Battery Life Calculator
Estimate how many hours or days a battery lasts from its capacity and the load current, including Arduino and IoT devices that sleep most of the time.
For a device that is always on, set time active to 100%.
How it's calculated
The average current is active current × share + sleep current × (1 − share), and the usable capacity is divided by it. Cutoff voltage, temperature and ageing mean you rarely get the full rated capacity, so 70 to 90% is a realistic figure. Self-discharge is not included, so leave extra margin for devices meant to run for months.
Frequently asked questions
How long will a 2000 mAh battery last?
At a steady 100 mA, about 17 hours if you plan on 85% of the capacity (1700 mAh ÷ 100 mA). Enter your own device's current above.
Why plan on less than the rated capacity?
Rated capacity is measured under ideal conditions. Real devices shut off at a cutoff voltage, run hot or cold, and batteries lose capacity with age, so 70 to 90% is a safer planning figure.