Battery Life Calculator
Estimate battery runtime from capacity and current draw.
This tool is for informational and educational purposes only. It is not a substitute for professional financial, medical, legal, or engineering advice. See Terms of Service.
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This calculator estimates how long a battery will power a device based on the battery's capacity and the device's current draw. The formula is simple: hours = mAh / mA.
- Enter battery capacity. Find this on the battery label or datasheet, in milliamp-hours (mAh). A typical AA alkaline battery is 2500 to 3000 mAh. A smartphone battery is 3000 to 5000 mAh.
- Enter current draw. This is how much current your device draws from the battery, in milliamps (mA). Check the device specifications or measure with a multimeter. If your device uses amps, multiply by 1000.
- Read the result. The calculator shows estimated runtime in hours and days.
Note that this gives an ideal estimate. Actual battery life is lower due to temperature, battery age, discharge rate efficiency, and the fact that most devices stop working before the battery is fully depleted.
About Battery Life Calculations
The Peukert effect means that batteries deliver less total capacity at higher discharge rates. A 2000 mAh battery discharged at 2000 mA may only deliver 1 hour, but discharged at 100 mA may deliver more than 20 hours due to improved electrochemical efficiency. This calculator uses the ideal formula; for high-current applications, apply a Peukert correction or use the manufacturer's discharge curves.
For devices with variable current draw (sleep modes, radio transmissions), use the average current consumption for a realistic estimate. If your device sleeps at 0.1 mA and transmits at 200 mA for 1 second per minute, the average is roughly 3.5 mA.
Frequently Asked Questions
How do I calculate battery life in hours?
Battery life in hours equals battery capacity in mAh divided by current draw in mA. For example, a 3000 mAh battery powering a device that draws 150 mA will last approximately 3000 / 150 = 20 hours. This assumes a constant current draw and ignores real-world efficiency losses.
What is mAh and how does it relate to battery life?
mAh stands for milliamp-hours. It represents the total charge a battery can deliver. A 1000 mAh battery can theoretically supply 1000 mA for 1 hour, or 100 mA for 10 hours, or 10 mA for 100 hours. Higher mAh means longer battery life for the same device.
Why does my actual battery life differ from the calculated estimate?
Several factors reduce actual battery life below the theoretical maximum: battery age (capacity degrades with charge cycles), temperature (cold reduces capacity significantly), the Peukert effect (higher discharge rates reduce efficiency), voltage cutoff (devices stop before the battery is fully depleted), and varying current loads. Expect real-world life to be 70 to 85% of the calculated estimate under normal conditions.
How long does a 18650 battery last?
A typical 18650 lithium-ion cell has a capacity of 2000 to 3500 mAh. At 100 mA draw (typical for a small IoT device), a 3000 mAh cell lasts about 30 hours. At 500 mA (a small flashlight), about 6 hours. At 3000 mA (a powerful flashlight on high), about 1 hour. Enter your specific capacity and current draw above for a precise estimate.