Battery Life Calculator

How it is calculated

Runtime is the capacity divided by the load: ideal hours = capacity (mAh) / load (mA). The tool then shows a second, more cautious figure: realistic hours = ideal hours × 0.85. The fixed 0.85 factor is a rule-of-thumb allowance for conversion losses and for a battery that cannot be drained completely; it is not measured for your cell, temperature or discharge rate. Both inputs must be greater than zero, and both results are given in hours with two decimals. Because the formula is only a ratio, the same arithmetic works for watt-hours divided by watts, as long as both inputs use matching units. It assumes a constant load, which real devices rarely have.

Common mistakes

• Mixing amps and milliamps, for example a 2000 mAh battery with a load entered as 0.15 for 0.15 A. The units must match, so enter 150 mA, otherwise the answer is off by a factor of 1000.
• Using the average current for a device that spends most of its time asleep and wakes in short bursts. Use the duty-cycle average, otherwise a battery lasts far longer than the peak current suggests.
• Treating the printed capacity as usable. Cheap or old cells often deliver less than the label, and cold weather reduces capacity further, so the realistic figure may still be optimistic.
• Comparing batteries of different voltage by mAh alone. Two packs with the same mAh but different voltages store different energy; convert to watt-hours (volts × amp-hours) first.
• Entering the charger or adapter rating as the load. Use the current the device actually draws from the battery, not the current its charger can supply.