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Comparing Power Tool Battery Specs – Watt-Hours vs Amp-Hours

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When cordless power tool brands switched to Li-ion battery technologies, they spec’ed and advertised batteries according to two fundamental characteristics – voltage and charge capacity, with units of volts and amp-hours, respectively.

Voltage is a widely-understood concept, and amp-hours less so. Still, with brands then offering compact batteries with lower amp-hour ratings, and larger batteries with higher amp-hour rating, thinking about charge capacity as the “size of a power tool’s gas tank” simplified things.

Most tool users should understand voltage (V) as being related to power, current (A) as a measure of electrical flow, and amp-hour (Ah) as a measure of charge capacity and how long a battery will power a tool or device.

See Also:
i) Cordless Power Tool FAQ: What Does “Ah” Mean?
ii) Revisiting What an Amp-Hour Means for Cordless Power Tool Batteries

Before some of you scream at me, yes – power is not a technically accurate ways of looking at voltage, but it makes things easier to understand.

When only talking about batteries of the same working voltage, such as comparing one 18V system against another 18V or 20V Max system, things are easy and simple.

But we’re no longer talking about just two battery sizes and only 18V – or 18V and 12V – cordless power tool systems.

Cordless power tool brands today have handheld power tools with voltages from 12V up to 60V, and some go beyond that.

Additionally, there are several very different technological generations of battery packs that are all still on the market.

Amp-hours aren’t as clear as they need to be.

An amp-hour is the duration a battery can deliver a current flow of one amp. If a battery can deliver a 1A continuous discharge current for 2 hours, it has a charge capacity of 2Ah.

With cordless power tools, we deal with battery packs – assemblies of multiple Li-ion cells with electronics, the battery management system (BMS) controlling things.

A battery’s amp-hour rating can change depending on the discharge current, but this is generally not paid attention to.

If you’ve seen the first Iron Man movie, consider the part where Stark builds his first arc reactor. As a fixed source of energy, it can power something small for a very long time, or something big for a very short time. Batteries are similar.

A few years ago, 4Ah batteries from Bosch, Dewalt, Makita, Metabo, Metabo HPT (Hitachi), Milwaukee, and others would all be built similarly. All of these brands had 18V (or 20V Max) cordless power tool systems, and 4Ah was 4Ah.

Now, a 4Ah battery can be built in very different ways.

Dewalt DCB240 20V Max Compact 4Ah Cordless Power Tool Battery

Shown here is a Dewalt 20V Max 4Ah battery, likely engineered with 5x 21700-sized Li-ion cells, each with 4Ah charge capacity.

Dewalt DCB204 20V 4Ah Battery

Here is an older-style Dewalt 20V Max 4Ah battery, engineered with 10x 18650-sized Li-ion cells, each with 2Ah charge capacity.

Makita XGT BL4040 Cordless Power Tool Battery

Here’s a Makita XGT 40V Max 4Ah cordless power tool battery, likely engineered with 10x 21700-sized Li-ion battery cells, each with 4Ah charge capacity. It has the equivalent energy capacity of an 18V 8Ah battery.

Makita XGT 40V Max 5Ah Battery

This Makita XGT 5Ah battery is physically larger than their 4Ah battery. To produce the 40V Max (36V nominal) output, groups of 10 cells are required, suggesting it’s built with 20x 18650-sized cells each with 2.5Ah charge capacity.

Dewalt PowerStack 5Ah Battery Internal Cell View

The Dewalt 20V Max (18V nominal) PowerStack 5Ah battery has 5x Li-ion pouch cells, the same count as their compact PowerStack 1.7Ah battery.

Dewalt FlexVolt 6Ah Cordless Power Tool Battery

Here’s the Dewalt FlexVolt 60V Max/20V Max dual-output battery, with 15x 18650 cells each with 2Ah charge capacity.

Things have gotten confusing, and even more so for anyone that hasn’t been following cordless power tool battery tech for the past few years.

Bosch and Milwaukee have stayed with 18V, Dewalt has 20V Max (18V) and FlexVolt 60V Max, Makita has 18V and XGT 40V Max (36V nominal), Metabo HPT has MultiVolt 36V/18V, and Flex and Kobalt both have 24V Max cordless power tool systems.

We need to switch over to watt-hours.

A Watt is a unit of power, with 1W being equivalent to 1V x 1A. One watt, or one volt-ampere, is the electrical work that is done when an electrical flow of one amp is applied across a potential difference of one volt.

A watt-hour is a measure of energy capacity. 1 Wh = 1 V x 1Ah.

Charge capacity is an important battery characteristic. Watt-hours takes the voltage into account.

Portable battery banks boast incredible charge capacity figures, such as 10,000 mAh for a $20-30 pocket-sized device charger. That 10Ah is not the same as 10Ah in the cordless power tool world.

Which battery has greater energy capacity, a 40V Max battery with 2.5Ah charge capacity, or a 24V Max battery with 4Ah charge capacity?

For the 40V Max battery, that’s 40V x 2.5Ah = 100 Wh. For the 24V Max battery, that’s 24V x 4Ah = 96 Wh.

It wasn’t necessary to talk about watt-hours in an 18V-dominant market. But now we have 24V, 36V, and even 60V cordless power tools the same general size and weight as 18V cordless power tools.

An 18V or 20V Max battery can be build with 5, 10, 15, 20, or even 30 Li-ion battery cells. 36V and 30V Max batteries can be build with 10 or 20 cells. 54V or 60V Max battery can be built with 15 or 30 cells. These can all be the same or different sized Li-ion batteries. 24V Max batteries have 6, 12, or 18 cells.

One could argue that voltage and charge capacity are all that are needed to describe a battery’s energy capacity.

Perhaps, but how many tool users still mistakenly believe that 20V Max delivers higher power than 18V, and the same for 40V Max compared to 36V?

With watt-hours, a higher value simply means greater energy capacity. The same is not necessarily true for amp-hours. With watt-hours, users can compare batteries across brands and cordless systems with less technical understanding.

Consider portable air compressors. If two models both have 2 gallon tanks, which one holds more air? The one that fills to a higher pressure. Using amp-hours as a comparative measure across different cordless voltage systems is like comparing air compressors only by their tank volumes.

Amp-hours are still useful for comparing batteries within any particular cordless power tool system, but watt-hours are better for comparing across different voltage systems, such as 18V vs 24V Max and 36V/30V Max.