Estimate battery runtime from capacity (mAh), load current, and efficiency factor.
Enter a battery's rated capacity in mAh, the current your device draws under load in mA, and an efficiency factor between 0 and 1, and the tool divides capacity by current and multiplies by efficiency to return an estimated runtime in hours and minutes. The efficiency factor is what people skip or misunderstand: it's meant to absorb real-world losses like voltage sag and higher-than-rated discharge current, not the battery's own capacity rating. The formula assumes capacity and current are both referenced to the same voltage; if the load current was measured on the output side of a voltage-changing converter, this calculation doesn't apply and you need a watt-hour based comparison instead. Leaving efficiency at 1.0 gives a best-case number that real devices rarely hit.
A concrete input and expected output from the current implementation.
Input
Capacity: 3000 mAh, Load current: 500 mA, Efficiency: 0.85
Expected output
5.1 hours (5 hours 6 minutes)
3000 mAh x 0.85 = 2550 usable mAh, and 2550 mAh / 500 mA = 5.1 hours.
The tool computes runtime (hours) = capacity (mAh) x efficiency / load current (mA), then converts the decimal result into hours and minutes for display. Capacity and load current must be in the same current unit (mAh and mA) for the division to be meaningful; the tool does not convert between Ah and mAh for you.
Efficiency is a single 0-1 multiplier you set to represent everything the raw formula ignores: internal resistance, regulator conversion loss, and capacity drop-off at higher discharge rates. It is a manual estimate, not a measured value, so the output is only as good as the number you enter - the calculator has no way to derive it from the battery chemistry or datasheet.
There is no single correct value - it depends on your regulator, wiring, and discharge rate. A DC-DC converter might run 0.80-0.90; a direct connection with minimal loss could be closer to 0.95. If you know an actual measured runtime for one scenario, back-solve for efficiency and reuse that number rather than guessing.
Likely causes are a higher effective discharge rate than assumed (capacity drops faster at high current, known as the Peukert effect), a cold environment, or an aged battery with reduced real capacity. Try lowering the efficiency factor to match observed behavior rather than treating the first estimate as fixed.
The example and behavioral notes were checked against the browser implementation. Standards and primary references below define the relevant format, formula, or platform behavior.
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