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.
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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.
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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.
Shown here is a Dewalt 20V Max 4Ah battery, likely engineered with 5x 21700-sized Li-ion cells, each with 4Ah charge capacity.
Here is an older-style Dewalt 20V Max 4Ah battery, engineered with 10x 18650-sized Li-ion cells, each with 2Ah charge capacity.
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.
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.
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.
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 built with 5, 10, 15, 20, or even 30 Li-ion battery cells. 36V and 30V Max batteries can be built 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.
Big Richard
“We need to switch over to watt-hours”
Even this has become more gray recently. Milwaukee uses nominal voltage to calculate Wh on their M18 packs, but they use max voltage to calculate Wh on their M12 packs. Similarly, DeWalt uses max voltage to calculate Wh on their cylindrical cell based packs (in the NA market), but nominal voltage on their pouch cell based PowerStack packs.
Stuart
That can be confusing.
Whenever I calculate watt-hours manually, someone always complains about my choice to use max or nominal voltage.
When comparing say 18V to 20V Max, it’s best to use max or nominal voltage for both. The actual values used aren’t important, as long as the comparisons are on even ground.
I tend to just use max voltage if any max voltage system is included in the comparison. I have no problem if 18V or 36V system brands use max voltage in their on-battery spec figure.
If brands start advertising watt-hours as a standard marketing spec, I would expect for the Power Tool Institute – the industry group in which most cordless brands are members – to develop a consistent way to do it.
Big Richard
Same, I find myself generally listing both to give the most comprehensive comparison, but as you said someone will always disagree with even that. Even if/when a standardization is agreed upon, it will still have its objectors.
Paul
The problem with using max voltage is that the max voltage is only really applicable at the start of the voltage curve, when it is fully charged. The battery will only hang out near that max voltage through most of its curve if you have extremely low current draw (like on the order of dozens or maybe a couple hundred mA depending on battery capacity). Li-ion has a discharge curve that flattens out around the nominal voltage with most common use (such as reasonable amp draw from power tools). You certainly aren’t going to see max voltage for more than a few seconds if you use an impact, for example. Maybe you’d see that for a while with a radio accessory at the lowest volume, and possibly not even then.
Nominal voltage is far more realistic for *estimating* Wh capacity of a battery pack. I think technically it should be measured as the integral of the voltage curve at 1C draw stopping at the low cutoff voltage, if you want to get technical with it and break out the calculus – at which point splitting hairs over the nominal or max voltage doesn’t really matter. But for estimation, I’m proudly a member of Team Nominal!
Stuart
Agreed.
However, for use in a comparison, consider there are 2 types of readers – those who are very well-informed about nominal voltages, and those who are not.
If I say a 20V Max 5Ah battery has 100 Wh, and an 18V battery has 100Wh, the more-informed consumer/enthusiast will know that it’s really 90 Wh vs 90 Wh. If I compare them as 90 Wh vs 90 Wh, less-informed consumers – and there’s nothing wrong with that, this is all beyond what average consumers are expected to know and understand – are more likely to get confused and wonder where the numbers come from.
What we want to avoid is comparing 100 Wh vs 90 Wh for 20V Max vs 18V batteries both with 5Ah charge capacities.
Because we’re looking at the figures comparatively, the quantitative accuracy is unimportant.
It’s like measuring the height of two pieces of wood to be cut to the same length for the sides of a free-standing cabinet in open space. If they’re 22.15″ long instead of 22″, that’s usually okay, as long as they’re the same length.
If quantitative accuracy is important, charge capacity is an unreliable figure anyway, as a single value doesn’t take into account the tested discharge value. As you mentioned, the discharge current can affect the voltage drop, and internal temperatures and resistance can alter that curve on the tail end to where runtime is shortened at higher discharge current. In a quantitative sense, you’re then compare batteries whose charge capacities might not necessarily hold true or change in a mirrored manner at higher discharge currents.
90 Wh is 25% greater than 72 Wh (18V 5Ah vs 4Ah), and 100 Wh is 25% greater than 80 Wh (20V Max 5Ah vs 4Ah). While not as accurate, the comparison is just as precise. Those who need the nominal voltage accuracy can extrapolate it. Those who don’t but can benefit from the comparison will have an easier time walking backwards from Wh to voltage and Ah with less need for extraneous explanations.
