Battery Life
Runtime from capacity and load, with a usable-capacity derating.
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The engineering
The 80% default covers what datasheet mAh ignores: cutoff voltage, temperature, self-discharge, and the regulator's tax. High loads make it worse — capacity falls as current rises (Peukert's observation for lead-acid; lithium suffers less but still suffers).
For duty-cycled loads, average the current over a full cycle including sleep — the standby microamps usually decide field life.
Where this math comes from
Alessandro Volta's 1800 pile started the whole problem of asking how long a battery lasts. Wilhelm Peukert measured the honest answer for lead-acid in 1897: available capacity shrinks as you draw harder, by a power law that carries his name.
A century of chemistries later — Gaston Planté's rechargeable lead-acid (1859) to the 1991 Sony lithium-ion cell (Whittingham, Goodenough, Yoshino's 2019 Nobel) — the derating row is still the difference between the brochure and the field report.
- 1800Alessandro VoltaThe voltaic pile — the first battery.
- 1859Gaston PlantéRechargeable lead-acid cell.
- 1897Wilhelm PeukertCapacity vs. discharge-rate law.
- 1991Sony (Whittingham/Goodenough/Yoshino lineage)Commercial lithium-ion changes the budget.
See the full timeline of the math behind every calculator →
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