I wrote most of this in an email to a reader, but realized a post would be an even better idea.
I posted about Amp-hours a while back, here. But here’s a revisit:
Amp-Hour Analogy
Consider a fixed container that can hold 5 gallons of water. You can attach a large spigot or a small one to fill drinking cups with. Either way, you get 5 gallons of water out of the container until you have to refill it.
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But now consider that the spigot size matters. That container might hold 5 gallons of water, but attach a very large spigot and you might not fill as many cups of water as if you had used a small spigot.
Clarification: Fill up a cup of water at a kitchen sink. Now try to fill one up at the bathtub faucet. How much water is lost when filling your cup at the sink? How much when filling it up at the bathtub?
With batteries, each cell has a measurable maximum capacity, measured in amp-hours. Amps are units for current flow. A battery cell with 1.0 amp-hour capacity can supply 1.0 amps of current for 1 hour.
If for the same cell, a more power-hungry tool is connected, and it draws 2.0 amps of current, the battery can supply 2.0 amps of current for 1/2 hour.
Attach an even larger motor, and it can supply say 10.0 amps of current for 1/10 an hour, or 6 minutes.
Let’s say there’s an even larger motor, still, and it draws 20.0 amps of power. In theory, we could expect 1/20th of an hour of runtime, or 3 minutes.
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Internal Resistance, or What Blocks the Spigot
But in reality, there are other forces at play. Have you ever felt a wire, motor, or battery that was warm or even hot to the touch?
As with water flowing through a pipe, current flowing through a wire can lead to energy losses. Akin to friction, resistance will convert some electrical energy into heat.
(Heat and noise are two signals that there is energy loss.)
Battery cells have something called internal resistance. Think of internal resistance as built-up in a spigot that restricts flow.
A battery’s rated charge capacity will decrease based on environmental and application factors.
In an over-taxed battery cell, you would likely see higher cell temperatures, faster voltage drops, and reduced runtime due to greater energy losses and higher internal resistance.
So while a battery’s amp-hour spec is in theory a constant rating, it is dependent on other factors.
Size, Capabilities, and Capacity
Now let’s consider different battery cell sizes. You have a smaller cell, such as 18650, with 3.0Ah rating, and a larger cell, such as 21700, with the same 3.0Ah rating. Assuming they are both modern high performance cells, the 21700 cell will be rated at being able to supply more current.
Why? Manufacturers push batteries to their limits and from that they can specify safe operating parameters for their cells.
A weight lifter can bench press 200 pounds and complete 6 reps. How do they know that? Testing.
Consider 2 battery packs, both compact, one built with the smaller battery cell and the other built with the larger cell. Connect the same tool, say a low-powered LED flashlight. You might see comparable runtime from both tools. Then connect a heavy duty tool, such as a reciprocating saw that’s being used to cut through a tough block of material.
The tools will draw high current from both battery packs. The smaller one, with lower continuous current rating, won’t be able to power the tool for long before it overheats. Or, if it doesn’t overheat, it might at least still lose more energy to internal resistance, and get hotter. Battery packs are designed to cool their cells, but there is a maximum thermal dissipation rate. If they can’t cool cells fast enough, temperature sensors will shut things down beyond a certain temperature, to avoid damage to the cells or battery pack.
Amp-Hours are Not Amps
Ben reminded about a common misunderstanding. Amp-hour ONLY refers to charge capacity, or the “size of the gas tank.”
Heavier duty power tools will benefit from a higher-rated battery pack, because they’ll run for longer before a battery needs to be charged or swapped out for a fully charged one.
When batteries of the same class are used, battery packs should perform similarly. With all things being equal, only the runtime is longer.
Consider the amp-hour to water jug analogy. The higher the number, the more it can hold. But the spigot should be similar or even the same.
In other words, a 4.0Ah battery pack should perform the same as a 5.0Ah battery pack, and vice versa. The 5.0Ah pack should in theory last 20% longer before needing to be recharged.
If all other considerations are not equal, such as if you’re talking about a compact battery pack (5 cells) and a high capacity pack (10 cell), or even larger packs (15 cells), that changes everything.
