?
For a gentle, steady load, 85–90% is a reasonable start. Push the battery harder, or run it in the cold, and you might drop to 70% or lower. When in doubt, estimate on the conservative side.
Does this work for a device that sleeps?
Only if you enter the average current. A device that sleeps most of the time and wakes briefly draws far less on average than its peak, so use the averaged figure for a realistic runtime.