What Does the Ah (Amp hour) Mean on Cordless Tool Batteries?

Last Updated:  July 1, 2021

Click above to watch the video

Wondering what in the world Ah (Amp hour) means? Is a 5Ah battery more powerful than a 2Ah battery? Does it last longer? The video above and this article give a quick and basic explanation of Amp hour and how it affects your cordless tools.

What is Ah (Amp hour)?

Amp hour (Ah) is a unit of measurement that has nothing to do with power and it’s a totally different unit than Amp (Ampere). Amps are used to describe amp hour, but they are different units of measurement.

Usually, if a battery has at least a 4Ah capacity, it will be prominently displayed somewhere on the battery like the 5Ah in the picture.

Amp hour describes the capable charge of a battery. Theoretically, we can draw 5 amps, continuously, for 60 minutes before our 5 amp hour battery is drained. Now, that’s under ideal conditions, perfect temperature, no vibration, and consistent power. We say a capable charge because in the real world, you never meet those ideal conditions. Just think of amp hour as a way to know that one battery holds more of a charge than another. You’ll be able to use your drills, circular saws, and jigsaws longer before the battery runs down.

You may hear something called Peukert’s law, which is applied to lead-acid batteries (the battery in your car), falsely applied to lithium-ion batteries like the ones shown in this article and video. Peukert’s law says that a battery’s amp hour rating is reduced disproportionately as more current (amps) is drawn from it. Lithium-ion batteries have a self-heating characteristic that raises voltage and counteracts the dissipation of power due to internal resistance.

Taking our 5Ah battery above it’s easy to say that we can draw 5 amps for 1 hour – 10 amps for 30 minutes – or 20 amps for 15 minutes, in a linear calculation, and in fact we can with a lithium ion battery.

Peukert’s law, however, explains why that is not true for lead-acid batteries and the more you draw from that type of battery the less charge time is available. So, you may draw 5 amps for 1 hour, but only be able to draw 10 amps for 13 minutes, for example.

Even if Peukert’s did apply to your cordless tool batteries, the companies that make those tools and batteries all source their cells from the same manufacturers, so they would all have the same charge/discharge rates anyway.

All that to say, you can pretty much make a straight comparison that any brand’s 18V (or 20V max) 5Ah battery will hold more than double the charge of any other 18V (or 20V max) 2Ah battery.

What makes a 2Ah or 5Ah battery?

An 18V (or 20V Max), 2Ah battery, for example, has 5 cells in it and each one of those cells is a 3.6V, 2Ah cell (FIG 2). All 5 are connected in series so their voltage is combined, but not their amp hour rating. Five 3.6V, 2Ah batteries connected in series results in an 18V, 2Ah battery (FIG 3).

The bigger battery (FIG 1) has two packs of 5 cells for a total of 10 cells. The cells are the same 3.6V, but have more capacity at 2.5Ah. Each pack of 5 is connected in series and then the two packs are connected to each other in parallel. When connecting cells in parallel the amp hour rating is combined, but not their voltage – the opposite of connecting in series.

The battery has two 18V, 2.5Ah packs connected in parallel which results in an 18V, 5Ah battery (FIG 4)

While the amp hour rating does not equal power, you will generally get a little more power from the larger battery. The reason is that 10 cells are doing the work that only 5 cells are doing in the smaller battery and it’s able to more consistently meet the demand of the tool.

If you’d like to see this statement in action, check out this video showing a test done by the guys at Pro Tool Reviews. They take a single drill and use it to drill various holes with a 2Ah and 5Ah battery to see if there’s a power difference. The results are pretty interesting.

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