More Muscle, Less Fatigue
Now consider a 2.0Ah compact battery pack and a 4.0Ah high capacity battery pack. The compact pack is built with 5 cells connected in series, while the higher capacity battery pack is built with 10 cells connected in series and parallel.
What happens if you have 2 weight lifters, with each able to lift 200 pounds? They can lift a 400 pound boulder together. Some brands program their tools to draw a little more power from higher capacity battery packs, because they can.
But let’s say you have a 200 pound boulder. One weight lifter can lift it, but the boulder is at the limit of what they can handle, leading to some huffing and puffing. Two weight lifters will make easier work of it, because they’re only working half as hard as their max output.
A similar analogy can be applied to individual battery cells. An 18650 cell that is rated to deliver a maximum continuous current of 15A will run hotter than a 21700 cell of the same capacity that is rated to deliver a maximum of 30A of continuous current.
Specific Examples
Here are battery discharge characteristic graphs for two batteries – Samsung IRN18650-30Q, and Panasonic NCR20700A. These graphs ignore battery pack design considerations and only look at individual cells.
This Samsung INR cell, with 3.0Ah charge capacity rating, has a max continuous discharge current of 15A.
It’ll reach 60°C after fully discharged at 10A. At 15A, it’ll reach 60°C after only 1.3Ah of discharge, and 80°C after full discharge. At 20A, it’ll reach 60°C after 0.95Ah, and 100°C after full discharge. There’s no operating temperature range rating that I can find.
This Panasonic 21700 cell, with 3.1Ah capacity, has a max continuous discharge rating of 30A.
At 10A, the Panasonic cell will fully discharge and reach ~42°C. With a 20A discharge rate, it’ll reach ~60°C when fully discharged.
The larger cell will run cooler and longer at higher current discharge rates.
When More Isn’t Better
While some brands used 18650-sized 3.0Ah cells in 6.0Ah battery packs, stepping up from 2.5Ah cells to 3.0Ah cells results in a significant drop in continuous current ratings. When I asked one product manager about 6.0Ah packs, they told me that they tested a competitor’s pack, and saw that their 6.0Ah packs were providing less runtime than 5.0Ah battery packs in demanding applications.
Cell Size and Cooling Considerations
Smaller Li-ion cells have reached a technology ceiling. Larger cells can achieve higher performance ratings, but there might be a limit on energy density since larger cells have less efficient cooling.
Surface area of a cylinder is 2πrh + 2πr^2, and the volume is πr^2*h.
Consider an 18650 battery cell, as a perfect cylinder with 9 mm radius and 65 mm height. Its surface area would be 4.2 cm^2, and its volume 16.5 cm^3.
Consider a 21700 battery cell as a perfect cylinder with 10.5 mm radius and 70 mm height. It surface area would be 5.3 cm^2, and its volume 24.2 cm^3.
Going from an 18650 cell to 21700 results in a 47% increase in internal volume, but only 26% increase in surface area.
Thus, when you hear people talking about reduced cooling efficiency of larger battery cells, that’s what they’re referring to. Battery packs are not fixed designed, and so brands can work on pack designs that make up for this.
BUT, as shown in the above plots, larger cells run cooler than smaller ones, at least in those two specific examples. The Panasonic 21700 3.0Ah battery is clearly a better choice than the Samsung 18650 3.0Ah battery in demanding applications. 2.0Ah or 2.5Ah battery cells in 18650 size factor might even be better choices too.
Measuring Ah
Lastly, I can measure amp-hours of any given 12V Max, 18V, or 20V Max battery pack. I have the equipment to do so – it doesn’t take much. An electronic load provides a steady load to a battery pack and measures how long it takes before the battery pack reaches its operating voltage cut-off point. From all that you get an amp-hour measurement.
Or, measure the power draw from something like an LED worklight, and time low long it will be powered by a selected battery pack.
Measuring amp-hour rating for batteries under heavy demand? That’s a different story. The necessary lab equipment sells for $4,000 to $5,000 (and up), and that’s not including what would be needed to create a safe testing environment.
