A Call for Revolution: Supercapacitors in Airsoft

[Pseudotectonic]

I think the rabbit hole of brushed DC motors and inrush currents goes deep and I'm not ready to jump into it, sometime to do with back EMF or something

 

[Sewdhull]

Inrush in this context if the current the motor would take when it's not rotating.

When you first apply power the current the motor will take is just the voltage applied divided by the resistance of it's windings.

As the motor rotates the magnets create an opposing voltage in the windings which effectively reduces the voltage applied to the windings.

As the motor rotates faster there's more opposing voltage until the point where the motor cannot go faster.

This is why more voltage spins the same motor faster.

The inrush current starts at it's max then reduces as speed increases, because the effective voltage in the windings reduces (the resistance remains the same) until the point is reached where the amount of current becomes stable.

Current then rises with load because the voltage reduces as the motor slows because of the load and speed becomes stable at the new load.

The power dissipated in the windings increases with the square of the current and this power heats the motor (with some help from the commutator and bearings from friction and contact resistance.

Torque, the turning force in the motor, is created the magnetic field generated by the current, interacting with the permanent magnets magnetic field.

More current, more torque...Stronger magnets , more torque...more turns, more torque.

Higher speed motors go faster than slower ones because they have fewer windings for the permanent magnets to generate an opposing voltage in and so for the same motor speed fewer windings generate less opposing voltage and a faster max speed.

You can fit fatter wire on the armature with fewer windings so that the motor can take more current to try to get more torque on a fast motor.

More windings is more efficient because the heat (waste) generated is linear with resistance, but exponential with current.

I have deliberately avoided things like commutator timing.

 

[Pseudotectonic]

I thought it is common knowledge that higher load means higher current, and that of course includes the initial spike, and the measurements confirm that. I am not sure what you are trying to get at, but I think if you are trying to theorise higher load will not induce higher current, not sure if that is a feasible exercise because I think it goes against conventional wisdom and also empirical data.

If you are talking about the micro events at speeds of electromagnetic waves (which approaches speed of light) maybe you are onto something but I don't think this affects the "macro" effects of higher load inducing a higher inrush current.

https://youtu.be/O-WCZ8PkrK0?feature=shared&t=666

 

[kadamski]

Standard IR measurements are either using DC in 2 stages or an AC meter. I use mine just to monitor battery health.

Can you elaborate on this standard IR measurements. What you mean by using DC in 2 stages?

In any case, the better chargers does show/measure the IR value. My standard method of measuring the IR value was to use 1Ohm load and check the voltage drop. The values displayed by my charger where roughly the same than the ones I got this way.

Firstly the variance I mention is from IR calculations, in the IR meter report, using the same current but over different time periods leading to different IR values and another IR value in the C rating report. IR values probably vary more than that depending on who knows what else. We have from about 12 to about 21mOhms. That variance is expected, but it limits the usefulness of IR since it is specific to a set of circumstances.

I did explain that difference in the IR on the "cheap IR meter" page. What I did not specifically mention, though, is that when I use "IR" term, I mean "IR at some specific conditions". Like, when the battery is fully charged, when the load is 1 Ohm, when it is kept under load for 1s, when it is new, etc. Changing any of this, changes the IR (although not always in significant way). I might have explained that more clearly.

Your table, for the C ratings says they are measurements, the last 3 columns, say nominal values. Nominal current, volts drop and %. You don't discuss your method so it seems like you have measured the IR ,measurements in the table, put it in the table and calculated the nominal values, I realise you have to calculate a %. Nominal is a word used to describe something in name only or a specification, not a measurement or used or actual value. You don't mention the voltage of the batteries but the volts drop of the GPX would indicate 7.4v was used which is not a fully charged battery, unless it was but you used the nominal 7.4v for 2S Lipos.                                                                                                                  

Then when discussing the GPX battery you say it 'would' drop its voltage by half at because of it's IR when 'does' indicate a thing that has happened. You use 'would' again for another battery. 

