Batteries for powered tools

[Dunc]

I notice that most of the kits - regardless of mfr - include small batteries, for eg, 1.5 amp hour. (Dealing with the li-ion 18-20 volt)
While this will "run" the tool, can it do it to the max? I am not thinking of how long the tool will run; instead,is the battery capable of driving the tool to its rated performance?
Do some tools require more power than that available from this size battery? What would be a more suitable size (amp-hour) battery to drive a powerful tool?
I am thinking along the lines of an SDS+ drill or angle grinder.

[eKretz]

Some tools have higher torque/power ratings with a bigger battery. It seems to be a more common thing with the newer tools. You'll probably have to check each individual maker and tool to find that out.

[Paul Alciatore]

EDIT: I went over this post after reading J Tiers' comment, "Paul A: You mean total energy contained........ not "power", which is a rate of energy delivery." I have crossed out any misusage of the term "power" and inserted "total energy" as needed. I believe it reads correctly now. My corrected post follows with the edits in red.


The power delivered by a battery depends on two factors: Voltage and current. (Edit: Voltage multiplied by current is the formula for POWER. No correction is needed here.)

The Amp-Hour rating of a battery speaks to ONLY ONE of those factors, the current that is being delivered. Amp-Hour ratings are more of a measure of the total amount of //power// energy that a cell (battery) can contain. It does not tell you ANYTHING about how fast or slow that current (//power// energy) can be released from the battery/cell.

The previous paragraph seems to be the only two places where I incorrectly used the term "power" instead of "total energy".

Current is given by Ohm's law:

I = V / R

Where I is the current, V is the Voltage, and R is the TOTAL resistance in the circuit. We can not change the battery's Voltage: that is controlled by the chemistry that is used. So the current and hence the power that is delivered is most dependent on the TOTAL resistance. And the TOTAL resistance will be the sum of the internal resistance of the battery itself and the external resistance of the item being powered.

OK, we can and do change the Voltage by adding more cells to the overall battery. We have 6 Volt batteries, 12 Volt batteries, 18 Volt batteries, 24 V batteries, and many others. But once the overall package size of the battery has been determined, then it is hard to add more cells. However this is why we have progressed from lower Voltage tools to higher Voltage ones. The increase in Voltage directly effects the power delivered (Edit: Power is the correct term. No correction is needed here.). Double the Voltage and you will be close to twice the amount of power delivered. This is why we have 18 and 24 Volt tools/batteries instead of 6 Volt ones. Side note, I had a 6 Volt, battery powered screwdriver and it was a real wimp; almost totally useless and a total waste of my money.

But back to that TOTAL resistance thing. It is the sum of the internal (battery) resistance and the external (tool) resistance. For a given make and model of tool, that external resistance is fixed and hard to change. So we are left with that internal resistance. Unfortunately the battery's internal resistance is the smaller of the two so any decrease in it may deliver more power to the tool, BUT the effect is a minor one. As an example if we move up to a 2 AH battery from a 1 AH one of the same Voltage, it will have a lower internal resistance. BUT it will not provide a two time increase in the power delivered (Edit: Power is the correct term. No correction is needed here.). It will be more like 5% or 10% and the user of the tool will be hard pressed to even notice it. This is due to the external (tool) resistance being the larger of the two and it is the sum that counts.

The Amp-hour rating has more to do with how long you can use the tool before the battery needs a recharge. The "hour" part of that term tells you that. A 1 AH battery can supply 1 Amp for 1 hour. Or 1/2 Amp for 2 hours. Or 2 Amps for 1/2 hour. Etc. Amps times hours equals Amp-hours. It is a product and that equation works within a range of values. It will however fail at very low or very high currents (Amps). (Edit: I included this last sentence because the AH rating does vary with conditions as others have been so kind as to report. But for relatively small changes, like 2X or 0.5X, the difference in the AH rating will not be that much.)

The Voltage is the thing that most directly controls the amount of power delivered (Edit: Power is the correct term. No correction is needed here.) I should have added that with the same total resistance, the current will rise in proportion to any increase in Voltage. Thus, the amount of power delivered to the tool will increase as the square of any Voltage increase. This is a big part of why higher Voltage tools are more powerful. But you can not exceed the power rating of the tool itself. If you force it to deliver more power than it is designed for, it will probably overheat and release some of that magic smoke. Tools with motors will already draw more current when they are heavily loaded and that alone produces more heat. To increase that heat is just asking for failure. I do not recommend raising the Voltage.

