Battery Life Calculator
How long a battery lasts at a given load, with a realistic derating.
How it works
Runtime is capacity divided by load, then reduced by everything that makes a real cell fall short of its label: internal resistance, the cutoff voltage of whatever it is powering, temperature, and age.
Eighty to ninety per cent of nameplate is a reasonable working assumption for a healthy cell at a modest discharge rate. Push the current high and the usable capacity drops further — a phenomenon named after Peukert, and the reason a battery rated over twenty hours delivers noticeably less over two.
Common questions
Why does my device die sooner than this predicts?
Peak draw, not average. A radio transmitting in bursts pulls far more than its average, and the voltage sag during those bursts can trip the low-voltage cutoff while charge remains.