# Battery Life Calculator- How long will it power a circuit?

How long will a battery power a circuit? Learn how to estimate battery life in this article.

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How long will a battery power a circuit? Learn how to estimate battery life in this article.

Scroll to the bottom to watch the YouTube tutorial.

We use batteries to power our circuits. But how long can a battery power a circuit for? When we look at the packaging or data sheet for a battery, we see a value with the letters Mah next to it. This is the milliamp. Our ratio rating.

For example, this one has a rating of 2500 Mac that tells us it could theoretically provide a current of 2500 M for 1 hour or 1250 milliamps for 2 hours, or 20 milliamps for 125 hours. However, in real life, it probably won’t actually last this long because the chemical reaction slows, so the internal resistance of the battery changes as it empties. There are lots of other things that affect this, such as the age and the temperature. There’s no real way to precisely calculate the lifespan. The best way is to simply test it.

We can, however, make an estimate of the lifespan with the following formula. The battery life equals the capacity in milliamp hours divided by the circuit current in milliamps. So for example, in this circuit we calculate a demand of 19 milliamps and the battery has a capacity of 3000 milliamp hours, so 3000 divided by 19 gives us 157.9 hours. But this really is the best case scenario, though, and in reality it almost certainly won’t achieve this. We have also built a free, simple calculator on our website where you can estimate the runtime of a battery as well as the required capacity. Check it out HERE.