Best Product – Dewalt 20V Max 10Ah vs. FlexVolt 9Ah Batteries

Dewalt 20V 10Ah vs FlexVolt 9Ah Batteries

I’ve been asked a tough question – how does the new Dewalt 20V Max 10Ah battery compare to their FlexVolt 9Ah battery?

To start, you can buy a 2-pack of 9Ah batteries for $299 on Amazon, and some retailers have it priced at $329. The Dewalt 20V Max 10Ah battery 2-pack is priced at $349 at Tool Nut.


With these two batteries similarly spec’ed at 10Ah vs. 9Ah, how do they compare?

Dewalt 20V Max 10Ah Battery (DCB210)

Dewalt 20V 10Ah Battery

I have not yet tested or even handled the new 10Ah battery, but it’s fair to assume that it’s engineered with 21700-sized Li-ion cells.

This is a 10-cell battery built with Li-ion cells presumably rated at 5.0Ah each. The cells are arranged in a 5S2P configuration, which means that you have 2 rows of 5 battery cells arranged in series.

Each battery cell has a 3.6V nominal voltage, or 4.0V Max voltage. So, 5x 4.0 = 20V Max. With two rows of cells arranged in parallel, the charge capacity adds up, so you have 5.0Ah x 2 = 10.0Ah.

If you need help visualizing this, take your two hands and put them together, with open palms facing and left and right fingers touching each other. You have 2 groupings of 5 fingers in a 5S2P configuration. Now, make two fists and stack them, one on top of the other. Your knuckles should line up in a single-file row. Excluding the thumb, this would be an 8S1P configuration.

Dewalt FlexVolt 9Ah Battery (DCB609)

Dewalt FlexVolt 9Ah Battery

Dewalt’s 9Ah battery, also presumably built with 21700-sized Li-ion cells, has 15 cells.

Dewalt FlexVolt Battery 9Ah Cell Configuration

When connected to a FlexVolt 60V Max cordless power tool, those 15 Li-ion cells are switched to a 15S1P configuration, where you have 4.0V x 15 = 60V Max. The charge capacity is not additive in a 1P configuration, and so it would be whatever the individual cells are rated at, or 3.0Ah for this configuration.

In parallel mode for use with a 20V Max cordless power tool, the same 15 cells are switched to a 5S3P configuration. 5×3 = the same 15 cells as in serial mode.

In 5S3P mode, 4.0V x 6 = 20V Max, and 3 x 3.0Ah = 9.0Ah.

Theoretical Differences

Generally, Dewalt’s 20V Max cordless power tools are designed around the power delivery capabilities of 20V Max batteries. In other words, the 10Ah battery and FlexVolt 9Ah battery should in theory deliver comparable performance but with just a proportional difference in runtime. After all, the same tools are also designed to be powered by Dewalt’s full spectrum of battery sizes, including their 1.5Ah to 5.0Ah batteries (and FlexVolt 6Ah) engineered with 18650 Li-ion cells.

Let’s remove that consideration for a moment. For the sake of this discussion there exists a hypothetical Dewalt 20V Max tool that can draw increasingly high levels of current from any connected battery.

We’re also speaking about the FlexVolt battery in parallel 20V Max-compatible mode exclusively.

Alright, so we have the 10Ah battery with its 5S2P configuration of 5.0Ah cells, and the 9Ah battery with its 5S3P configuration of 3.0Ah cells.

Let’s change things a bit. Instead of the two batteries being engineered with different Li-ion cells, let’s say that they both featured the same Li-ion cells, either 3.0Ah or 5.0Ah, it doesn’t matter.

With all things being equal, a 5S3P battery can deliver more power than a 5S2P battery.

Imagine that there are 5 people with identical strength and endurance, and they are split into two groups of 2 and 3. When faced with light work, the two groups will perform their tasks with comparable effectiveness. When faced with medium-duty work, you might start to see some differences. But with heavier work, you’ll see an increasing performance gap between the two groups as you approach or surpass the limit of what a group of 2 such people can do.

When composed of identically-capable members, a team of 3 can outperform a team of 2.

The tricky part here is 20V Max tools are designed to be powered by 5S2P and even 5S1P (compact 5-cell) batteries. And so, the real-world differences likely come down to proportional differences in runtime.

But wait, there’s more to consider. What we were talking about just now involves imagining that both batteries were engineered with the same Li-ion cells, but they’re not. The 10Ah battery is engineered with 5.0Ah cells, while the 9Ah battery is engineered with 3.0Ah cells.

Generally, and I say this in case there are ever exceptions, when considering cylindrical batteries of comparable size and technologies, and within the same or similar product families, higher capacity batteries with higher energy densities will have lower constant discharge current ratings. Battery cooling and heat dissipation come into play here as well.