My "C rating" post was in the "opinions" section. Probably that is why I was so short and not well described. Even I couldn't figure out what exact method I used to produce this table, which is bad. But my other page "Batteries review" describes this a little bit better.

Anyways, the table has "Nominal Current", which means the current the C rating is allowing (I do understand this term might not be the best, but this is how I called it). So the Rhino battery is 2.2Ah and has 50C so the "nominal current" was assumed to be 110A. Of course measuring anything at that high current isn't easy so I did the IR measurements at lower currents but I expect the IR to only grow at higher currents, not drop, so this is still the "best case scenario". I also did not see a big variance in the IR (as the ratio of Vdrop and current) when the current was doubled for a given battery. So the calculations, for the sake of simplicity, are assuming the IR is similar for any current up to the "nominal current as specified by C rating". So, I calculated the IR by measuring Vdrop at say, 10A or 20A and then used that value to extrapolate the Vdrop that would happen if the battery was loaded with the "nominal" (again, as allowed by C rating) current.
This was a quick test just to show that C-rating is just arbitrary and one can't, contrary to common myth, use this value to reason about the battery behaviour under load. The voltage the battery has under load is an important factor for me. So saying a battery is OK to be used with currents of 40C while at this current it drops its voltage by at least 50% is inappropriate in my opinion and that was all I wanted to show in this short article.

Having said the above if you did indeed draw 72A from that pack and halve its voltage and calculated the IR from that, I apologise for the comments. This bit ' not possible to drop voltage and maintain the current'  is wrong, I meant something else which is irrelevant if the IR is calculated.

Something went wrong with this paragraph as I can't understand the meaning of that. In any case, I can modulate how much current my load takes and obviously the bigger the current, the bigger the Vdrop. But some batteries will drop more voltage at the same current, some will drop more, depending on their IR.

I think the C rating can't be used either, it has been my experience that only occasionally has it reflected the current supply ability of the cells.

Yes, that was the exact conclusion of my article there - C rating does not reflect the current supply ability of the cells. IR is much better at that. But it is also not a constant value in the function of the load or time. The manufacturers could come up with some value that reflects the situation a little bit better, like "how much current can I roughly take from this battery for a period of 1s so that the voltage does not drop more than 10% at full capacity" or something like that. But nothing beats proper graphs showing the situation at different conditions.

The key when conducting some sort of experiment is for the reader to be able to replicate your experiment so all the necessary information must be present for that to happen. A precise duplication of your results probably wont be possible but the same basis of your experiment must be possible.

Standard terminology, a repeatable framework ( aim, hypothesis, method , results, discussion, conclusion). Where you have diagrams or pictures etc, they have to be clear.

That is completely true. But writing such an article requires not only much more work but also usually involvement of some other people, like reviewers. Like, it's often hard to asses what is obvious and what is not, if you are the one who writes the article. Reviewer may easily spot that, though. That being said, this "C-rating" article was in the "opinions" section, the articles in "experiments" section are a little bit better at that.

When looking at the motor current experiment you did, which I really liked over all, there's no baseline motor speed measurement shown but you refer to it and it's relevant as this will tell you where the motors sit in the speed or torque camp. You may not have had the means to measure it, but without it it's harder to make meaningful comparisons.

One thing that puzzled me was why the inrush current changed with a stiffer spring, it's not as if the motor windings reduce in resistance to draw more current. Did you do multiple runs or just one of each set up, because you have anomalous results. But great work anyway and valuable.

You know, I was not aiming at writing a scientific paper grade article and my time and resources where limited. I did do multiple runs and choose the most common/typical one for each configuration. To me, this change of current with different springs was expected - the more stiff spring creates higher resistance so the current must be bigger to start the motor. If you disconnect the motor from the gearbox and let it run freely, the "inrush" current is also much smaller.
Now, I guess, the problem is again in the usage of "inrush current" term. My measuring equipment has a limited resolution/bandwidth so the actual, theoretical current peak may be the same for each setup (and it lasts some fractions of millisecond), but I can't measure that. Instead I can in practice see just the "average" current over some short periods and this is what I see on the scope. This is what I am in fact more interested in - how much current the motor takes in the first few tens of milliseconds after starting but before settling on a max speed. That takes many revolutions of the motor and this obviously is influenced by the resistance the gearbox and the spring gives.

