Battery Life Calculator
Estimate battery runtime from capacity and load current, or calculate average current for active/sleep duty-cycle devices. Includes mAh, Ah, Wh, efficiency, and usable-capacity derating.
Battery life formula
The simplest estimate divides usable battery capacity by load current. For devices with sleep modes, use average current across the full active/sleep cycle.
Worked example
Real runtime can be lower because battery capacity changes with temperature, discharge rate, age, cutoff voltage, and regulator behavior.
Battery capacity is not always fully usable
A rated 2000 mAh battery may not deliver 2000 mAh in your circuit. High current, cold temperature, old cells, boost converters, and device cutoff voltage all reduce usable energy. For practical estimates, derating the capacity is usually better than assuming perfect lab conditions.
Trusted references
For low-power runtime modeling, see Microchip – Battery Life Estimator and Microchip – nanoWatt XLP low-power design material.
Common questions
-
Divide usable battery capacity in mAh by load current in mA. For example, 2000 mAh ÷ 100 mA = 20 hours before derating.
-
Temperature, battery age, discharge rate, cutoff voltage, regulator losses, and battery chemistry can reduce usable capacity.
-
Usable capacity is the portion of the rated battery capacity your device can actually use before voltage drops too low or performance becomes unreliable.
-
Calculate average current across the active and sleep portions of the cycle, then divide capacity by that average current.
-
mAh measures charge capacity at a given voltage. Wh measures energy and is often better when comparing batteries with different voltages.
-
Not fairly. Convert both to Wh if the voltages are different.
-
Use measured converter efficiency if available. For a rough estimate, 80% to 95% is often used depending on the regulator and load.
-
Yes, many batteries deliver less usable capacity at high discharge rates, especially small cells and alkaline batteries.