There’s a common trope where a smaller guy is fighting a larger guy, and all they have to do is wait for the larger guy to tire themselves out. It’s kind of like this.

We don’t know what lithium ion battery cells are used in the 10Ah or 9Ah batteries right now, so let’s look at two off-the-shelf batteries.

The Samsung INR21700-50E has a 5Ah charge capacity and is rated with a maximum constant discharge current of 9.8A. The Samsung INR21700-30T has a 3Ah charge capacity and is rated with a maximum constant discharge current of 35A.

Let’s say you have a device that runs at 3.6V (the nominal voltage for Li-ion cells of this type) and draws a continuous 9.5A of current. Both of these 3.0Ah and 5.0Ah batteries will be up to the task, and at the end, the higher capacity 5.0Ah battery will likely run quite a bit hotter. Crank up the power demand slightly, and the 5.0Ah battery might exceed its operating conditions if it cannot be cooled fast enough. Cooling is an important part here, because this is greatly influences how much sustained power a battery can deliver.

Brands shifted from 18650 cells to 21700-sized cells for similar reasons. 18650-sized 3.0Ah cells were not quite up to the power delivery capabilities as 1.5Ah to 2.5ah cells. Shifting to 21700 meant that 3.0Ah charge capacity (and higher) were possible with lower charge density, which meant higher maximum continuous current discharge rates and cooler batteries under load.

Dewalt FlexVolt DCB615 15Ah Battery

Dewalt has a new FlexVolt 15Ah battery coming out, and features a different battery configuration than their FlexVolt 9Ah and 12Ah batteries. This is likely because 5.0Ah cells cannot deliver sufficient power in a FlexVolt 60V Max 15S1P configuration.

Batteries with higher energy densities have lower max discharge current specs, and they also often run hotter. At lower demands, the higher charge capacity battery delivers proportionally longer runtime, but at greater demands the lower capacity battery will often have a much higher power and performance ceiling.

Consider, a race between a sprinter and a long distance runner. In slow and long territory, with both runners matching each other’s strides, the long distance runner will last longer. But in hard and fast territory, the long distance runner might get winded and pass out as the sprinter surpasses them.

Okay, so let’s add things up. In theory, a 5S3P (FlexVolt 15 cell in 20V Max mode) battery configuration can deliver more power than a 5S2P battery configuration (20V Max) when using the same exact cylindrical form factor Li-ion cells – 21700-sized in this case.

And, as far as I have seen, a 3.0Ah battery cell can deliver more power than a 5.0Ah cell.

So, in a comparison between a 5S3P battery with 3.0Ah cells and a 5S2P battery with 5.0 cells, you have 15 battery cells capable of greater power deliver compared to 10 battery cells of lower capability.

There is going to be a huge gap between the theoretical capabilities of a FlexVolt 9Ah battery and a 20V Max 10Ah battery.

However, this is all theoretical. Realistically, Dewalt designed and tested the 10Ah battery to match the power needs of 20V Max cordless power tools. The 10Ah battery is smaller and lighter than their FlexVolt 9Ah battery, which also delivering around 11% longer runtime. If the 10Ah battery runs hotter than the brands’ other 20V Max batteries, that could potentially affect things, but I trust that Dewalt knows what they’re doing.

It’s difficult to draw conclusions about any of this, which is why I stressed it’s all in theory. In theory, the 9Ah FlexVolt battery looks a lot better when it comes to potential on-paper electrical power capabilities compared to the 10Ah 20V Max battery. However, the engineering around all this means that the 10Ah battery should be capable of delivering the power requirements of any of Dewalt’s 20V Max cordless power tools. As long as what it’s being asked to do falls within its capabilities, do the theoretical performance ceilings matter?

If you’re 6 feet tall, does it matter if a doorway is 7 feet tall or 10 feet tall?

It remains to be seen whether Dewalt’s new 10Ah battery can sufficiently power all of their 20V Max cordless power tools without breaking a sweat, but keeping in mind that their higher performance tools are powered by 15-cell FlexVolt batteries, I don’t feel this to be a major concern.

As an aside, given the specs I’ve seen for 21700-sized 5.0Ah lithium-ion battery cells, it seems that we’ve about reached the limits as to how high of a practical charge capacity can be achieved with this form factor. Some brands are better-rated than Samsung’s INR, such as Panasonic (15A max continuous discharge current), while others are worse, such as LG (~7.3A max continuous discharge current at room temperature).

Which One to Buy?

If you want to save a little money and are okay with a little more size and weight, or if you need FlexVolt tool compatibility, go with the FlexVolt 9.0Ah. If you want a slightly smaller and lighter battery and want a little more runtime from 20V Max cordless power tools, go with the 10.0Ah battery.

If you don’t want to be an early adopter, wait for others to put the 10Ah battery through its paces. I would caution against getting hung up on theoretical limits.