Battery Life Calculator
Runtime from capacity and load — constant or duty-cycled IoT profiles with sleep current, plus realistic derating.
mAh ratings are measured at gentle currents (often C/20). Alkaline cells lose up to half their rated capacity above ~250 mA — that is the “derating” input. For coin cells, pulse current matters more than average: a CR2032 sags badly above ~15 mA pulses even when the average is tiny. Lithium rechargeables hold capacity at high drain far better.
Battery Life Calculator computes how long a battery lasts under constant or duty-cycled (IoT) loads — free, instant and private in your browser. IoT developers, product designers and anyone sizing batteries for field devices use it to skip the datasheet algebra: type your numbers, read the answer with the substituted formula shown step by step, and share an exact permalink of the calculation.
About Battery Life Calculator
Battery Life Calculator computes how long a battery lasts under constant or duty-cycled (IoT) loads using the standard engineering relation: t = (C·derating)/I(avg), with I(avg) = Ia·D + Is·(1−D) for sleep/wake devices. Worked live: a 2000 mAh cell at 50 mA runs ~34 h at 85 % usable; the same cell on an ESP32 sleeping 99 % of the time lasts months. The result recalculates on every keystroke, the worked-example panel shows your numbers substituted into the formula, and the Copy permalink button encodes the inputs in the URL so a colleague opens exactly your calculation. Everything runs client-side — nothing you type leaves your device.
How to use Battery Life Calculator
- 1Enter your values — the tool starts with realistic defaults for this exact use case, so the worked example is meaningful immediately.
- 2Read the live result and the worked-example panel, which substitutes your numbers into the formula step by step.
- 3Adjust any input to compare scenarios, then use Copy result or Copy permalink to share the calculation.
Why use Battery Life Calculator?
- ✓Implements the real formula — t = (C·derating)/I(avg), with I(avg) = Ia·D + Is·(1−D) for sleep/wake devices — with the substitution shown, not a black box
- ✓Built for IoT developers, product designers and anyone sizing batteries for field devices
- ✓Copy result and permalink buttons — share the exact calculation in a README, forum answer or design review
- ✓100% free, no sign-up, runs entirely in your browser (works offline once loaded)
Frequently asked questions
How do you calculate battery life?+
How long a battery lasts under constant or duty-cycled (IoT) loads follows t = (C·derating)/I(avg), with I(avg) = Ia·D + Is·(1−D) for sleep/wake devices. For example, a 2000 mAh cell at 50 mA runs ~34 h at 85 % usable; the same cell on an ESP32 sleeping 99 % of the time lasts months. The calculator applies the same relation and shows the substituted arithmetic so you can verify every step.
Why does my device last far less than capacity ÷ current predicts?+
Three usual suspects: mAh ratings are measured at gentle currents (high drain shrinks alkaline capacity up to half), cold halves it again, and the device cuts off above 0 V — capacity below the cutoff voltage is unreachable. Hence the derating input.
For sleepy IoT nodes, what dominates — active or sleep current?+
Below about 0.1 % duty, sleep current rules: at 10 µA sleep, a 2000 mAh cell's ceiling is ~20 years before active bursts even count, but a leaky regulator drawing 1 mA caps everything at ~3 months. Measure sleep current first; optimise wake time second.
Is the Battery Life Calculator free and private?+
Yes — completely free with no sign-up or usage limits, and it runs entirely in your browser: the values you enter are never uploaded or stored on a server.
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