Ampere-hour
From Wikipedia, the free encyclopedia
An ampere-hour (abbreviated as A·h) is a unit of electric charge. Although it is not a direct measure of the energy in a battery (like the joule (J) or watt-hour (W·h)), it is a common measurement of how long a battery will last (or in the case of a rechargeable battery, how long it will last when fully charged).
One ampere-hour is equal to 3600 coulombs (ampere-seconds), and indicates the amount of electric charge that passes either terminal of the battery when it provides one ampere of current flow for one hour. The commonly seen milliampere-hour (mA·h) is equal to 3.6 coulombs.
However, in reality, the available capacity of a battery depends on the rate at which it is discharged. If a battery is discharged at a relatively high rate, the available capacity will be lower than expected. Therefore, a battery rated at 100 A·h (360000 coulombs) will deliver 20 A (20 coulombs per second) over a 5 hour period, but if it is instead discharged at 50 A (50 coulombs per second), it will run out of charge before the theoretically expected 2 hours. For this reason, a battery capacity rating is always related to an expected discharge time, which is typically 5 or 20 hours.
The relationship between current, discharge time and capacity is expressed by Peukert's law.
In general, the higher the ampere-hour rating, the longer the battery will last for a certain device. Installing batteries with different A·h ratings will not affect the operation of a device rated for a specific voltage.
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[edit] Battery Life
The amount of time a battery will last can be calculated like this:
- <math>t=\frac{C}{I}</math>
<math>t</math> is the discharge time in hours, <math>C</math> is the battery's current capacity rating, and <math>I</math> is the current draw from the battery. If the batteries are connected in parallel, then multiply the capacity rating by the number of cells.
For example, a CD player draws a constant current of 200mA from two rechargeable AA batteries that have a capacity of 2300mAh each. So, 2.300Ah / 0.200A = approximately 11.5 Hours of battery life. Remember that most electronic devices stop functioning when the battery voltage drops below a certain voltage, so the actual battery life will be less than calculated.
The actual time will be much less than calculated becase this assumes that the battery voltage remains constant, which it does not, so its only a rough approximate. To learn more, check a Datasheet for a standard Duracell battery (code starts with MN####).
[edit] Conversion to energy
The A·h rating of a battery is related to, but not the same as, the amount of energy it stores when fully charged. If two batteries have the same nominal voltage, then the one with the higher A·h rating stores more energy. It would also typically take longer to recharge.
The energy E available from a battery is approximately given by:
- <math>E= QV\,</math>
where
- Q is the charge, and
- V is the nominal voltage.
This yields:
- number of joules = number of ampere-hours × number of volts × 3600 seconds per hour, or
- number of watt-hours = number of ampere-hours × number of volts.
This is only an approximation though, due to the fact that the voltage during discharge is not actually constant.
[edit] Other units
The SI unit of electric charge is the coulomb. One ampere-hour is equal to 3600 coulombs.
[edit] See also
fr:Ampère-heure it:Amperora nl:Ampère-uur pl:Amperogodzina fi:Ampeeritunti sv:Amperetimme
