Battery Runtime Calculator

How to use the battery runtime calculator

1. Fill in the fields — they start with a worked example.
2. Read the answer, and the arithmetic shown beneath it.
3. Copy the results, or adjust the inputs to compare scenarios.

About this tool

Divide capacity by current and you get runtime: a 2000 mAh battery supplying 100 mA lasts nominally 20 hours. That is the arithmetic, and it is optimistic in every real case.

Several things eat the difference. A battery's rated capacity is measured under gentle, controlled discharge; drawing harder yields less, an effect that grows sharply at high currents. Voltage falls as the battery depletes, so a regulated device draws progressively more current to maintain the same power. Cold reduces available capacity substantially — a lithium cell at freezing may deliver well under half its rated figure. And most devices refuse to run down to a truly empty cell.

An efficiency allowance is therefore included and set to 85% by default, which is a reasonable figure for a moderate, steady load at room temperature. For a high-current or intermittent load, or anything used outdoors in winter, expect worse.

Treat the result as an upper bound that a real device will fall short of, rather than a prediction.

Common uses

• Sizing a battery for a microcontroller project.
• Estimating how long a power bank will run a device.
• Comparing two batteries for the same load.

Frequently asked questions

Why does my battery last less than calculated?

Rated capacity assumes gentle discharge at room temperature. Higher currents, cold, and devices that cut off before empty all reduce it — which is why an efficiency allowance is included.

What efficiency figure should I use?

Around 85% for a steady moderate load at room temperature. Use 70% or less for high-current, intermittent, or cold conditions.

How do I convert Wh to mAh?

Divide watt-hours by the nominal voltage, then multiply by 1000. A 37 Wh pack at 3.7 V is about 10,000 mAh.