Chuck
For deep cycle batteries, the Ah rating is the 20 hour amp rating, so a 0.05C draw. I wasn’t able to find a solid number for these cells, but saw from 1C to 0.2C. I assume the typical draw is much higher than the draw used to test capacity.
From what I am reading here, the discharge curve doesn’t change too much for higher amp draws with the right cells.
https://batteryuniversity.com/article/bu-501a-discharge-characteristics-of-li-ion
Fowler
Yeah, using max voltage for comparisons isn’t much an issue (although it can cause the gap between some batteries to seem bigger than it really is), but it will give you an inflated Wh value, so you wouldn’t want to use it if you were estimating a tool’s runtime or something like that.
Li-ion batteries tend to drop from their max voltage fairly quickly so the nominal voltage provides a much closer match to actual voltage across the entire discharge curve.
MM
That’s a good point.
I’d also add that while watt-hours gives a good idea of how much energy a given battery pack contains it does not tell the whole story: how quickly the battery can deliver that energy is also important. For example, a 5ah Powerstack pack and a 5ah conventional Dewalt 20V max pack both have similar energy capacities in terms of watt-hours, however the Powerstack can deliver much higher peak current which means they will perform very differently in a high-power-demand tool.
Now all that said, Stuart is making a valid point. It’s basically just a different way of saying “don’t look at amp-hours alone, you have to consider voltage too”. And that’s sound.
Big Richard
Agreed on all points.
Philip
Don’t forget the powerstack can have twice as many charge cycles vs cylinder 5 amph. This is the factor for me when choosing.
Lyle
It’s too bad they don’t have a longer warranty. I feel like they should have a 5 year warranty if they’re going to say it can support more charge cycles. They are premium batteries but I think Dewalt should get it their warranty where they marketing is.
Jared
Watt hours would make more sense, though I presume most people are comparing batteries within a system. It would help explain why a batteries in different configurations cost so much more. Occasionally I’ll see Flexvolt kits advertised as having “3ah” batteries, for example, and that probably makes them look unreasonably priced versus a “9ah at 20v” pack.
Jason
Wh distinction is important for flying too! Was coming back to the states from Canada a few years back and had a couple M18 5.0 batteries with me.
Depending on carrier some are fine with less than 150 Wh, some it’s 100. Regardless I knew mine were 90 Wh each and were fine.
I explained to the security agent they are M18 5.0 batteries to which she responded “how am I supposed to know M18 stands for 18 Volts and 5.0 stands for amp hours. Took about 15 minutes but eventually they let me through. There is a Wh distinction on the serial plate of most batteries but of course that’s usually where it gets rubbed off most easily
Big Richard
Good point, this is very overlooked advantage of FlexVolt packs compared to other brand’s HO packs. Because their transport cap separates and isolates the banks of cells into separate “batteries”, you can carry a 12Ah FlexVolt battery on a plane, but you cannot say the same with a 12Ah M18 battery.
Jason
That’s a great point, I do remember seeing that. Probably not a huge demographic but I’m sure there’s plenty of folks that need to fly with their tools and would be maxxed out at 8AH. I wonder if their 15 Ah is the same.
Along those lines though Dewalt lists their 8 Ah you linked above as 160 Wh so in theory you couldn’t fly with it but could with their European marked battery even though it’s the same?
Big Richard
They likely will go with what is on the battery, so in the case of the 20v 8Ah XR, it says 160Wh so that is what they would consider it. Oddly enough, I don’t think DeWALT offers 18v 8Ah XR packs abroad (or the 6Ah XR for that matter), BUT if they did they would be labeled as 144 Wh and would be allowed – assuming the airline allowed 150 Wh batteries – even though it is exactly the same as 20v version that is not allowed. I think 100Wh is the FAA limit, though bigger batteries are allowed with airline permission.
The 20v/60v 15Ah is labeled as 3 x 100Wh, so it *should* be allowed on just about any airline. Always check with them first, and keep in mind TSA can confiscate anything they want if they question it.
SV
You make a good point. Resantly I’ve found myself checking WH more frequently to getter compare battery’s.
