Battery stores charge in the form of chemical energy and then converts it into electrical energy to utilize for a specific time. The amount of available charge is the capacity of a cell or battery which may be expressed in Ah (Ampere-hour). Moreover, in a charged battery, the numbers of molecules are limited to create a flow of electron in electric circuits, so, there must be a limited number of electrons in a cell/battery which they motivate through a circuit to fully discharge.
Now we have the option to rate the battery capacity in Number of flowing electrons for a specific time, but, it would be a headache, because there are a vast number of electrons in it. So we have another option (1C (Coulomb) = 6.25 x 1018 electrons, or 6,250,000,000,000,000,000 electrons.
In addition, 1A (Ampere) = 1 coulomb of electrons per second and,
1 hour (h) = 3600 Seconds
Therefore;
1Ah = (1A) x (3600s) = (C/s) x (3600s) = 3600 C.
∴ A (1 Ampere) = 1 Coulomb per second = C/s
But,
Why make up a new unit for battery capacity rating when an old one unit is doing just fine? Of course! To make your lives as technicians and students more difficult ;). As they do for electricity units… i.e. 1 Unit of Electricity = 1kWh = 1 board of Trade Unit.
Why Battery is Not Rated in VA (Volt Ampere) or Watts?
VA stands for volt amps. This is the measurement for apparent power one of the three types of power in AC circuits. The other two are true power, measured in watts and reactive power measured in VARS (Volt Amperes Reactive). The relationship between the three determines a high power factor which is what you want in an AC circuit. Batteries are DC where power factor is unity (i.e. P.f is not not applicable in DC circuits due to zero frequency).
In addition, battery is not a constant voltage source i.e. we only interested in battery capacity which is only for specific time, hence we can’t use Wh instead of Ah for battery as well. That’s why battery is not rated in VA or W instead of Ah.
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