The bottom line is this: a battery’s amp-hours are based on manufacturer’s ratings, real-world conditions, and usage behaviors.
Questions?
rmkilc
Actually, your amp-hour analogy is flawed. A 2.0Ah 12V battery pack isn’t going to have the same amount of water in the bucket (energy) as a 2.0Ah 18V pack. The amount of water in the bucket is actually watt-hours, not amp-hours. The size of the spigot is current and the water pressure is volts.
Stuart
Most of the discussion is about individual battery cells, with 3.6V nominal voltage.
For a 12V Max battery, you’ll have 3 jugs stacked and connected to one spigot. For an 18V battery, you’ll have 5 of the same sized jugs stacked and connected to one spigot.
For simplicity’s sake, the reader is driven to assume that voltage is an arbitrary and constant value.
So while I agree that you’re right, I’m not wrong. =)
Andrew
Maybe the 18V bucket is higher up , and the spigot connects to a longer hose so there’s more pressure.
fred
Your weight lifter analogy might be getting apropos to the “Popeye-the-Sailor” forearms we are starting to need to use some cordless tools. That cordless drill – coupled with a 9Ah battery starts to get pretty heavy when working all day overhead. I’m glad to see that manufactures have introduced compact batteries and smaller 12V tools – as bigger is not always better.
But for the “bigger battery” what do you think is next? Is some sort of forced cooling (air or liquid circulation) on the horizon? How about adding surface area (e.g. fins) to cell cans? Or do we need to wait on the next paradigm shift to new battery chemistry?
Stuart
Right now the push is to larger cells. While in theory larger cells might not cool as effectively as smaller ones, new chemistry or technologies allow those cells to run cooler. Larger packs might allow for different cooling configurations inside, which could compensate.
Smaller battery packs are still great for powering compact tools, and higher capacity battery packs are great for powering heavier duty tools.
These latest battery trends are pushing forward the “corded performance” envelope, where we have cordless table saws, larger bladed miter saws, and things like air compressors.
I can almost see what the next few years will bring in regard to heavier duty tools, but I can’t see what will happen with compact and “core” tools.
J M
Your spigot / water analogy doesn’t quite hold water (at least for me – pun intended):
But now consider that the spigot size matters. That container might hold 5 gallons of water, but attach a very large spigot and you might not fill as many cups of water as if you had used a small spigot.
Spigot size shouldn’t impact how _many_ cups, but how long it takes to fill those cups.
Stuart
True, but here’s how I was visualizing it:
Kitchen sink faucet vs. bathtub faucet. If you’re filling cups of water at the bathtub, you’re going to lose a lot of water as it splashes over the side of the cup. It’s a messy analogy that doesn’t quite fit, but I couldn’t visualize a better way to put it.
In theory, yes, the spigot size shouldn’t matter. In practice, it absolutely does.
Cr8on
Stuart, I was with you all along, I thought your analogies worked!
Stuart
Thanks!
I do agree that some of them are imperfect. The problem is that it started off as an email. If I were to approach this as a perfectly clear post, it’d be on my to-do list for months, and the final post would be twice as long. I hoped that the more conversational nature would help with flow, and that it was clear nothing explicitly mentioned could be assumed to be momentarily ignorable, such as voltage.
JoeM
I think your analogy works if you don’t use “Water” as the item flowing, but rather “Fluid” instead. That way, Barrel, Spigot, and Fluid are all understood to be Container, Flow Rate, and Voltage. But, the same size Barrel filled with Motor Oil represents a different motor type than a Barrel filled with Water or Alcohol. They all flow at different speeds, representing the different voltages, and their Amperage draw out of the battery as a container.
You get more work done with an 18 volt class tool, drawing from a 3Ah battery, so it’s the higher flow-rate Alcohol. The 12 Volt class tool would be more akin to Water, just as easy to fill, but slower, and don’t get as much done. Then there’s Motor Oil, which would probably represent what happens to either one when they’re under too high a load to do the job. Powering a Circular Saw with a 1.5 Ah Battery, for example. You get one cut, maybe two, and then you have to refill the battery, or change it out. The flow rate has too much volume for the tiny spigot.