Aaron+S
The “3.0 AH” M12 pack that I have in reach from my office desk in small print lists 36Wh, pretty sure all my m18 packs include it. need to get it into large print!
Stuart
Most if not all batteries have this info on the physical housings. But when have you ever seen a cordless power tool battery advertise this on its packaging or online product pages?
Big Richard
DeWalt actually does. Listed online on the product page and on the package itself, see fifth picture – https://www.dewalt.com/product/dcb208/20v-max-xrr-8ah-battery
Stuart
Thanks, I stand corrected!
Nils
Related note- some of my 7-8 year old Dewalt flex volt batteries are dying.
What’s the best w-h deal I can hope for?
Seems like the 6ah batteries are on sale (as in yesterday’s flash sale) maybe those are making way for new power stack versions?
My guess is battery deals are best around November? Any thoughts?
Stuart
Here are the deals from Black Friday 2022: https://toolguyd.com/dewalt-tool-deals-black-friday-2022/#dewalt-battery-deals
Generally, yes the best deals are available in November/December , but there are occasionally good deals year-round.
9Ah 2-pack for $279 was the best deal in terms of Wh or Ah per dollar.
If you’re not fixed on a particular size, Acme has the same deal as Black Friday – a 9ah and 6Ah battery for $249. That’s not quite as good as 2x 9Ah for $279, but it’s the best available right now. In comparison, 2x 6Ah are $269.
After that, the next best pricing right now is 9Ah for $169.
In this context, since you are looking at Dewalt FlexVolt batteries specifically, amp-hours work well for comparison purposes. If you were debating between cost-of-ownership between FlexVolt and Milwaukee M18 or Makita XGT, that’s when watt-hours is a better measure to compare.
Clay
There may be some Father’s Day deals coming up, but they are frequently not as good as Black Friday deals.
MKY
Stuart –
Typo
“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.”
Stuart
Thanks! *fixed*
Sometimes I wonder what goes through my mind when writing and proofreading a post.
Philip
Love most tools and there different set ups.
But I focus on staying away from 80 percent of Milwaukee High output tools and batteries. At 18v … so far at any amph or watth… the tools are pretty light duty and over state there capacity capability. And they are super expensive. All they do is trip out from heat load
Kingsley
I have both 20V and 18V branded Dewalt 5ah batteries. What’s weird is that the stated Wh printed on the bottom of the USA ones is 100Wh. On the UK 18V ones it’s 90wh. Obviously they’re actually the same, but to conform with things like aircraft regulations they use the branded voltage.
Scott K
I generally understand voltage and ah, but this post is really informative and highlights something that I was not very informed on- thank you.
This kind of makes me think of Home Depot’s FPR which tried to create a way to compare hvac filters across various brands who each had their own system.
Stuart
Thank you, I appreciate it!
Interestingly, Dewalt’s UWO (unit watts out) is a good measure of comparative cordless drill power, but is widely panned for not being as clear as max torque. I haven’t seen any other tool brand outside of Stanley Black & Decker use UWO.
Aaron+S
UWO seemed like it would be a great idea but was too late in the proliferation of standards and ended up in the classic scenario: https://xkcd.com/927/
Dave
The first mistake was believing battery specs. The second mistake was doing math to show how well it will work. *Engineer.
Stuart
Someone asks you why a 40V Max 4Ah battery costs so much more than an 18V 5Ah battery. How would you explain this without talking about watt-hours?
Skalamanga
Don’t mention the voltage at all, refer to the battery by cell configuration, capacity, and safe discharge rating. RC batteries are referred to this way: A 2S 5000mAh 20C battery has 2 cells in series, it can produce 5A for 1 hour, or 100A for 3 minutes
Fowler
I’ve been thinking this for so long. I work in consumer devices and for the most part you’re kind of forced into listing a spec in amp-hours because that’s what people know and that’s what all the other products use. Powerbanks and tool batteries are in Ah, so if you throw out a Wh number no one can compare it to anything, even though it’s the real metric for how much energy the battery holds.
I’ve been researching a product lately that has a big 15Ah battery, but all the cells are in parallel, so the output voltage is only 3.7V. It only has much energy as a 3Ah M18 battery.
I also guarantee that all the engineers and designers who work on power tools refer to battery capacity in Wh. Ah is just not helpful if you’re using different voltages inside the tool.
