What is Ampere Hour (Ah) ? (Chapter 2 Lesson 3)

If you’ve ever looked closely at a battery, you’ve probably noticed a number printed on it, such as 2000 mAh, 5000 mAh, or 5 Ah.

This number is called the battery capacity, and it tells us how much electrical charge the battery can store.

Understanding battery capacity is important because it helps us estimate how long a battery can power a device before it needs to be recharged or replaced.

What is battery capacity?

Battery capacity is the amount of electrical charge a battery can store and deliver.

It is commonly measured in:

• Ampere-hour (Ah)
• Milliampere-hour (mAh)

For larger batteries, the capacity is usually expressed in Ah, while smaller batteries, such as those used in smartphones and portable electronics, are typically rated in mAh.

For example:

• 1000 mAh = 1 Ah
• 5000 mAh = 5 Ah

A common misconception about batteries

Before learning about battery capacity, let’s clear up one common misunderstanding.

Many beginners think that if a battery is capable of supplying 5 A, it will always force 5 A through any device connected to it. This is not true.

The amount of current drawn from a battery depends primarily on the load, not the battery. For example, suppose a battery can safely deliver up to 5 A, and you connect a device that only requires 2 A. Only 2 A will flow because that’s all the device needs to operate.

Now imagine connecting a device that requires 10 A.

The battery cannot provide more current than its safe limit. Depending on the battery and protection circuitry, the voltage may drop, the protection circuit may disconnect the battery, or the battery may overheat if overloaded.

The important point is that the load determines how much current it tries to draw, while the battery determines how much it can safely supply.

What does Ah mean?

The Ampere-hour (Ah) rating tells us how much electrical charge a battery can deliver over time.

For example, a battery rated at 5 Ah can ideally supply:

• 5 A for 1 hour
• 2.5 A for 2 hours
• 1 A for 5 hours
• 0.5 A for 10 hours

In each case, the battery delivers approximately the same total amount of charge before becoming discharged.

Similarly, a 5000 mAh battery is simply another way of writing 5 Ah.

Calculating battery runtime

If the battery capacity and load current are known, the approximate operating time can be calculated using:

Battery Runtime (hours) = Battery Capacity (Ah) ÷ Load Current (A)

Let’s look at a few examples.

Example 1

A battery has a capacity of 1.3 Ah and powers a load that draws 2.6 A.

Runtime = 1.3 Ah ÷ 2.6 A = 0.5 hours

So the battery will last for approximately 30 minutes.

Example 2

Now suppose the same battery powers a circuit drawing 100 mA.

First, convert the current into amperes:

100 mA = 0.1 A

Runtime = 1.3 Ah ÷ 0.1 A = 13 hours

So, under ideal conditions, the battery can power the circuit for approximately 13 hours.

Does this calculation always give the exact runtime?

Not always.

The runtime calculated above is an ideal estimate.

In practice, the actual runtime depends on several factors, including:

• Battery age
• Temperature
• Discharge rate
• Battery chemistry
• Efficiency of the connected device

As a result, the real operating time may be slightly lower than the calculated value.

Ah vs Wh

It’s important not to confuse Ampere-hours (Ah) with Watt-hours (Wh).

• Ah tells us how much electrical charge a battery can store.
• Wh tells us how much energy the battery stores.

Two batteries may both have a capacity of 5 Ah, but if one is 3.7 V and the other is 12 V, the 12 V battery stores much more energy.