Going back and forth between conversational analogy and the actual equations gets really, REALLY, difficult.
Stuart
I considered that, but I’d have to use exaggerations such as water vs maple syrup, and it seemed too messy.
JoeM
Well, without the actual equations, it is REALLY messy. I don’t blame you for trying to find a metaphor. You’re not wrong about the metaphor, or how it works, it’s just that we’re talking about a system that has more variables than the Metaphor does. Electricity and the conversion between Amperage, Voltage, and Wattage as it applies to both Cordless AND Corded tools is extremely varied across the board.
Your Metaphor works with 3-4 variables at most. But Power Tools are using 3 base variables, and 3-6 resistance variables. It’s not fair to say any one metaphor is better than the other when this is the case. At least the metaphors you used make some sense. It’s all you can ask for, really.
You need Ohm’s Law, Faraday’s Laws, and Maxwell’s Equations all together in order to really break it down. The equation needs to be substituted with other equations in place of their standard variables, just to cover the use of Time as one of the conditions you’re measuring. And for those of us who just look at the tool, the battery, and the job, and say “Yeah, this’ll do it, just like last time” the equation, or the thought of such a unified equation, makes one think their eyes and nose might spontaneously start to bleed if they saw it.
Jim Felt
So when will we return to good old days of coal fired steam powered belt drive tools? Not all this throwaway imported “modern” plasticky stuff.
Asking for a friend.
JoeM
Not to put too fine a point on this… but we do have theoretical technology that releases energy from splitting Water into Hydrogen and Oxygen, then recombining the vapor from the resulting combustion back into Water again. Plus Hydrogen Fuel Cells, and eventually some of the more exotic Nanotube or Graphine based energy cells that actually work more like the coal fired steam engine and belt drive than we’d like to admit. Using a Nanotube to conduct large currents to a bath of electron-rich fluid, through which a belt of graphine naturally “Pushes” itself through using the inertia of the electrons passing through the Graphine as a small motor of sorts… The resulting electron flow would be great… if we could make Carbon Nanotubes and Graphine cheap enough, and ABUNDANT enough to actually make a battery out of it.
Problem is… By the time you diagram the thing out, it does end up looking an awful lot like a nano-sized belt-driven dynamo, fueled by the strange behaviour of these materials when in contact with certain electrolyte solutions… And you do end up looking crazy for suggesting it. It’s real, and the laws of physics say it’s efficient, but the people it would help most won’t be the ones with the PhDs that understand HOW it works, it’ll be the ones who just want a cheap, light, reliable battery. Attempting to market something that requires a PhD to understand is kinda shooting one’s self in the foot, no matter how well it may work.
Paul
I hate to burst anyone’s bubble on this but fuel cells have 2 major flaws.
While the H+ (not H2) transport across a membrane to react with O2- is efficient, how are you going to produce enough H+. It comes from H2, but graphine is not a great H+ material. It does transport the the electrons well. You also need a platinum [family] catalyst to form the 2H+ from H2.
The second more pressing issue which I do not think that you are going to fuel cell is that you need a huge amount of hydrogen gas. Where are you going to get it? The hydrolysis reaction to break water into hydrogen and oxygen AND back into water (via a fuel cell) is only about 10-15% efficient. The only places that we where we have enough energy to do that reaction on a large scale is nuclear power (where H2 is free) or solar power (where you have a large usable energy source that you need to store up regardless of the loss).
To some this up, fuel cells have tremendous potential as an energy storage but and significant overall loss. It is not economic on an industrial scale and I do not see that changing in the near future.
Paul
Hmm, you’re right. it does kind of look like a dynamo.