So get some cells that have higher AH ratings if you want and install them in the battery pack. But do not expect great results. One thing they will do is maintain the rated power for a longer time in use. And more time before needing the charger may be a good thing.

PS: I wrote an article about rebuilding the NiCad battery packs of a decade ago ("The Cheap NiCad Rebuild", Dec 2015-Jan 2016, Machinist's Workshop ). One thing that I discovered while doing that was the use of sub-sized cells in the OEM battery pack. Instead of standard C cells, they used ones that had the same diameter but were SHORTER. That decreased the AH rating of the battery while not changing the Voltage. So the OEM battery pack had to be charged more often. Those short cells were installed in the battery pack with spacers to make up the full length of a standard C cell. I replaced the sub-sized cells with standard ones by simply throwing the spacers away. This did increase the AH rating. The full sized cells did hold more energy and I did get more usage time per charge, but I did not get any more power at the tool.

[PStechPaul]

Most new battery powered tools now use Li-Ion or LiPo batteries, which have very high power density as well as very high maximum current capacity, as much as 50 to 100 times the nominal rating. A 2 A-h battery will typically handle about 10x rating, or 10C. For a 20 volt tool this will be 400 watts or 1/2 HP. Modern tools most likely have current limiting built into the battery as well as the motor and controller. So it is possible that a higher A-h battery will deliver more power (as torque), the current limiting circuitry may ultimately determine what it can do. There are also low voltage sensors that disable output when depleted, and temperature sensors that prevent damage to battery or motor.

[J Tiers]

Paul A: You mean total energy contained........ not "power", which is a rate of energy delivery.

There is a rating of energy delivery rate (power in joules/sec), and there is a rating of energy capacity, which is generally ampere hours.

Ampere hours is the most useful check of energy and delivery capacity, as it will vary with current, for the reasons Paul mentions. A good data sheet gives the ampere hour capacity at different rates of discharge (currents).

The nominal ampere hour capacity is the capacity at a certain current, often the "20 hour rate", the rate which drains the battery in 20 hours to whatever the "endpoint" is defined as.

But, the capacity in ampere hours at the 10 hour rate (2x the current) may not be the same as at the 20 hour rate. and the 1 hour rate (20 x the current) is almost sure to be significantly lower, since some power is dissipated in the battery, and that may depend on the the square of the current, or some function generally following that, depending on how much like a true resistor the battery acts.

From the various rates, you can estimate what the battery will give in terms of time in use. You do have to know what the current is when the tool is used in your application, so that you can determine where you are operating in terms of discharge rate..

[PStechPaul]

Lithium batteries differ significantly from older NiMH, NiCd, and Lead-Acid chemistry.

Lead-Acid capacity is famously determined by the Peukert effect, which allows the full rated A-h rating at 1/20 C, so a 20 A-h battery will deliver 1 amp for 20 hours before depletion. But it is recommended to limit discharge to no lower than 20% or 10% of full charge. At 1C, only about 1/2 rated A-h can be expected. Lead acid batteries will typically supply up to about 10C for short duration current surges such as a car starter, and internal impedance will cause a voltage drop to about 1/2 nominal voltage. That is why older cars used a resistor in series with a 6 volt ignition coil, and the ignition switch bypasses the resistor when starting to get full spark voltage.

Lithium batteries have extremely low internal impedance so they can supply 10x to 100x their nominal rating and still have most of their full charge voltage available. And unless internal or external current limiting is provided, a short circuit can cause dangerously high current that may melt internal components and result in lots of heat and fire. I have a lithium pouch cell that still reads about 3.7 volts after at least 10 years of storage.

[mattthemuppet]

power tool packs use li-ion chemistry, typically the high current draw type (INR?) hence the low Ah ratings. Most of them are rated to 20-25A current draw per cell, some up to 30A. So if you have a small pack which will have 5 cells in series for an 18-21V pack, that will be able to supply say 25A (x20V) continuously. If the power draw is higher than that, you either need to increase the voltage (more cells in series) or increase the number of cells in parallel. For a given tool line, you can't increase the voltage otherwise the battery for your drill won't work in your leaf blower. So they add cells in parallel. Cells added in parallel add to the current capacity, so for a single string of 5 cells you get 25A max continuous, so 2 strings of 5 cells in parallel you get 50A max continuous (and double the Ah). The bigger packs run the lower power tools just fine, but the small packs struggle with the higher power tools, to the point where you can kill them if you do it enough.