You've probably looked at the Airsoft Trajectory project, a large experiment, a good example. A lot of work tho.

I also read many scientific papers. Of different quality. Creating a very good quality one is just *tons* of work. I did not aim at that. My aim was to do some measurements and describe them so that they are not lost. This was, in my eyes, much more than most people in airsoft do and that is why there are so many myths here. But I didn't care too much about someone being able to reproduce my experiment but I did want to describe my experiments properly and I did not succeed fully. Even if scientific method should aim for that, this is often not met in the officially published scientific papers so I feel excused for my results published on some random website

All in all, the most important conclusion from the perspective of the original discussion here is that the trigger response and RoF in classic airsoft gun (without precocking, etc) depends highly on the capabilities of the battery. The biggest struggle for the battery is at the spinup of the motor as the current is very high and this may make the battery drop a lot of voltage. The smaller the voltage, the slower the motor reaches full speed and thus the worse trigger response. The max speed will also vary between batteries because the speed depends on the voltage and this depends on the voltage drop caused by the current.
I can see how the supercapacitor could help the battery at the motor spinup time, when the current draw is the highest, providing the ESR of the capacitor is low enough for the current to be taken from the capacitor and not the battery itself in large portion. As was said here several times, this would have the biggest impact with NiMH batteries, and I expect it to be much smaller with beefy LiPO. I someone prefers using NiMH battery, such an approach with capacitors may give you some noticeable benefits. Would that help with LiPO? That would have to be checked. It is possible, though, especially with cheaper ones. Problem is - space. The batteries tend to be able to provide more current if they are physically bigger. So by the time you add the size of those caps to the size of the battery, you might get similar or better results by just switching to some bigger battery. Depending on the cost of the caps, it might also be cheaper. The caps, on the other hand, could be reused after switching the battery in very long games and may have a bigger lifetime. All in all, it would be good to measure if there is a noticable difference when using caps with LiPO batteries.

 

[Sewdhull]

I thought it is common knowledge that higher load means higher current, and that of course includes the initial spike, and the measurements confirm that. I am not sure what you are trying to get at, but I think if you are trying to theorise higher load will not induce higher current, not sure if that is a feasible exercise because I think it goes against conventional wisdom and also empirical data.

If you are talking about the micro events at speeds of electromagnetic waves (which approaches speed of light) maybe you are onto something but I don't think this affects the "macro" effects of higher load inducing a higher inrush current.

https://youtu.be/O-WCZ8PkrK0?feature=shared&t=666

Inrush current or peak current, not to be confused with current under load. 

So the maximum current a motor can take is when it's not turning. Inrush begins here and is not dependant on load, unless ofcourse that load stops the motor turning. Once turning the current drops until some sort of equalibrium is reached.

I can't describe that more clearly than in the previous post.

 

[Sewdhull]

You can measure or calculate IR using voltsdrop as you have, apply an oscillating signal using a meter to measure it or a small and larger current to get a better idea of the IR.

I'm curious as to what could raise the inrush current then drop it as spring rates rise, then raise it again as the spring rate rises again.. My thoughts are that it's a limitation of instrumentation, but I don't know why that would be.

This happens for the non specna motors..

The other currents rise with increased loads on all the motors.

https://www.benzoenergy.com/blog/post/battery-internal-resistance-and-its-measuring-method.html/

There's a description the AC measurements there.

 

[Egon_247]

I have come to the conclusion that years of drugs/drink/women has destroyed my knowledge of most things capacitor based other than how to charge up a big one with the legs bent round and chuck it to your mate...

?

So, my hat is off to you sir, much respect for getting this far and good luck. Hope it pays off for you. ?