JoeM
Oh, no bubble burst here. I’m just playing off the connection Jim Felt there made between the old fashioned goal fired steam engine and belt-driven technologies of the industrial revolution, and the oddly similar movement of the new Nanotech research has to those old devices. Put a series of sheets of Graphine on edge like a cog, around a pivot point, and submerge it in an Electrolyte Solution (Even Gatorade has common Electrolytes in it. High-Ion saturated fluids. There’s hundreds of them out there, not saying it’s easy, but the experiments have been done.) and that cog spins. Because the free ions in Electrolytes pass through any number of the sheets of Graphine, and create a very simple electron flow that passes through the Graphine in waves, which exerts a force on the sheet. This does one of two things very well, either pushes the sheet forward through the fluid like a motor unto itself, or, given the pivot point, and other sheets of Graphine, turns the pivot point, just like an old fashioned Steam Boat, or Water Wheel.
Is that a battery? No. Will we use it for Tools? Possibly. There are ways of using Nanotubes and Graphine to increase the efficiency of our tool batteries, and they’re already being put into place. Which, given Jim’s comments… Is a little eerie in its resemblance to old industrial tech.
As to Fuel Cells, such as the Water To Hydrogen Fuel Cell… That’s a giant battery, not for tools. But, the fact that it generates Water Vapor the way Coal Fired steam engines created Water Vapor, makes it another interesting parallel to those industrial technologies of the past century.
So, it’s less about these technologies totally replacing our tool batteries, and more about how much the Future looks like mechanisms from our past. I mean, if a battery was using an Electrolyte Solution inside a battery, the standard way, with the Anode and Cathode transferring electrons through the solution, then it isn’t hard to at least IMAGINE that a series of Nano-Dynamo Graphine wheels might be slowly-but-constantly turning along the bottom of the battery compartment, where the pivots are all driving what is essentially a belt drive to a larger magnetic dynamo somewhere on the battery itself. Giving these batteries a HYPOTHETICAL self-healing ability, or self-charging ability. Perhaps we end up changing from electric recharge on these types of batteries, to a simple fluid change. I don’t know. I doubt it’s all that efficient, but all jokes aside, our Future Tech is alarmingly similar to our Past Tech, just on a much smaller scale.
Chances are better that we’ll use Carbon Nanotubes to replace some of the Copper and other Metals used in our batteries’ wires and coils. Knowing how much the Tool industry relies on their Batteries for their bottom line, that will probably be the only concession we get any time soon. It’s the only cost-effective way to keep the batteries disposable and consumable, while still making efficient changes the Consumers and Users can see and feel.
All that, and I just enjoy Scientific Research. I’m a total Nerd, really. An Academic far more than I am any kind of Testosterone Junkie. Despite the audience sometimes booing me, I do find these discussions interesting to have here on ToolGuyd. Getting the wide perspectives of all the industrial types here really puts some fresh ideas in my head, for what could be done with such materials, should I ever miraculously become some real-world version of Tony Stark, or manage to work for Elon Musk in some wacky alternate reality.
Paul
My PhD was to try am make the “cogs”. I know others have but mine never spun:-(
JoeM
If it was fully workable, we’d already be using them, Paul. Your PhD work not achieving the goal is every bit as valuable as those that achieved the spin action. You proved how delicate and precise this method is, and probably advanced our knowledge of the margin for error by decades with your version.
Think about it. Say to a tool company “This is an easy solution! It works every time!” and they’ll instantly try to jump on a way to cut corners and costs. But tell them, “You just have to keep them within these tolerable parameters. If you’re off even by an electron in all this, it’s dead.” and they’ll stop and reconsider the manufacturing process.
Which means you’re exactly as important to the progress of this technology’s creation as everyone else’s work. You pushed the limits, and provided data that can act as control for these things. Someone will pick up your thesis and say “Okay, not going to let it go that way” and try something different.
That, sir, is awesome work on your part.
fred
Ha! that modern coal stuff? Why not go back to water wheels or windmills?
I’ve visited old mills in England (Coldharbour in Devon) and New England (those in Pawtucket and Lowell are highly recommended as places to visit ) – built when falling water then steam had become more practical than beasts of burden (donkey or oxen power) as the prime mover.
