BatteriesQuestions - Answers

Why is a Battery Rated in Ah (Ampere hour), Not in VA?

Why are Batteries and Cells Rating in Ah (Ampere hour) and not in VA or Watts?

A 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.

Why is a Battery Rated in Ah (Ampere hour), Not in VA?

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


1Ah = (1A) x (3600s) = (C/s) x (3600s) = 3600 C.

∴ A (1 Ampere) = 1 Coulomb per second = C/s


Why make up a new unit for battery capacity rating when an old one 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.

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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, the battery is not a constant voltage source i.e. we are 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 the battery is not rated in VA or W instead of Ah.

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  1. I really appreciate your sense of humor ………. the last line … make tech. student life difficult. ……………..excellent site for tech help and guidance ……sharing knowledge is jut increasing your knowledge.
    thanks .

  2. How to calculate battery charge?
    Type of motor needed for it and is there a board that I can easily build for it?
    Can you maybe guide me to a site or a book that is available?

    Thank you!

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