# eli5 why battery capacity is measured in mAh in phones but Wh normally?

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eli5 why battery capacity is measured in mAh in phones but Wh normally?

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Ah is the capacity, Wh is the amount of stored **energy**.

Problem here is that the amount of energy you can *take out* of the battery depends on the discharge rate. The faster you discharge the battery, the lower the total amount of energy in the end.

So imagine you had a water bottle that played by the same rules: It contains water, but how much water you get out of it depends on how fast you pour. Would it be better to print “1L” on the bottle (even though some folk will get 0.7L or 1.4L out of it) or would it be more useful to print “the flow of water will last this many hours if you pour it this quickly”?

Watt-hours allows you to compare battery chemistries that have different voltage. At higher voltage, or if you string together multiple cells, they can obviously pack more punch. Amp hours are only good for one cell of the same type. Knowing the amp hours makes it easier to calculate the charging current an time.

The base unit for energy contents in anything is Joules otherwise known as Watt-seconds. An hour is a more convenient casual unit of time.

mAh is milliamp hours, a measure of current over time. The ampere (amp) is the basic unit used to measure electric current. A milliamp is one one thousandth of an amp. Cell phones (as far as I know) all use the same 3.7 volt lithium battery technology. Only the amps are different, not the volts.

Wh is watt hours, a measure of power over time. Power is amps times volts. So when you’re comparing things with different voltages, you need to use watts. Comparing just amps is not enough if the voltages are different.