Hilton
From a realistic point of view all this doesn’t really matter. If you have cordless Dewalt tools you buy their batteries and carry on with your life. As far as I know manufactures don’t offer battery packs with a choice of cell be that 18650 or other.
Your only decisions are “What is the minimum battery pack size my tool requires?” and then “Can I afford a bigger one?”.
Understanding Stuart’s post is of course fantastic for overall knowledge gain and I applaud your effort. Thank you.
Stuart
There is a lot of confusion out there, with many users not perfectly understanding which battery packs in any give platform are right for their needs.
I get emails asking about which amp hours are better, faster, or more powerful.
With the newest battery pack designs and technology, runtime isn’t the only benefit, and that requires a good explanation.
Hilton
I certainly wasn’t criticizing you, it was a good post.
Stuart
I didn’t take any offense. Simply sharing more of the “why” behind the post. =)
You gave me a good segue.
Joe
I can say with certainty,that my Flexvolt tools such as the circular saw will cut more lumber and rip more lumber with its 9ah actually 3ah on 60v Tools then any 9ah on a 18v tool…….Not only that but it does it with ease as compared to an 18v circ saw they will struggle tremendously ripping stock……
I’m not a tech guy but I own all brands and use mostly 60v tools and M12 for small stuff……
ToolBoxHero
Over in Lawn & Garden we are quickly switching to Watt-Hours as the only way to compare one battery to the next.
There are so many different voltages and so many brands trying to “fudge” the numbers to make you think their 56 volt is better than the competitor’s 54 volt and another’s 60 volt. Watt-hour is the only realistic way to sort them out.
It’s now required labeling. https://www.lion.com/lion-news/march-2016/how-to-find-watt-hour-rating-of-lithium-ion-batter
Tool Of The Trade
Thanks for clearing that up Mr Stuart.
Hilton
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JoeM
I’m just going to leave this here.
https://www.graphene-info.com/graphene-batteries
https://en.wikipedia.org/wiki/Carbon_nanotube_nanomotor
Having a proper education on a subject is always helpful and interesting. Then the big words don’t sound so scary, and the future is actually something to look forward to.
James
Stuart, do you think there would be some value in testing or calculating battery performance? Maybe it would require the largish capital expense you mentioned but perhaps you could monetize it by doing something no one else does. From a laymen perspective if we could see everything in the same standard that would be nice. Even better if we could see the units be comparable to the tools. Maximum continuous watts (usable power) out and total watt-hours at say 3 different power draw ratings. Tools could be rated by power input/output ratings as well. Maybe the manufacturers would pay you for certifying battery packs or tools. Call it the Toolguyd Rating. All over youtube you can see people posting about tools. Some are better at reviewing than others. Head to heads comparisons are fun but I don’t think I’ve seen any standardized testing.
Stuart
I’ve considered different testing projects in the past, but things get complicated fast, and the takeaway get a muddied. In other words, yes, but every time I visit the idea, I end up concluding that there are better uses of my time and resources, where there’s greater benefit to readers.
ACC
I thought I had a pretty good understanding of batteries, but why do people report getting more power from Milwaukee’s larger extended batteries over the lower capacity ones? Are the cells combined to yield higher voltage? Are they using a DC voltage booster?
Personally, if the power is the same, I love using the smallest batteries for their light weight and small size. Easy to carry a few and charge time is fast. Holding a 6 amp battery of your head all day, or hanging from your belt seems like a pain.
I suspect if they are boosting voltage and power it will help them move the bigger more expensive batteries.
Stuart
10 cells vs. 5. Think “D” battery instead of “AA”.
If 1 cell can deliver say 20A max, 2 cells in parallel can deliver 40A max. In theory.
Milwaukee designed some of their tools to receive a ~10% bump up in performance when paired with an XC battery pack instead of a compact pack.
Let’s say Rich is helping you move boxes. You give him average weight boxes to take to the truck. Let’s say Joe and Dave are helping you, and they’re working together. You can give them heavier things to carry to the truck.
Lance
Stuart, I applaud your efforts, but simple analogies are not going to work. The math is dead simple, why not just stick with that?
