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How will you describe battery capacity and discharge and its ampere-hour?

A battery's capacity is the amount of electrical charge it can deliver at the rated voltage. The more electrode material contained in the cell the greater its capacity. A small cell has less capacity than a larger cell with the same chemistry, although they develop the same open circuit voltage. Capacity is measured in units such as amp-hour(A-h).

The rated capacity of a battery is usually expressed as the product of 20 hours multiplied by the current that a new battery can consistently supply for 20 hours at 20'C while remaining above a specified terminal voltage per cell. For example, a battery rated at 100Ah can deliver 5A over a 20-hour period at room temperature.

The fraction of the stored charge that a battery can deliver depends on multiple factors, including battery chemistry, the rate at which the charge is delivered, the required terminal voltage, the storage period, ambient temperature and other factors.

The higher the discharge rate, the lower the capacity. The relationship between current discharge time and capacity for lead acid battery is approximated(over a typical range of current values) by Peukert's law,
          Cp is the capacity when discharged at a rate of 1amp
           I is the current drawn from a battery (A)
          T is the amount of time (in hours) that a battery can sustain
          k is a constant around 1.3

Batteries that are stored for a long period or that are discharged at a small fraction of the capacity lose capacity due to the presence of generally irreversible side reactions that consume charge carriers without producing current. This phenomenon is known as internal self-discharge. Further, when batteries are recharged, additional side reactions can occur, reducing capacity for subsequent discharges. After enough recharges, in essence, all capacity is lost and the battery stops producing power.

Internal energy losses and limitations on the rate that ions pass through the electrolyte cause battery efficiency to vary. Above a minimum threshold, discharging at a low rate delivers more battery's capacity than at a higher rate.

Installing batteries with varying A-h ratings does not affect device operation rated for a specific voltage unless load limits are exceeded. High- drain loads such as digital cameras can reduce total capacity, as happens with alkaline batteries. For example, a battery rated at 2000 mAh for a 10 or 20-hour discharge would not sustain a current 1A for a full two hours as its stated capacity implies.

C Rate:
The C-rate is the multiple of the current over the current that a battery can sustain for one hour. A rate of 1C means that an entire 1.6 Ah battery would be discharged in one hour at a discharge current of 1.6 A. A 2C rate would mean a discharge current of 3.2 A over one half-hour.

An ampere-hour or amp-hour(A-h or Ah) is a unit of electric charge, 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 electrical batteries. The commonly seen milliampre-hour(mA-h or mAh) is one thousandth of an ampere-hour(3.6 coulombs).

A milliampere-second(mAs) 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 thr 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.

An AA size dry cell has a capacity of about 2 or 3 amperes-hours. Automotive car batteries vary in capacity but a large automobile propelled by an internal combustion engine would have about a 50 ampere-hour battery capacity. Since one ampere-hour can produce 0.336 grams of aluminium from molten aluminium chloride, producing a ton of aluminum requires transfer of at-least 2.98 million ampere-hours.


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