[mattthemuppet]

what Jerry said also applies - the capacity of the pack depends on the current draw per cell. So a 1.5Ah pack with 5 cells (25A max current) will give you 100% of rated capacity at 1.5A but perhaps only 70% at 25A, whereas you'll get say 85% of capacity from a 3Ah pack with 10 cells. You also have voltage sag too, where you can drag down the voltage of the pack enough that it'll trip the undervoltage protection even though there's plenty of capacity left in the cells.

[eKretz]

Yeah the AH rating isn't what determines how much power the pack can put out. There are absolutely "high discharge rate" and "low discharge rate" cells. These have differing internal resistance and are capable of very different maximum power outputs. Some tool makers limit output on the smaller AH packs so they don't exceed their maximum discharge rate and/or go dead in a matter of single digit minutes with heavy use. This is possible because they don't just use a straight battery to motor through a switch connection anymore - there are microprocessors involved these days...Some of you guys might want to do a little more reading.

[J Tiers]

Actually, Peukert's "law" has limits. It relates to a single constant discharge to a given end point. The voltage sag issue Matt pointed out is really what is happening.

The "capacity" is "recovered" if the discharge is stopped. and/or a different lower discharge rate is used. This is because the voltage drop is different at a lower current, and because time at no discharge (time out) gives diffusion a chance to move electrolyte to the plates, etc, etc.

The "time out" recovery is a real "recovery", which will even allow added capacity to be "found" at the previous discharge rate.

Since diffusion varies with temperature, Peukert's law does not work as well when the battery warms as a result of discharge, or simply at different ambients. The exponent may vary with temperature. It definitely varies with battery construction type. So thee is not "one" exponent, nor is there "one" capacity at all temperatures.

Yeah the AH rating isn't what determines how much power the pack can put out. There are absolutely "high discharge rate" and "low discharge rate" cells. These have differing internal resistance and are capable of very different maximum power outputs. Some tool makers limit output on the smaller AH packs so they don't exceed their maximum discharge rate and/or go dead in a matter of single digit minutes with heavy use. This is possible because they don't just use a straight battery to motor through a switch connection anymore - there are microprocessors involved these days...Some of you guys might want to do a little more reading.

It IS an indicator that will tell you a lot about the battery. If you can get info about the A-h capacity at different rates, AND different temperatures, then you are getting the real deal.

That's not to mention that the discharge capacity is what you ask from a battery, so it is pretty relevant. And, possibly for that reason (yah think?), it is generally the main information that you will be provided by the battery manufacturer.

ONE A-h rating is only useful for "those" specific conditions. But with 2 or more, you can derive the Peukert exponent (at the test temperature), or just get useful information more specific to your application and planned discharge rate. Getting temperature curves at different rates is really helpful, and you can select suitable batteries on that basis for many applications.

That information WILL allow you to determine a good estimate of the total energy you can expect to get from the battery under your conditions. That may not be the most useful data, but it is derivable if you do want it.

Most applications want to know how long the battery will sustain a given discharge rate. That allows determining how long a product will work, at given conditions, per charge. Normally that is the important factor. It will determine "miles per charge", time at temperature per charge, number of standard holes drilled per charge, etc.

Some other factors, such as for products used intermittently, are modifiers to the number from the A-h curves. Those do not, however, invalidate the A-h as the basic useful measurement of effective capacity.

[eKretz]

It IS an indicator that will tell you a lot about the battery. If you can get info about the A-h capacity at different rates, AND different temperatures, then you are getting the real deal.

That's not to mention that the discharge capacity is what you ask from a battery, so it is pretty relevant. And, possibly for that reason (yah think?), it is generally the main information that you will be provided by the battery manufacturer.

ONE A-h rating is only useful for "those" specific conditions. But with 2 or more, you can derive the Peukert exponent (at the test temperature), or just get useful information more specific to your application and planned discharge rate. Getting temperature curves at different rates is really helpful, and you can select suitable batteries on that basis for many applications.

That information WILL allow you to determine a good estimate of the total energy you can expect to get from the battery under your conditions. That may not be the most useful data, but it is derivable if you do want it.

Most applications want to know how long the battery will sustain a given discharge rate. That allows determining how long a product will work, at given conditions, per charge. Normally that is the important factor. It will determine "miles per charge", time at temperature per charge, number of standard holes drilled per charge, etc.

Some other factors, such as for products used intermittently, are modifiers to the number from the A-h curves. Those do not, however, invalidate the A-h as the basic useful measurement of effective capacity.

I'm not arguing against any of that... All accurate.