Skalamanga
What we need is regulation so batteries can be cross compatible. Take the marketing nonsense away and replace it with scientific fact.
Force OEMs to comply with a modular system that means I can buy one charger, and replace dead cells instead of entire battery packs.
A 72v pack could be a modular pack containing 6x 12v packs
This should have gone to the root threads not you directly.
Stuart
(If a comment ever goes in the wrong place, place a copy where you want it or intended for it to go, and I can move things around. If it’s not clear which placement is the preferred one, add a note.)
Kalos Orisate
Reminds me of the early days of rocketry, when the Soviets used Power and the Americans used Pounds of Thrust. The two measures could not be directly compared.
Franco
I remember back in 1998, I lived through the Ice Storm we had here in Quebec. I was without electricity for 6 days, but some people were without power up to 48 days.
At the time, I knew very little about electricity (by no means an expert now). I was at a hardware store in the electrical department. It was swamped with people looking for ways to hook up generators and other electrical questions.
There was an older sales person, possibly in his early 60’s, explaining all kinds of things as a group of 15 to 20 people were huddled around, needing answers. At one point the topic was 120v versus 240v and is it true you can save 50% on your electricity bills. He explained that maybe 50% was optimistic, more like 20%.
I look behind me and a young kid, also an employee there, was fixing stuff on the shelves, looks at me, and says to me that what he (the older sales person) was saying was not true.
I was a bit stunned. he went on to say that he is currently in university, studying electrical engineering. He said that your basic 1500w baseboard heater, whether powered by 120v or 240v, will cost the exact same amount to the penny. He said it is a very big misconception that 240v is more economical than 120v (you find 240v heaters in homes because it is cheaper to wire higher voltage/low amp). At the end of the day, we all get charged by the KilloWatt/Hour (almost all regions are charged by the KW/h). Basically something that consumes 1000 watts for 1 hour, is using 1 KW/H, where I am that is about $0.10, tax and all in.
He went on to explain that a baseboard heater, ovens, water kettles…anything that uses a constant current, will always consume X watts/ hour. Motors, like a circular saw, can be rated at 15 amps, but being a motor, will have spike and peaks when it is started or when it needs more amps, like when the blade is binding.
He explained many other things, which I went on to ask a professor of Chemistry I knew plus contacted the POCO, to confirm all of this.
SO, to get back to the topic of what Stuart is talking about. All the issues about 18v, 20 max, or nominal versus max, can be confusing. Years ago, I am not sure what entity, because I am pretty sure it wasn’t the MFR’s that as a group decided this, lawnmowers and snow blowers dropped HP ratings and went to CC’s size of the engine (at least in Canada but pretty sure in the USA also) because too many false and misleading claims were made.
I am pretty sure that the power tools industry as a whole, is governed by 1 or more entities (govt?) that check that safety or other protocols are in place.
What would be very good is if they standardized battery wattage and consumption. A really simple way would be to use something, like mentioned above. Almost every power tool is motorized and will have spikes. But something like a flashlight, or any test method that has a simple constant flow of current, this would give an absolute number. When you see that battery X has 250 watts, and there is an asterisk or a symbol, like the UL symbol, we know it is tested and accurate. This could be mandated that al MFR’s measure in this manner, so the consumer knows exactly what it is. An independent entity, like UL as an example, can test batches here and there to make sure the MFR’s are following the prescribed method, and what is shown on the battery itself is exact.
Of course, this does not change that a 250 watt battery used in a drill will last longer than in a grinder, recip saw or other hi-amp draw tool. But it would at least lessen the confusion when comparing batteries.
After all, a battery is essentially a gas tank. We all understand that 20 gallons of gas in a Honda Fit will go farther than your neighbors F-150. But at the pump, 20 gallons will cost the same for both vehicles.
So battery technology can change, but a watt will always be a watt, regardless of 18650 vs 21700, or Lithium Poly vs Li-ion vs whatever other technology comes out…until someone rewrites Ohm’s law and the physical laws of electricity, the watt remains constant.
**** While they are at it, laws should be imposed to drop “max” and an 18v is 18, not 20v, or a 36v is not 40v…just more BS to make it confusing for the end-user that does not care to know all the technical data.