Battery Life Calculator
Runtime with a derating factor, because the ideal figure never happens.
Runtime with a derating factor, because the ideal figure never happens.
0.7 to 0.85 is typical for consumer cells.
Realistic life (hours)
16
0.67 days · ideal figure 20 h
Capacity divided by draw gives 20 hours, and that figure is always optimistic. Real cells lose usable capacity to self-discharge, temperature, internal resistance and the voltage cutoff of whatever they are powering — so the derating factor is applied openly rather than left implicit in a number that will not be met.
How the Battery Life Calculator works
Capacity divided by draw gives the ideal runtime, and that figure is always optimistic. Real cells lose usable capacity to self-discharge, temperature, internal resistance and the voltage cutoff of whatever they power, so a derating factor is applied openly rather than hidden.
Also known as: how long will my battery last · mah to hours calculator · battery runtime calculator · battery capacity calculator
Where to go next
The Battery Life question rarely arrives on its own. These are the ones that usually come with it:
- LED Resistor Calculator — The series resistor, rounded up to a standard value.
- Power Calculator — Watts, kilowatts and horsepower, with a running cost.
- Transformer Calculator — Turns ratio, secondary voltage and current, with power conserved.
- Speed Distance Time Calculator — Any one of the three from the other two, in four units.
Frequently asked questions
How do I calculate battery life?
Divide capacity in mAh by draw in mA. A 2,000 mAh battery at 100 mA lasts 20 hours in theory, and around 16 in practice.
What derating factor should I use?
0.7 to 0.85 for most consumer cells. Lower for cold conditions, high discharge rates, or devices that cut off well above the cell's minimum voltage.
Why doesn't my battery last as long as the maths says?
Capacity ratings are measured under gentle, warm, laboratory discharge. Real use is none of those, and the gap is routinely 15–30%.
Does a higher current draw reduce total capacity?
Yes. This is Peukert's effect. Drawing harder gets less total energy out of the same cell, which is one of the things the derating factor stands in for.
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Related calculators
LED Resistor Calculator
The series resistor, rounded up to a standard value.
OpenPower Calculator
Watts, kilowatts and horsepower, with a running cost.
OpenTransformer Calculator
Turns ratio, secondary voltage and current, with power conserved.
OpenSpeed Distance Time Calculator
Any one of the three from the other two, in four units.
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