All I was saying is that some of the tool manufacturers limit the discharge rate/current of the battery through the tool via the microprocessor control. The AH rating of the battery doesn't actually control that, except in these cases, the tool's microprocessor "brain" per se, allows a high discharge rate/current with a large AH battery pack, but only a lower one with a small AH battery pack. There are identification chips on those types of battery packs that tell the tool whether to allow higher current discharge or not.

[Paul Alciatore]

I have edited my post above to correct a couple of errors in terminology and to add some additional, hopefully clarifying remarks. I believe it is correct as far as it goes.

That post did not address any "intelligent" controls that may be present in the tools or the batteries. These controls can go far beyond the basic theory involved here and they can do so in many different ways. I know that OEMs can do things for different reasons. The story about sub-sized C cells in my previous post shows that these reasons may not be completely without ulterior motives. Unfortunately that is often the case. The OEM in that story released a more expensive version of the same batteries which was said to offer longer life per charge. A switch to full sized C cells in the same plastic battery case, but without the spacer, would explain how they could so easily do so. They did not improve the batteries, they just used larger ones. I call that a sneaky trick, if not a dirty one. But they did it. And I am sure that they and other OEMs can and do use other tricks in their sales tactics. "Intelligent" controls are a tremendous opportunity for incorporating such tricks into a product. So, YES, it may very well be possible to improve the performance of a tool/battery combination if a way of getting around these "tricks" can be found. That may or may not involve a battery pack with greater ratings in AHs or whatever other spec. they may be playing around with.

I welcome any proven improvements that can be shown.

[Willy]

The Torque Test channel on YouTube is a gold mine of some real world results using various ratings of batteries in cordless tools, They cover a lot of this subject matter in a lot of the tool tests done each week on a regular basis just to demonstrate exactly how various battery sizes and technologies affect tool output, some surprising results at times.
I think that they've only been online for about a year so you won't have hundreds of videos to chose from, but have a good look at their lineup and I'm sure you'll find some interesting material to whet your appetite.
Like I said though much of the information you are looking for is covered in a lot of the regular tool tests, especially the impact gun tests.
Rather than offer video suggestions I'll just leave the link below.

https://www.youtube.com/c/TorqueTestChannel/videos

[BCRider]

Without getting into all the theory about this I can report that this is an issue. Not with the lighter duty tools but certainly with the two Ryobi angle grinders running on the 1.8 and 4amp-hr pack options.

The smaller packs do load down and run a few RPM slower under a given load than the bigger 4A-hr packs. And if pushed much harder than that the smaller packs self cutoff and I have to release to reset the cutoff controller in the pack. So for myself I much prefer the big packs for the angle grinders I have and the little circular wood saw which is also a power hog.

The Ryobi line does not include an SDS style hammer drill (that I know of) yet so I can't offer anything on that side of your question.

_____________

OK, one bit of almost theory.... The cells in the packs have a maximum CDR (Continuous Discharge Rate) where the cell won't see the voltage drop too far or the cell overheat. This is typically related to the amp-hr capacity. The little safety circuits in the packs limit this by measuring the voltage or voltage and current. Mostly it is so the pack is cut off before the cell voltage becomes too low and damage is done. But it also provides a current safety limit since the output voltage is also related to the current being drawn. Pull high current and the pack voltage will slump due to the load.

That's why the smaller packs can be pushed so hard that they cut off even with a half charge if loaded too heavily. This forces us to work more lightly or to buy the higher capacity packs which can deliver the current needed without sucking down the voltage too far.

I'm sure someone above mentioned this but with all the other theory being thrown in (good but pretty much way off the topic of the question asked I'm thinking) it's in there but perhaps not so obvious.

[darryl]

Pretty much all the cordless tools are capable of drawing 20 amps and more when under load. If the battery pack is weak or has a low C rating, it will heat up and waste that part of the power. At the same time, the reduced voltage to the motor will have you trying to draw even more current to keep the rpm up. The motor will dissipate more of the battery power as heat also. Add in the tendency of people to pulse the trigger and you have a recipe for short battery life, and probably tool life too. And that's not to mention the speed controller element as well. When the voltage drops there isn't enough to fully turn on the mosfet, and it will heat up- you would be lucky to smell it and stop using the tool. If you push it too far it will burn up.

All in all, a cordless tool should have a battery with a high C rating, and you should be careful if you're rebuilding a battery pack to only use that type of cell. The exception might be a flashlight, where it never draws lots of amps. Then, a lower C rated battery might have a lower self-discharge rate.