Battery Life Calculator

The formula in brief

Runtime is the battery capacity divided by the current draw, multiplied by the efficiency. With capacity in milliampere-hours and draw in milliamperes, the result comes out in hours.

For example, a 4000 mAh battery drawing 500 mA would ideally last 8 hours. At 85 percent efficiency the realistic estimate drops to about 6.8 hours.

What you enter and what you get

You enter the battery capacity in milliampere-hours, the device's current draw in milliamperes and an efficiency in percent. The result shows the estimated runtime in both hours and minutes.

Efficiency accounts for losses in voltage conversion and heat. A realistic value is often around 80 to 90 percent, which trims the theoretical runtime a little.

When it helps

The calculator is useful for comparing batteries, sizing a power bank or estimating how long field sensors will keep running. It is also handy for hobbyists and DIY electronics builds.

Good to know

The figure is indicative only. In reality temperature, battery ageing and fluctuating load shorten runtime noticeably. Treat the estimate as a starting point rather than a guarantee.