 

[Pseudotectonic]

@kadamski I agree with more testing with LiPo. In actual fact, rather than me buying a bunch of LiPo and just simply measuring the trigger response in milliseconds, it would be more economical and informative for me to make another unit and simply send it to you to be tested with your LiPo and looking at the currents and voltages with the scope, if you don't mind the work and sharing the results of course. That will be more insightful than me measuring with Audacity.

In fact the offer is up for anyone happy to test it, the condition is you need to have a scope to look at the currents and voltages (with LiPo) and share the results. (I will pay for postage and everything)

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And btw, I have uncovered the ancient texts from the "Airsoft Mechanics" website (2005) somehow stored on the Internet Archive. There is an article and some forum posts which some interesting discussions that somewhat mirror the discussions here...

https://web.archive.org/web/20110511090621/http://www.airsoftmechanics.com:80/home/content/view/16/30/1/0/

https://web.archive.org/web/20090726234228/http://forums.airsoftmechanics.com/index.php?topic=78. (sadly only the first page was saved)

In the article they say they tested with some salvaged PC motherboard "2200mFD 10V" capacitors which I think is a mislabelling on their part. Because a 2200 mF or 2.2 F 10 V capacitor would be firmly in the realm of a supercapacitor (rather than a normal capacitor) but I don't think you can find these on PC motherboards. Also looking at their small picture it seems to be rather a 2200 uF (which is 0.0022 F) capacitor which is more plausible coming from a PC motherboard. In their text they refer to "microfarads" instead of "millifarads".

Although their caps were in the millifarad territory, which I don't think would work, somehow they say they have improved ROF by around 3% and an unknown amount of reduced voltage drop (I think their image is missing).

I don't know what to make of this. Maybe it is down to the fact they were using some 8.4 NiCd on some TM motor. The forum did confirm the OP didn't improve trigger response, although another person say it did but with unknown capacitors. All without any actual numbers so I don't think it means anything other than confirming my maths about they need more farad than a few millifarads to actually improve trigger response.

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And for the sake of continuity here is the Reddit post for future reference: https://redd.it/18k2324

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Also semi-relevant: https://www.youtube.com/watch?v=tKki89sq0XY

 

[kadamski]

@kadamski I agree with more testing with LiPo. In actual fact, rather than me buying a bunch of LiPo and just simply measuring the trigger response in milliseconds, it would be more economical and informative for me to make another unit and simply send it to you to be tested with your LiPo and looking at the currents and voltages with the scope, if you don't mind the work and sharing the results of course. That will be more insightful than me measuring with Audacity.

I will try to check if I can setup a measurement environment next week. I'm not entirely sure if shipping the unit to me would be the most economical option, maybe it would be simpler if I build one myself. You didn't specify which exact supercaps you use, though. Or I missed it?

 

[Pseudotectonic]

The ones I'm using are PTV-6R0V305-R

https://www.digikey.co.uk/en/products/detail/eaton-electronics-division/PTV-6R0V305-R/13404670

(They are 3Fs)

Resistor is https://www.digikey.co.uk/en/products/detail/yageo/FMP100JT-52-2K2/9107445

LED is https://www.digikey.co.uk/en/products/detail/broadcom-limited/HLMP-1601/637567

Wire is 16 awg

Deans is deans

Btw I'm studying the pros/cons of omitting the resistor because the LED alone draw more than enough milliamps to function as draining resistor (with its built-in resistor), perhaps this is something you can try

 

[Sewdhull]

I think if you have a supplementary power source, esp something like capacitors, you will take some of the load off the primary power source.

Those smaller caps are likely to have much lower esr than the larger ones and may be able to supply a burst to help the battery whose voltage is sagging which is significant. Getting enough energy out of them to matter would be the issue.

There are some some fairly standard caps of 47000uF 16v with 13mOhm ESR. Sadly they would only give you 47mA for a msec for each volt they were above battery voltage, but could supply 77A. getting the right cap for the job, esp given what is available its tricky and you may have found the best cap already for the task.