Once that’s clear, explain that power is equal to voltage x current; increase voltage or current and you get more power. Keep the current the same and double the voltage, you get double the power.
Then the only other concept you need to drive home is that wires of a certain gauge (thickness) can only carry so much current efficiently, that is, without too many losses due to heat. Therefore higher voltage allows for more efficient transfer of higher power because current is always the limiting factor, be it with wires, circuits, motors, whatever.
Give all those confusing analogies a break and make people actually LEARN something if they want to understand it. You also have to be OK with realizing not everyone is going to get it. That’s fine. At the end of the day, if you put a battery on a tool it will do work, and that’s all anyone really needs to understand.
Stuart
Focusing on voltage is a whole different topic.
An 18V 2.0Ah battery has more stored energy potential than a 12V 2.0Ah battery, but power output depends on the Tool.
There are 12V tools that will outperform certain 18V tools.
The point of the post was to focus on Amp-hours and other characteristics tied to that. Vary voltage as well, and now I’d need to write a test book chapter to convey everything clearly and thoroughly.
JoeM
I think the closest we’ll ever get is if someone made a spreadsheet document that does the math for you. You enter all the known variables, and it solves what it can solve in the spaces below it using said equations.
It’s pretty safe to say UWO and Ah are too far apart mathematically to equate them, or use them in the same equation. You need an equation to extract the values from each term, and put them into terms the other equation can use.
By the time you substitute all the added math you do to make each equation replace each variable, you’ve got a migraine, no matter who you are. Ohm’s Law, Maxwell’s Equations, and Faraday’s Laws are all involved in power tool usage, but if you line up their equations, and substitute their bits and pieces to fill in the values for each set of rules, it’s not just “Throwing the Equation up there” anymore, you have to solve things in order, or it all goes wrong.
Paul
My PhD was to make those cogs. Got the degree. Couldn’t make them spin.
Paul
Sorry for the repeat post.
Joe Smith
Amp hours are not even that useful of a measurement. They are only good for comparing batteries of a similar voltage.
The important numbers are watt-hours for comparing the capacity of dissimilar batteries and max discharge current.
JoeM
If the goal is to choose a new tool platform, yes. If the goal is to compare your needs based on your CURRENT platform, then no.
I think the DeWALT 20 Volt Max/XR/FlexVOLT family is pretty ideal in demonstrating this. You pick up one of their brand new 20 Volt series saws, maybe that new brushless wormdrive saw, or the deep-throat bandsaw. You buy a kit with a 4Ah Battery or two in it, and they’re starting to run down. Now… You see a sale on all 20 Volt Max/XR batteries somewhere. So now you have choices.
Using the Time per Ah on your tools calculation, you can compare the 4Ah battery price to the 5Ah battery price, and see if the Price per Ah scales for your budget, and your average use. It may well come up that, due to some promotion going on, that magically the Per/Ah amount is somehow much lower for a much higher power battery, like the 6Ah, or a pack of FlexVOLTs. It helps you decide the per unit price to get the most out of your purchases. And comparing the per-unit price is much easier to do while shopping than it is across entire lines and platforms.
Ben
I enjoyed that!, good, intelligent information/conversation- Thanks to all.
Robert Adkins
Battery labeling is a hot mess. The industry better clean it up before Big Brother forces them to. It’s shameful that you have to dig out your battery tester and patiently test to find out how many Ah or WH the battery holds at low, medium, and high drain… AFTER you have bought the battery. Some people even go to youtube and watch some savage plop the batteries into a massive leaf blower, which he states the longest run time wins… regardless of whether the battery has thermal cycled.
JoeM
I think another “analogy” discussion should have been started. this has led away from the actual topic of batteries capacities v ah ratings and all that.
Analogies aren’t an issue unless you don’t understand the point being made, everyone understands the point, right?
This has been helpful for me as I am considering cordless circular saws and batteries required v. drill drivers requirements. Hell, if you have enough charged battery at the start of the day there really isn’t an issue. but changing them every hour or two v. every 10-15 minutes can make it an issue.
Thats my analogy.