Battery Life Calculator
Estimate battery runtime from capacity, device current draw and usable efficiency.
Assumptions used in this calculation
- Constant current draw: Assumes a constant average current draw, real devices often draw variable current (bursty in idle/active states), so actual runtime will vary.
About this calculator
A battery's mAh rating printed on the label looks like it should divide cleanly by a device's current draw to give runtime in hours, but real batteries don't deliver their full rated capacity under load, internal resistance, voltage sag, and the cutoff voltage at which a device stops working all eat into what's actually usable, which is why devices routinely run out sooner than a naive calculation predicts. This calculator multiplies the battery's rated capacity by a usable-efficiency percentage (typically 80-90% for consumer cells) before dividing by the device's average current draw in milliamps, giving a more realistic runtime estimate in hours. It's useful for comparing battery options for a portable project, estimating how long a power bank will keep a device running, or sizing a battery pack against a known load current, without assuming the unrealistic best case of 100% usable capacity that a simple capacity-divided-by-current calculation implies.
Worked example
3000mAh battery, 200mA draw, 85% efficiency
Result: 12.75 hours
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