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What Do Ah and Capacity Mean on Your Power Tool Battery?


If you're a novice and you've just bought a cordless power tool, you may have noticed that the battery has some numbers on it. One of these is the battery voltage. However, the other number has the letters "Ah" after it. So what does this mean, and what does "Ah" stand for? Even professionals seem to be a bit confused by this spec, so in this short guide, we'll explain what it means.

Bosch cordless combi drill.

Bosch cordless combi drill.

What Does the Voltage Mean on a Power Tool Battery?

This is the easy bit. All electrical supplies and batteries have a voltage, measured in, you guessed it, volts. Voltage is analogous to pressure in a water hose, the higher the voltage, the greater the electrical pressure. The symbol for volts is "V". Power tool batteries typically have voltages of 12 V, 14.4 V, 18 V, 36 V and 54 V. The voltages differ a bit between the USA and the UK and other countries because of how the voltage is specified.

So, for instance, an 18-volt battery in the UK is equivalent to a 20-V battery in the USA. In general, a higher voltage battery gives more power output and is used with tools that demand more power, e.g. circular saws with large diameter blades, demolition hammers and battery lawnmowers. These devices typically use a 54 V battery. This is for technical reasons, because at a higher voltage, less current is required to give the same amount of power. It's more practical to design motors that can use lower currents because the copper windings can be thinner gauge to carry the lower current.

For a more detailed explanation of this, see my guide on basic electricity:
How to Understand Electricity: Volts, Amps and Watts Explained on Appliances

What Does Ah Mean on a Battery?

Batteries store electricity in the form of chemical energy. Basically you charge them and the chemical energy gets converted back to electricity later when you use your power tool. The amount of charge in a battery (not necessarily a power tool battery) is measured in amp-hours or "Ah" for short. The amp-hour capacity can be thought of as the current that the battery supplies multiplied by the time for which it can supply that current before it becomes flat.

Let's take a 4.0 Ah battery.

This can source a current of 4 amps for 1 hour before it goes flat.
How did we work that out?

We know that:

Amp-hour capacity = current x time for which current can be supplied.

Therefore from simple maths:

Time for which current can be supplied = Amp-hour capacity / current

So we divide the Ah capacity by the current drawn:

4 Ah / 4 amps = 1 hour. So it's that simple. Well, not exactly!

Imagine if the current drawn was 40 amps by a tool requiring a lot of power.
Again we divide the Ah rating by the current:

So 4 Ah / 40 = 0.1 hour = 6 minutes.

However, the available energy is lower at higher currents. So the battery would source 40 amps, but voltage would fall faster and reach a lower limit requiring a recharge. Battery manufacturers produce charge and discharge characteristics for their batteries showing the Ah capacity of the batteries at various discharge currents. As the discharge current becomes higher, the Ah rating falls. On these curves, the capacity is shown for various "C-rates".

A 1C rate is when the current drawn is equal to the value of the battery capacity. So for a 2.0 Ah battery, A 1C discharge rate would be 2 amps, and a 10C rate would be 20 amps. The capacity quoted on a battery is typically when the battery discharges at a 0.5C rate or lower. Power tools typically discharge a battery at a 10C to 15C rate, depending on the power of the motor in the tool. So for a 4.0 Ah battery, this is a current of 4 x 10 = 40 amp for a 10C discharge rate.

Bosch 2.0 Ah, 12 V lithium-ion battery, fitted to a cordless drill

Bosch 2.0 Ah, 12 V lithium-ion battery, fitted to a cordless drill

What's the Difference Between mAh and Ah?

mAh means milliamp-hour.

One Ah = 1000 mAh

So a 2100 mAh battery has a capacity of 2.1 Ah

AA battery with a 2100 mAh or 2.1 Ah amp-hour capacity.

AA battery with a 2100 mAh or 2.1 Ah amp-hour capacity.

What Does Watt-Hour or Wh Mean?

While amp-hours is a measurement of charge in a battery, Watt-hours (abbreviated to Wh) is a measurement of energy storage capacity of a battery. To calculate the watt hour rating of a battery, you simply multiply the Ah rating by the battery voltage.

Example 1: What is the Wh rating of a 4.0 Ah, 12 V battery?
Answer: Wh = 4.0 x 12 = 48 Wh.

Example 2: What is the Wh rating of a 4.0 Ah, 18 V battery?
Answer: Wh = 4.0 x 18 = 72 Wh.

So as you can see, higher voltage batteries store more energy for the same Ah capacity.

What Are the Settings for on a Cordless Drill?

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

© 2022 Eugene Brennan