Battery Life Estimator from a Duty-Cycled Load Profile
Enter each operating mode with its current, duration and how often it happens. The calculator returns the average current, the runtime, and which mode is actually spending your battery — including self-discharge, which often exceeds the circuit itself.
| Mode | Current (mA) | Duration (ms) | Times per hour | Duty | Average (µA) | Share |
|---|
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Average current is what empties a battery
Peak current sizes your decoupling and your regulator. It has almost nothing to do with runtime. What drains a cell is the average, and on a duty-cycled device that average is usually dominated by something with a duty cycle well under one percent.
Load the example and look at the share column. A radio transmitting 120 mA for 25 ms twelve times an hour has a duty cycle of one hundredth of a percent, and still contributes more than the microcontroller that is awake far longer. Meanwhile the 5 µA sleep current runs continuously and quietly matters more than either.
Iavg = Σ ( Imode × duration × rate ) per hour
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runtime = usable mAh / (Iavg/η + Iself-discharge)That structure is why firmware optimisation so often disappoints. Halving the processing time of a mode contributing 8% of the drain buys 4%. Finding the mode at 60% is where the runtime is.
Self-discharge sets a ceiling you cannot code around
Every cell flattens itself. At 2% a month a 2000 mAh cell is losing about 55 µA continuously — more than many well-designed sleep modes draw. Once your circuit is quieter than the chemistry, further firmware work changes nothing at all, and the tool says so explicitly when that crossover happens.
This is the honest ceiling on multi-year battery products, and it is why chemistry selection is a design decision rather than a purchasing one. Lithium primary cells self-discharge at a fraction of a percent a year; some rechargeables lose several percent a month.
Everything here is optimistic
- Rated capacity is measured under laboratory conditions at a modest constant current and room temperature. Pulsed loads, cold, and a higher cut-off voltage all reduce what you actually get, which is what the usable-fraction field is for.
- Regulator efficiency at microamps is usually far worse than its headline figure, which is quoted near full load. A buck converter that is 92% efficient at 500 mA may be 40% efficient at 50 µA, and its own quiescent current may exceed your sleep budget. This is frequently the single largest error in a battery-life estimate.
- Temperature. Capacity falls in the cold and self-discharge rises in the heat. A cell rated at 20 °C can deliver a fraction of that at −20 °C.
- Ageing. Capacity fades with cycles and with calendar time whether you use it or not.
- Peak current still matters — not for runtime, but because a cell’s internal resistance turns a transmit burst into a voltage dip that can brown out the device. High internal resistance at low temperature is a classic cause of a product that works all summer and resets every winter morning.
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