Battery Discharge Time Calculator

The ampere-hour as a standardized capacity unit entered engineering practice in the 1890s alongside Gaston Planté's lead-acid batteries, which powered the first electric vehicles and street-railway emergency lighting. Capacity ratings were originally specified at a single discharge rate — usually 10 or 20 hours — because rate strongly affects delivered capacity. Wilhelm Peukert's 1897 empirical correction gave engineers the first quantitative model of that rate dependence, still used unchanged in modern solar and off-grid calculations.

End-of-discharge cutoff voltages emerged from cycle-life studies in the 1930s. Running a lead-acid cell below ~10.5 V (on a 12 V pack) accelerated sulfation and permanent capacity loss; limiting discharge to 50% depth-of-discharge (DoD) roughly quadrupled the usable cycles. Lithium-ion cells in the 1990s inherited a similar rule: they survive far more cycles if discharged only to 2.8–3.0 V per cell rather than the absolute 2.5 V minimum. Every modern Battery Management System enforces chemistry-appropriate cutoffs in firmware to preserve long-term capacity.

Watt-hours as the preferred energy unit (rather than amp-hours alone) took over once variable-voltage loads became common. A laptop draws 15 W regardless of battery voltage; specifying battery energy in Wh lets you compute runtime directly. Modern data-sheets for cells, packs, and portable devices all report Wh alongside mAh for exactly that reason.

Enter battery capacity (accepts mAh, Ah, or Wh with suffixes) and nominal voltage. Specify the load as either current (mA/A) or power (W). Set a usable-capacity percentage based on your chemistry's recommended DoD. The calculator returns runtime, total and usable energy, and the instantaneous draw — useful for sanity-checking spec sheets and sizing backup batteries.

Real-world runtime typically falls 5–15% short of the ideal number because voltage sags under load (converters draw more current to compensate), temperature reduces capacity, and the load may not be constant. For final designs, measure with your actual load on the exact battery you intend to use. All math runs client-side.