An ampere-hour ar amp-hour (SI symbol A·h or A h; also denoted Ah) is a unit of electric charge, having dimensions of electric current times time, equal to the charge transferred by a steady current of one ampere flowing for one hour, or 3600 coulombs. The ampere-hour is frequently used in measurements of electrochemical systems such as electroplating and incorrectly, the capacity of electrical batteries(note: first example below) (a battery constituent material's specific capacity is commonly expressed in mA·h/g). The commonly seen milliampere-hour (mA·h or mAh) is one-thousandth of an ampere-hour (3.6 coulombs). This is commonly used for marketing purposes, but it does not express the real capacity of a battery. A milliampere-second (mA·s) is a unit of measure used in X-ray imaging, diagnostic imaging, and radiation therapy. This quantity is proportional to the total X-ray energy produced by a given X-ray tube operated at a particular voltage. The same total dose can be delivered in different time periods depending on the X-ray tube current. The Faraday constant is the charge on one mole of electrons, approximately equal to 26.8 ampere-hours. It is used in electrochemical calculations. An ampere-hour is not a unit of energy. In a battery system, for example, accurate calculation of the energy delivered requires integration of the power delivered (product of instantaneous voltage and instantaneous current) over the discharge interval. Generally, the battery voltage varies during discharge; an average value or nominal value may be used to approximate the integration of power.

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