Battery Life Calculator

For a simple constant-current load, battery life in hours is capacity in milliamp-hours (mAh) divided by load current in milliamps (mA). A 3,000 mAh cell supplying 150 mA ideally runs about 20 hours before depletion.

Real runtime is shorter — voltage sag, temperature, age, and cutoff circuitry stop devices before zero volts. Peaks above average current (Wi-Fi bursts, motors) reduce effective capacity versus a steady bench load.

Example: 2,500 mAh at 500 mA average draw → 5 hours nominal. At 50 mA for a low-power sensor node → about 50 hours. Size upstream for peak current, not average alone.

Lithium-ion phone packs list mAh at the nominal cell voltage — actual usable mAh drops as the cell ages and after hundreds of charge cycles. Budget 80–90% of label capacity for multi-year projects.

Peaks matter: a device that averages 150 mA but spikes to 800 mA during radio transmit may brown out on a supply sized only for the average. Add bulk capacitance or headroom in the regulator rating.

Worked example with defaults: 3,000 mAh capacity ÷ 150 mA draw = 20 hours nominal. Doubling draw to 300 mA halves runtime to 10 hours — the relationship is linear for this ideal model.

Duty-cycled devices need a weighted average current, not peak alone. A Wi-Fi sensor drawing 150 mA for 2 seconds every 60 seconds averages roughly 5 mA — far longer runtime than 3000 mAh ÷ 150 mA would predict if you sized for the burst. Burst Wi-Fi at 150 mA for 2 s every 60 s averages ~5 mA — linear 3000 ÷ 150 overstates life unless duty cycle is modeled.

Peukert's law (for lead-acid) and lithium discharge curves mean mAh ratings assume a specific C-rate. Drawing 3000 mA from a 3000 mAh phone cell (1C) often yields less than one hour; at 150 mA (0.05C) you may slightly exceed the linear estimate. 1C draw from a 3000 mAh cell often finishes under one hour — 150 mA at 0.05C may slightly beat the ideal linear estimate.

Parallel identical cells add mAh while series adds voltage. Two 3000 mAh cells in parallel at the same voltage still supply 150 mA load but share it — each cell sees ~75 mA, extending cycle life compared to one cell carrying the full draw. Parallel 3000 mAh cells share a 150 mA load (~75 mA each) — series adds voltage, not runtime hours at the same current.