Battery Life Calculator
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How long will a battery last? Runtime ≈ capacity (mAh) ÷ current draw (mA).
Battery & load
Estimated runtime
20h 0m
20 hours at constant load
- Total minutes
- 1200
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Battery runtime
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.
Common questions
Why is actual life shorter than calculated?
Battery chemistry, temperature, and state of charge curves reduce usable capacity. Old cells also lose mAh rating over cycles.
Can I use Ah instead of mAh?
Yes — convert: 1 Ah = 1,000 mAh. Enter mAh in this calculator for phone and Li-ion pack labels.
Does this work for coin cells?
Same formula applies if you know mAh and average µA/mA draw. Watch self-discharge on very low loads over months.