Best Product – Dewalt Cordless Mower Question: Why 20V Max and Not FlexVolt?

Dewalt 20V Max vs FlexVolt Battery Comparison 2021

A reader wrote in with an interesting and tough question about 20V Max and FlexVolt batteries.

Chris wrote:


I am a DeWalt customer from Australia and am wondering if you know why they do not call any of the large Ah 18V/20V batteries Flexvolt. They have just launched the new mower and it is paired with two 5.0Ah batteries instead of the 10.0Ahs. It seems very strange given the batteries are the same except for the voltage printed on the casing.

To start, I must respectfully correct Chris on two points. FlexVolt and 20V Max batteries are not the same, and Dewalt’s 5Ah and 10Ah batteries are both 20V Max batteries; the 10Ah battery is not a FlexVolt battery.

I could not find much about a new Dewalt cordless mower coming out in Australia, and so I will generalize my answers based on my familiarity with the mowers Dewalt launched in the USA.

Also, as most of you know, Dewalt’s cordless power tools are marketed as 18V and FlexVolt 54V in most parts of the world, and 20V Max and FlexVolt 60V Max in North America. I’ll stick to 20V Max and 60V Max for simplicity.

This is a very tricky topic, and there are actually several good questions here, and so I’ll break things down a bit.

Dewalt 20V Max vs. FlexVolt Batteries

A lot of users still mix things up when it comes to Dewalt 20V Max and FlexVolt, and the introduction of new higher performance and runtime 20V Max batteries complicate things further. However, the basics remain the same, and so we’ll start there.

FlexVolt batteries work with Dewalt’s 60V Max and 120V Max cordless power tools, as well as most of their 20V Max tools.

Dewalt’s 20V Max tools only work with their 20V Max tools.

Dewalt designed their FlexVolt batteries with internal physical switches that allow it to convert over from 20V Max to 60V Max voltage output depending on the tool it is connected to.

Additionally, all FlexVolt batteries so far (excluding the upcoming 15Ah) are built with (15) Li-ion battery cells. The 20V Max batteries are built with (10) Li-ion cells (5) for their compact batteries.

FlexVolt batteries can handle the power needs of 20V Max and FlexVolt tools, while the 20V Max batteries are only designed for the maximum power requirements of 20V Max cordless power tools.

A Loose Analogy

Consider 5 people – siblings that are identical in all regards, such as their strength capabilities – that are split into two groups.

One team has 2 siblings and the other has 3. Each sibling can lift 50 pounds, and so (2) can in theory lift 100 pounds combined, and (3) can lift 150 pounds. If you have a 50 pound weight, either team can lift if. If you have an 85 pound weight, either team can lift it, but the team of 2 will strain a little more than the team of 3. Move up to 100 pounds, and both teams will lift the weight, but with very different levels of easiness and speed. Move up to 120 pounds and only the 3-person team is up to the task.

Now, let’s say you have a 65 pound weight to be moved. If it can be lifted and moved by the 2-person or 3-person team, you have options. Both teams might move that weight with the same proficiency, but with added muscle, the 3-person team will take longer to fatigue, getting more work in before they need to rest and recharge their energy.

Dewalt’s Cordless Mower

Dewalt DCMWP233U2 Cordless Mower

Dewalt designed their 20V Max cordless lawn mowers around their 18V/20V Max platform, and so they can be powered by either 2x 20V Max or 2x FlexVolt batteries.

With the mowers able to operate using 20V Max batteries, they provide a more flexible power solution as it means 20V Max and FlexVolt tool and battery users can potentially power the mower with existing batteries beyond what it’s kitted with.

The decision to include new Australian mowers with 5Ah batteries instead of 10Ah can be due to supply constraints (the 10Ah is still very new and hard to find on its own), cost considerations, the average size of lawn most users in this region might expect to mow, or a combination of these and potentially other factors.

Dewalt’s new cordless mowers that launched in the USA, which Ben recently reviewed here, are bundled with 10Ah batteries, presumably because Dewalt wanted to provide specific minimum runtime performance.

Dewalt 5Ah vs. 10Ah Batteries

The 20V Max 5Ah and 10Ah batteries feature different Li-ion form factor technologies, with the 10Ah battery being built with larger cells. Thus, the 10Ah battery is physically larger than the 5Ah by a bit.

Dewalt’s 20V Max 6Ah, 8Ah, and 10Ah batteries all feature the larger Li-ion cell sizes, similar to what can be found in FlexVolt 9Ah and 12Ah batteries.

Why 2x 20V Max for the Cordless Mower?

Let’s consider watt-hours for a moment. For simplicity, we’ll also ignore that different Dewalt cordless mowers are kitted with different battery sizes.

Watt-hours is voltage x charge capacity. We’ll use 20V Max and 60V Max as the voltage, instead of 18V and 54V nominal.

To help eliminate qualifications and special considerations, let’s consider just (2) battery sizes – Dewalt 20V Max 6Ah and Dewalt FlexVolt 12Ah.

Dewalt 20V Max 6Ah

Each battery delivers 20V Max x 6Ah = 120 Watt-hours of energy.

