Two batteries of the same make and same voltage have one that costs nearly twice as much as the other; the difference between them is just two letters: Ah. Here's what those letters actually get you.
Ah stands for amp-hour and is a measure of capacity, indicating how much charge the battery can hold; it's similar to the size of a fuel tank rather than the size of the engine. At the same voltage, a battery with a higher Ah rating will last longer when the power demand is the same, and that's the only difference.
This brief guide covers the meaning of Ah on a battery, the changes that occur when you choose a higher Ah rating, when Ah ceases to be a comparable measure, and which band you should select. Further information on batteries can be found in our electric tool battery guide.
Ah, translated
The amount of charge stored is measured in amp-hours. For example, a 5Ah battery could in theory supply a current of 5 amps for one hour or a current of 1 amp for five hours. In reality, the figure varies with temperature and with how heavily the tool is working, but the basic idea remains: amp-hours is the equivalent of the tank.
Voltage is the pressure and is the component which determines how forcefully the tool strikes. Ah stands for the fuel tank, that is, the amount of fuel carried. A larger tank does not make the engine more powerful; it only extends the length of the journey. The figure on the label is the one you should remember because it's the one that affects your workday.
Higher Ah at the same voltage
The voltage is the same but with a higher capacity the running time is longer. The relationship is roughly proportional since a 5.0Ah battery contains about two and a half times as much energy as a 2.0Ah battery and so the same tool will last about two and a half times as long on it.
The amount of power remains the same. Increasing the capacity from 2.0Ah to 5.0Ah will not make your drill spin any faster. If you need more power, then that is a question of voltage, and in that case you should buy a different tool, not one with a higher Ah rating.
Apart from runtime, the weight and the price change. Larger packs are heavier, so you can feel this during overhead work, and they are more expensive. The support guidance provided by the manufacturers also settles the question of compatibility: a higher Ah pack works just as well in the same tool when the voltage and the connector are the same.
People think that buying the highest Ah rating makes the tool more powerful and then find that they've paid for hours which they never use. The figure you should choose is one that corresponds to how long your longest task actually takes.
When Ah can't be compared
It only compares fairly within the same voltage class; an 18-volt 2.0Ah bundle contains 36 watt-hours, while a 40-volt 2.0Ah bundle contains 80 watt-hours. The amount of amp-hours is the same, but the energy is more than double since the voltage is higher.
That is the reason why watt-hours have been introduced: since watt-hours are equal to voltage multiplied by amp-hours, they are the appropriate unit to use when the voltages are different. Another unit to mention here is mAh, or milliamp-hours, which is used with small electronic devices. As one thousand milliamp-hours make up one amp-hour, a phone battery with a capacity of 2,000 mAh is equivalent to a 2 Ah battery when expressed in the language used for power tools.
Picking an Ah band
For a drill or driver, the practical bands look like this:
| Band | For |
|---|---|
| 1.5 to 2.5Ah | Light jobs, occasional use, keeping the tool light |
| 3.0 to 4.0Ah | The middle ground most people should buy |
| 5.0Ah and up | All-day work and power-hungry tools |
The middle band is where the value lives. It's the one we suggest most, and the one people regret skipping.
The full walkthrough of choosing a pack, from the model number to the seller, is in our guide on how to choose a replacement power tool battery.
Common questions about amp-hours
What does Ah mean on a battery?
Ah is short for amp-hour, a measure of capacity, how much charge the pack holds. It's the fuel tank size. Voltage is the engine.
Does a higher Ah battery give more power?
No. At the same voltage, a higher Ah pack gives more run time, at the same power. More muscle comes from voltage, not from Ah.
Can I use a higher Ah battery in my tool?
Yes, when the voltage and connector match. A 5.0Ah pack runs the same tool as a 2.0Ah pack, just longer, and it's heavier and pricier.
Is a higher Ah battery always better?
No. It's better only if you use the runtime. Otherwise you're carrying weight and spending money for hours that never happen. Match the Ah to your longest typical job.
The tank is represented by Ah, the engine by voltage, and the only question that Ah addresses is how far you get on a single charge. You should match it to the longest task you have, not to your ambitions, and thus the cost of the two letters will not be incurred.
References & Sources
- Black & Decker support (battery interchangeability guidance at the same voltage)
Need a replacement?
If you've got your Ah band picked, browse replacement batteries by brand. We stock compatible packs across the capacity range, and any question goes to a real person through our contact page.