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Battery Life Calculator
Calculates how long a battery will run a device from its capacity in mAh and the average current draw, with a realistic usable fraction.
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Enter your figures and press Calculate.
In short
Battery Life Calculator is a free online tool on inputtesthub.com that calculates how long a battery will run a device from its capacity in mAh and the average current draw, with a realistic usable fraction. It needs no account, no download and no payment, and there is no daily usage limit. Everything is processed inside your own browser, so the data you paste is never uploaded to a server.
How to use the Battery Life Calculator
- Enter your figures in the input fields.
- The result updates as you type, together with the full working.
- Check the breakdown table to see exactly how the number was reached.
About the Battery Life Calculator
The Battery Life Calculator calculates how long a battery will run a device from its capacity in mAh and the average current draw, with a realistic usable fraction. It runs as plain JavaScript inside the page, which means there is no upload step, no queue and no size limit imposed by a server — the only limit is your own device.
What people use it for
- Estimating how long a project will run on a battery.
- Comparing two cells for the same device.
- Working out whether a device will last a working day.
Details
| Healthy lithium cell | ≈ 85% usable |
|---|---|
| Few years old | ≈ 70% |
| Cold weather | Lower again |
| Energy | Wh = mAh × V ÷ 1000 |
The maths behind it
This tool uses Ideal runtime = capacity ÷ draw. Realistic runtime multiplies that by the usable fraction, because a cell cannot be run flat and loses capacity with age and cold. Every intermediate value is shown on screen, so the result can be checked by hand.
Capacity ÷ draw is the ceiling, never the answer. Voltage sags under load so the regulator pulls more current as the cell empties, and capacity falls with age and temperature.
Frequently asked questions
Is the Battery Life Calculator free?
Free, unlimited, and no email address required. You can use it a hundred times a day and nothing changes.
Does anything I enter leave my computer?
No. The Battery Life Calculator runs entirely in your browser using JavaScript. Your input never reaches inputtesthub.com or any third party — open your browser's Network tab while you use it and you will see no request go out.
How is the result calculated?
It uses Ideal runtime = capacity ÷ draw. Realistic runtime multiplies that by the usable fraction, because a cell cannot be run flat and loses capacity with age and cold. The full working is printed under the result, so you can follow every step rather than trusting a black box.
What is the Battery Life Calculator used for?
The jobs it is built for: estimating how long a project will run on a battery. Comparing two cells for the same device. Working out whether a device will last a working day. It is built for exactly those cases rather than being a general-purpose tool with this bolted on.
What do people usually get wrong here?
Capacity ÷ draw is the ceiling, never the answer. Voltage sags under load so the regulator pulls more current as the cell empties, and capacity falls with age and temperature.
What are the key numbers for the Battery Life Calculator?
Healthy lithium cell is ≈ 85% usable. Few years old is ≈ 70%. Cold weather is Lower again. These are printed on the page itself, so you do not have to take them on trust.
Can I rely on this for exact work?
The calculation runs in double precision and the result is shown to ten significant figures, so converting a value and converting it back returns the original. What is displayed is rounded for readability; what is copied is not.
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