Battery Life Calculator

The battery life of a small device is roughly its battery's capacity divided by the current the device draws:

Hours = Capacity / Consumption

with the capacity in milliamp-hours (mAh) and the consumption in milliamps (mA), or Ah and A. Enter any two of the three, and the calculator finds the third. It suits clocks, remotes, sensors and microcontroller projects on AA, AAA, coin or lithium cells.

Examples

1. How long a battery lasts: a clock drawing 10 mA from 2,000 mAh.

   2,000 / 10 = 200 hours, about 8 days

2. The capacity needed: 50 hours at 20 mA.

   50 * 20 = 1,000 mAh

3. The current allowed: 60 hours from 3,000 mAh.

   3,000 / 60 = 50 mA

Devices that sleep

Most battery devices sleep most of the time and wake briefly, so use their average current. A sensor that draws 20 mA for 1 second each minute, and 0.05 mA asleep for the other 59 seconds, averages:

(20 * 1 + 0.05 * 59) / 60 = 0.3825 mA

On 2,000 mAh that is 2,000 / 0.3825 = 5,229 hours, about 218 days.

Real batteries

The result is an upper limit. In practice, plan on about 70–85% of the rated capacity: capacity falls with cold, high currents, age, and the cut-off voltage at which a device stops working.

A capacity given in watt-hours or milliwatt-hours is turned into mAh by dividing by the battery's voltage: mAh = mWh / V. A 10,000 mWh battery at 3.7 V is about 2,700 mAh.

Frequently asked questions

How many hours does a 3,000 mAh battery last?

It depends on the current: at 100 mA, about 30 hours; at 300 mA, about 10; at 10 mA, about 300. Divide the capacity by the current.

Why does my device run out sooner than calculated?

The current is usually higher than the label or the sleep figure suggests, for example while a radio transmits or a display is on. Measuring the real average current gives the best estimate.