In a 2x 20V Max configuration, this means 240 Watt-hours of energy is available to the cordless mower.

Dewalt FlexVolt 12Ah

With FlexVolt batteries, the charge capacity often refers to the 20V Max value. That is, if you want to use 60V Max, its charge capacity is 1/3 that of the 20V Max value.

So, there are two ways to compute this:

20V Max x 12Ah = 240 Watt-hour

60V Max x 4Ah = 240 Watt-hour

The energy in the battery is the same either way, you just have to keep in mind what the advertised charge capacity is referring to.


Dewalt’s 20V Max 6Ah battery is their smallest battery that uses larger form factor Li-ion cells. This is why it’s messier to talk about Dewalt’s 5Ah batteries, because those use different Li-ion cells which require a separate discussion. You can refer to my post on Milwaukee’s battery sizes for further information on this.

See Also: Milwaukee M18 Cordless Power Tool Battery Sizes Explained

Okay, so the 20V Max 6Ah batteries is their smallest next-gen battery, with 8Ah and 10ah offering greater charge capacity and runtime. Dewalt’s FlexVolt batteries only go up to 12Ah. Yes, there’s a 15Ah battery on the way, but we’re ignoring it for now since it uses different battery tech.

So, if the choice is between 2x 20V Max and 1x FlexVolt 60V Max, going the 2x 20V Max route is better as it raises the runtime ceiling.

We’re not even talking about performance here, just runtime. But, that’s not say that performance isn’t a factor. When you have 2x 20V Max 6Ah batteries, you get (20) Li-ion cells combined, vs. (15) cells for the FlexVolt battery.

Ignoring complicated matters such as thermal management and Li-ion max current datasheet specs, let’s go back to the analogy from above, where you have 5 siblings in groups of 2 and 3. Which configuration is going to be able to do a little more work, a battery pack with 20 cells or one with 15?

Wrapping Things Up

2x 20V Max vs. 1x FlexVolt

Single-battery FlexVolt tools can generally outperform single-battery 20V Max tools. But when it comes to runtime, 2x 20V Max tools would hold a runtime advantage compared to single-battery FlexVolt solutions. There could also be performance differences, depending on how the the tools are engineered.

A single FlexVolt battery solution, with 12Ah as the largest battery currently on the market, would have a 240 Watt-hour ceiling when it comes to the maximum amount of energy available to that mower.

With a 2x20V Max battery solution, you can have 2x20V Max or 2x FlexVolt batteries, with up to 480 Watt-hours of electrical energy.

With 2x 8Ah or 2x 10Ah batteries, you can provide the mower with greater energy from 20V Max batteries than with a 12Ah FlexVolt battery.

Dewalt designed their mowers within the power delivery capabilities of their 20V Max batteries.

2x 20V Max vs. 2x FlexVolt

If a Dewalt cordless power tool can be powered by 20V Max batteries, there’s no reason to narrow their user base by requiring FlexVolt batteries.

Sure, Dewalt could also design a 120V Max 2x FlexVolt mower, with larger cutting blade, but they haven’t, at least not yet.

I see that Dewalt offers their Australian market cordless mower with 20V Max 5Ah or FlexVolt 6Ah batteries. The mower is the same 20V Max-powered model for both, with just differences in how they’re kitted.

A FlexVolt-specific mower would lock out 20V Max users and presumably also cost more than a 20V Max mower.

With the tool being 20V Max, Dewalt can offer the same mower with different battery kit options, with some practical constraints based on desired performance, runtime, and retail pricing.

Dewalt 20V Max vs. FlexVolt User Base

Let’s say there are just four types of tool users:

  • Potential users who do not own any Dewalt cordless power tools
  • 20V Max cordless power tool users
  • 20V Max and FlexVolt cordless power tool users
  • FlexVolt 60V Max/120V Max cordless power tool users

There’s absolutely a need for higher performance FlexVolt cordless power tools, but if you can power a tool with 20V Max batteries, isn’t that a better option, presumably making it available to a broader user base?

Dewalt launched several FlexVolt Advantage cordless power tools that operate on their 20V Max platform, with FlexVolt batteries delivering a performance boost. And, Dewalt recently launched a new FlexVolt Advantage air compressor that will be replacing their FlexVolt-exclusive model. Why is that? I believe it’s so that Dewalt can make these tools available to a broader user base than if they were FlexVolt-specific models.

There’s always the potential for Dewalt to launch a FlexVolt-specific cordless mower, similar to how their FlexVolt lawn and garden tools have greater capabilities (and power requirements) compared to their 20V Max cordless outdoor power tools.

But since the 2x 20V Max mowers can be comfortably powered with 20V Max tools, there would only be disadvantages in making them FlexVolt-specific.

I’m sure there are marketing reasons factored into all of this as well. Hypothetically, you manage a chain of retail stores and have a set amount of square footage at each store to display Dewalt’s cordless mowers (and other outdoor tools). What do you think would sell better, a 21″ cordless mower that’s priced at $400, or a 25″ mower that’s $600? Ideally, there could or would be both, but that’s always possible.