# Simple question about power usage

My question is simple for humans but hard to search for in Google or somewhere else, so forgive me if this isn't the best place to ask.

I was looking up the power consumption of an electronic device on several sites and all of them stated "this device takes 100ma" or something like this.

My question: If a device uses "100mA" how much mA per hour does it use?

So if I have a battery of 10.000mAh, how long will it last if the device uses "100ma" as they say?

• Divide capacity by current . So it comes to around 6 mins, which is 0.1 hours. Assuming rest are same. – Umar Mar 12 '17 at 9:50
• Your first question makes no sense: mA per hour is a meaningless unit in battery context. The answer to your second question is 100 hours. – Dmitry Grigoryev Mar 13 '17 at 8:48
• If you use 100mA for 1 hour then you have used 100mAH of energy. It depends on how much time you run it for. – Voltage Spike Mar 13 '17 at 15:52
• Please don't forget to accept an answer if you think it successfully answers your question. – Daniel Tork Mar 15 '17 at 16:53

If you have a battery of 10.000mAh it means you can theoretically run a 10.000mA load for 1 hour until it runs out of juice. If you want to find out how many hours your device can run on that battery you simply divide the capacity of your battery by how much the device sucks up. 10000mAh / 100mA = 100 h of runtime.

• Thank you, that's what I assumed. But then I don't understand why someone said: "...the device will last over 4 hours on 1,400mAh!" But he said, that the device only "uses 120mA". Shouldn't the device then run for more than 10 hours? – Akito Mar 12 '17 at 8:31
• (10 hours is over 4 hours. :-) – skvery Mar 12 '17 at 8:32

As a general rule, you can find out how much will a device's battery last by dividing its capacity by the current consumption (assuming you know exactly what that is) without taking into account the gradual decrease in EMF(electromotive force) or supply voltage as the battery is drained.

In your case, that would be: $\frac{capacity}{current \ consumption}=\frac{10000 mAh}{100 mA}=100 \ hours$

What does the phrase in bold mean? It points out that the device might stop working before the battery is completely empty. Each of them requires a minimum $V_{DD}$ to work, but manufacturers make the most of this capacity. As the battery loses its energy, its EMF drops until a certain point. Also, rechargeable batteries perform better than the alkaline type, because their discharge curve does not show a sharp drop, but instead they have a smoother fall.

You can compare the 2 graphs for yourself and find out which of the two is more efficient. The second image shows a flat curve. It does have a sharp drop, but it appears later, when closer to using the entire capacity.

Batteries have a nominal capacity which will diminish when in certain cold temperature conditions. That's the explanation behind the fact that your car won't start during winter if you keep it outside and don't use it for a while.

The 100 mA estimation might not be accurate enough. The device might enter sleep mode, consume more or less depending on the application and you might want to consider self discharge, too. That's why it helps to know exactly what kind of device it is.