Here’s a question that actually comes up quite often: “what does Ah mean?” When talking about cordless power tool battery packs there are a number of important specifications to keep in mind, but it mainly comes down to voltage and capacity.
Amperes and Amp-Hours
Ah is an abbreviation for ampere-hour, or amp-hour. This describes the charge capacity of a battery, or how much current can be delivered at a constant rate as the battery is depleted over the course of one hour.
For example, consider a hypothetical cordless power tool that continuously draws 1.0A of current. At this rate, it will completely drain a 1.0 Ah battery pack in one hour (under ideal conditions).
This means that a 2.0Ah battery pack can power the same tool for longer than a 1.0Ah battery pack can, assuming the current flow remains at 1.0A and there are no other differences.
So, a 4.0Ah battery pack lasts twice as long as a 2.0Ah battery pack, right? Sometimes, yes. Other times, no. Sometimes it can deliver more than double the runtime, but this depends on how the battery packs are designed.
Series vs. Parallel
Batteries connected in series are conductively coupled end-to-end. Take two 1.5V AA batteries and attached them so the positive terminal of one is connected to the negative terminal of the other. The voltage measurements taken across the connected batteries should read as 3.0V.
Batteries connected in parallel are connected side by side. Take those same two 1.5V AA batteries and cross a piece of copper across both positive terminals and then another piece across both negative terminals. The voltage will still be 1.5V but they will last twice as long when powering a device.
Today’s 10.8V and 18V battery packs, also called 12V Max and 20V Max in the US, are built with 3.6V lithium-ion cells. 10.8V/12V Max battery packs are built with three cells each, and 18V/20V Max packs have five cells each.
A high-capacity battery pack, such as a 3.0Ah or 4.0Ah 18V pack, has double the number of battery cells as 1.5Ah and 2.0Ah packs. A 2.0Ah battery pack will have five 3.6V cells – each with 2.0Ah capacity – connected in series, and a 4.0Ah pack will have two sets of five batteries connected in parallel.
But what could it generate? If my math is right, and it always is, 3 gigajoules per second. That could run your heart for 50 lifetimes. Or something big for 15 minutes.
(Scientist Dr. Ho Yinsen and Tony Stark, Iron Man, 2008.)
Power tool battery packs are not so simple where 4.0Ah = 2.0Ah times two. Yes, a 4.0Ah battery pack has twice as many battery cells as a 2.0Ah pack, but the circuitry and controls are usually different.
This is how Milwaukee’s RedLithium higher capacity (XC) lithium ion battery packs can deliver a performance boost over compact battery packs, and why Makita’s heavier duty LXT tools must be powered by high capacity battery packs and not their compact ones.
It’s not just about runtime. Higher capacity battery packs can deliver power via additional channels. Li-ion batteries can be damaged by overdrawing current from them, so modern battery packs have safety measures in place. If a tool wants to draw more current than it should, control circuitry will shut the tool off until it cools down or the load is reduced.
If you have twice as many cells to draw current from, there’s going to be greater headroom.
Let’s say a 2.0Ah battery pack is limited such that continuous current flow cannot exceed 1.0A in order to protect the cells. Theoretically, a 4.0Ah battery pack could be designed where each grouping of cells are set to deliver a maximum continuous current of 1.0Ah. With the two groupings coupled together in parallel, the battery pack could then support a maximum current draw of 2.0A.
But you’re not going to see battery packs advertised as delivering 100% more power than smaller same-voltage packs. Ignoring real-world engineering limitations that govern battery pack designs (such as heat buildup), power tools are not designed to be able to handle 100% more power.
Realistically, if a compact battery pack has a maximum continuous current draw of 1.0A, a higher performing extended capacity battery pack might be designed where it can support a maximum current draw of 1.2A or so. These numbers are all for example purposes. Realistic shut-off limits are likely quite a bit higher.
If a power tool doesn’t draw the extra power extended capacity packs can provide, total runtime is further extended. Since battery cells in parallel share the total load, losses (such as those due to heating) could be reduced. Thus, greater efficiency could enable double-capacity battery packs to deliver more than double the runtime.
4.0Ah vs. 3.0Ah and 2.0Ah vs. 1.5Ah
I discussed this a bit before, but it’s worth another mention. Lithium-ion battery technology has advanced to where battery cell manufacturers can build same-size cells with greater charge density and better performance.
Most of the tool brands I’ve spoken to don’t make their own battery cells, they source them from elsewhere. Panasonic is one that does their own thing, hence their new 4.2Ah battery packs.
Basically power tool brands can – and please someone correct me if I’m wrong – swap in higher capacity cells for lower capacity ones in current battery packs. Change battery cell suppliers or select a different line and whoosh 1.5Ah packs become 2.0Ah and 3.0Ah packs become 4.0Ah. But of course it’s not this simple.
The latest generation of higher capacity battery cells perform better and last longer. But research costs and potentially more complex manufacturing means higher costs for power tool manufacturers and end users.
Some manufacturers are simply building higher capacity battery packs, others are taking the opportunity to incorporate additional changes to power delivery and control circuity. So even if different brands use the exact same battery cells, there will be differences in battery packs.
Ah is simply a measure of the total charge capacity of a battery pack. Higher values can mean longer runtime AND additional power. Power tools, at least those compatible with both compact and high capacity battery packs, are designed with a current draw ceiling, so you typically won’t see double the power if you pair them with beefier battery packs.