Battery Life Calculator

Estimates how long a battery will run a device, or how big a battery a target runtime needs. For a steady load the answer is capacity divided by current. Most battery-powered electronics are not steady, though: a sensor wakes, measures, transmits and sleeps again, so the tool also takes an active current, how long it lasts, the sleep current and how often the cycle repeats, and works out the average current the battery really sees.

That average is where the surprises are. A sensor drawing 20 mA for 100 ms every 10 seconds and 5 µA asleep averages about 205 µA, so a 2000 mAh cell lasts roughly 9,760 hours — a little over 13 months. The tool shows what share of the charge goes to the active bursts and warns when sleep current is the bigger drain, because at long intervals a few microamps of leakage, a regulator's quiescent current or a forgotten pull-up resistor can outweigh the radio entirely.

Real batteries rarely deliver their full rating. The usable-capacity field derates it for the cut-off voltage of your circuit, cold temperatures and ageing; 70–90 % is a common planning range, and the tool leaves it at 100 % until you choose. Capacity can be entered in mAh, Ah or Wh; Wh is converted with the nominal voltage, which is also used for the energy and average-power figures.

For lead-acid batteries, Peukert's law corrects for the capacity lost at high current. A battery rated C amp-hours over H hours lasts H·(C/(I·H))^k at a current I, where k is typically 1.1–1.3 for lead-acid and close to 1.05 for lithium-ion. A 100 Ah battery rated over 20 hours with k = 1.2 runs a 20 A load for about 3.8 hours, not the 5 hours plain division suggests. The law describes a constant current, so with a pulsed load it is only an approximation.

The reverse mode takes a target runtime and returns the capacity needed, in Ah and Wh, applying the same derating and Peukert correction. The model ignores self-discharge, which matters over years (a few percent per month for NiMH, much less for lithium primaries), and the voltage sag that can reset a device before the cell is empty. All figures are computed in your browser.