Electric current

What Is An Ampere-hour?

An ampere-hour is a unit of electric charge. It is represented by Ah. The ampere-hour is not part of the International System, since the hour is not either.

What is an ampere-hour?

The ampere-hour indicates the amount of electric charge that passes through the terminals of a battery (or an electrical conductor) providing an electric current of 1 ampere (A) for one hour (h).

The ampere-hour is used to measure the capacity of a battery, that is, the amount of electricity it can store during charging and return during discharge.

A battery that has, for example, a capacity of 100 Ah, theoretically can give a current of 10 amps for 10 hours, or 1 amp for 100 hours.

At a practical level there are certain performance losses conditioned by the battery discharge rate, the faster a battery discharges, the more energy is lost due to internal resistance. Therefore, the load capacity is usually given with reference to a standard discharge time (10 or 20 hours), and for a specific final voltage.

The Faraday constant is used in electrochemical calculations. The Faraday constant is the charge in one mole of electrons, approximately equal to 26.8 amps per hour.

How many Coulombs is an ampere-hour equivalent?

One hour of amps equals 3,600 coulombs (amp-seconds) of electrical charge transferred to a constant current of 1 amp for 1 hour, that is. In 1 second at 1 amp, current will flow 1 load pending.

The unit of electric charge in the International System is coulomb C. Coulomb is a derived unit that is defined in terms of the ampere: 1 coulomb is the amount of total electric charge that passes through a cross section of a conductor through which a electric current of one ampere for one second.

What is the amp-hour used for?

The Ampere-hour is used in batteries, in solar batteries or in electric batteries in general to indicate the capacity. In automobile batteries it is maintained that the 12 V battery must be capable of supplying a discharge electric current of 1 / 20º of the nominal capacity indicated in A · h for 20 hours at a temperature of 25 ° C at a voltage higher than the cutoff which is 10.5V.

The amp-hour is also frequently used in measurements of electrochemical systems, such as electroplating.

The milliampere-second (mA · s) is a unit of measurement used in X-ray imaging, diagnostic imaging, and radiation therapy. This amount 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 administered in different periods of time depending on the current of the X-ray tube.

Watt-hour conversion

Often battery manufacturers indicate in their specifications only the  charge  stored  in mAh (mAh), others  indicate  only the  energy  stored  in  Wh · h  (Wh). Both characteristics can be called the term "capacitance" (not to be confused with  electrical capacitance  as a measure of a conductor's ability to accumulate charge, measured in  farads  ).

In general, it is not easy to calculate the stored energy from the stored charge: the  integration of the  instantaneous power generated by the battery is required for the entire duration of its discharge. If greater precision is not needed, instead of integration, you can use the average values ​​of voltage and current consumption, for this, use the formula that follows the fact that  1W = 1V · 1A  :

1 W · h = 1V · 1A · h.

That is, the stored energy (in watt-hours) is approximately equal to the product of the stored charge (in ampere-hours) and the average voltage (in volts):

E = q · U ,

and in joules it will be 3600 times bigger

E = q · U · 3600

Examples where the amp-hour unit is used

  • An AA size battery has a capacity of approximately 2 to 3 amp-hours.
  • The average smartphone battery generally has between 2,500 and 4,000 milliamps-hour of electrical capacity.
  • Car batteries vary in capacity, but a large car powered by an internal combustion engine would have a battery capacity of about 50 amps per hour.
  • Since one hour of amps can produce 0.336 grams of aluminum from molten aluminum chloride, producing one ton of aluminum requires a transfer of at least 2.98 million hours of amps.

Published: May 13, 2015
Last review: March 28, 2020