Why batteries are rated in Ah

Why Batteries Are Rated in Ampere-hour (Ah)?

The Ampere-hour Rating of a Battery.

When you check the side of your rechargeable battery, you would observe some ratings in Ah. That is referred to as the charge or coulometric capacity of the battery. And you might wonder why it is in Ah and not Ampere, Watt, VA and so on.

It will interest you to know that batteries are not just rated in Ah. They can also be rated in some other ways. First, let’s understand what battery ratings and capacity means.


Understanding Battery Rating and Capacity


Battery rating is used to rank or classify batteries. This rating can be done in terms of voltage, capacity, cycle life and so on. But a lot of people are more interested in the rating based on its capacity.

A battery’s capacity shows the amount of energy that can be contained in a battery or how much energy can be delivered as an output. The battery capacity changes as the battery ages, gets discharged/charged or exposed to unsuitable temperature. That is, a battery that was once a 100Ah will not remain that forever. Yes, the same applies to your phone battery as well.

So, a battery’s capacity is a way of rating a battery.

Battery capacities can be in Ampere hour (Ah), Watt (W) or Watt-hour (Wh). However, Ampere hour (Ah) is most commonly used for rating batteries.

Key points: Battery rating is a way of ranking or classifying batteries while a battery’s capacity is the measure of how much energy can be stored or delivered by the battery. Battery capacity can be measured in Ah, W, and Wh, but the most used is the Ah.

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Why Does Battery Have to be Rated in this Ampere-hour?

Do note that it is not all about the ampere-hour of the battery. The ampere-hour is very important but so is the voltage rating, C-rating, and other ratings of the battery.

Batteries store electrical energy in the form of chemical energy and discharge in the form of electrical energy. The electrical energy that can be discharged by a battery is determined by the amount of charges the battery can ignite to flow through external circuits. This amount of charges that is discharged from the battery is called the Ampere-hour (Ah).

The amount of charge is given by the formula: Q =It.

Q = amount of charge, I = current (A) and t = time (h)

So, the Ampere hour (Ah) simply shows the charge that is available in a fully charged battery to deliver one amp of current for one hour.

For example, if a 12V battery is rated 48Ah, it theoretically means the fully charged battery can discharge 48 Amps for one hour or 12A for 4hours and so on. Thus, Ampere hour (Ah) is about the current(A) discharged by the battery and its discharge time(h).

Also, the current discharge limit is measured by a rating called C-rating. The C-rating tells you the current that can be used to charge the battery or rate at which the battery can be discharged. And it can only be derived from the Ampere-hour rating.

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For example, a 48Ah battery doesn’t necessarily mean it will discharge 48amps for one hour. If its C-rating is 0.5C, it would mean that it can discharge 24A for 2hours. So, the Ah serve as a guide to knowing batteries’ continuous discharge current and time.

Without Ah, it might be difficult to determine the steady amount of current that can be drawn from a battery at a specific time.

Since Ampere-hour shows how long a battery can keep delivering a certain amount of current, it became the most important to a lot of people. Even though there are other ways of rating a battery.

So, the reason why batteries are rated in Ampere-hour is that it indicates the amount of charge (discharge current and time) it can deliver. And it guides users to knowing batteries discharge current limit.

Key points: The amount of charge in a battery has the formula, Q = It. Batteries are rated in Ampere-hour because it practically shows the amount of charge available and can be delivered by the battery. Ah helps determine the steady amount of current that can be delivered by a battery.

Why is Battery not Rated in Watt-hour (Wh)?

Actually, batteries are also rated in watt-hour . Watt-hour describes the energy capacity of the battery but is not as popular as Ah. That is, it describes how much energy can be delivered from a fully charged battery. Sometimes, you see both the Wh and Ah rating written on a battery. And even if you see only one of them, you can derive the other by using the formula: Energy (Wh) = Ah * V. Where V is the Battery’s nominal voltage.

For example, if a 12V battery is rated 125Ah, you can get the Wh rating by multiplying the Ah by the batteries voltage. That is, 125Ah * 12V = 1500Wh

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So, it is a matter of choice from the battery manufacturers. Batteries can be rated in Watt-hour too.

Key points: Batteries can be rated Watt, Watt-hour, Ampere-hour, and so on. it all depends on the battery manufacturer.

Why Batteries are not Rated in KW or KVA?

Some batteries are now rated in W but never in KVA. It is never rated in KVA (apparent power) because batteries stores and delivers direct current (DC), and in DC, there is only active power. Or you can say that in DC, apparent and active power is the same. Thus, power factor is 1. You can learn about active and apparent power

The Watt rating in batteries isn’t common because it signifies power which is instantaneous. Batteries are about energy, which isn’t instantaneous.

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