RANT: The Battery Wars Rage On

Ah yes. A classic hot-button issue, and a very divisive one. I have felt like doing a post about it for a while, but I did some recent stalking around on the interwebs, and let's just say that shots are still being fired, and the war is definitely still alive, so now is as good a time as any, I guess. In this post I really hope to clear some things up about the battery firestorm.

So first, a little history for the uninitiated. The "battery wars" primarily concern the power systems for electric Nerf guns and the efforts to improve on longstanding status quo practices, which for reasons unknown (but maybe all too familiar) have been met with tremendous resistance.



I speak of course of the heated discussion over things like TrustFires, stock wiring, battery packs, soldering, connectors, current draw, resistance, torque, trigger response... this strange, tense and undesirable hostility about anything involving foam and amperage. You have probably seen it in the wild even if you don't mod.

Image credit: http://reddit.com/u/Temstar - /r/nerf

You also probably, unless you live under a rock, know me specifically based on the subject of electrical modding, and perhaps you know me as a notorious combatant in the battery wars. I do understand that my contributions to the hobby are not uniformly positive or well received, and that is unfortunate, but I am absolutely not ashamed of what I have done and I retract nothing I have said on the subject. It is in the name of progress, and making the nerf hobby a better place for all of us. I am aware my reputation is not a clean one and I am viewed variously as a quack, a bully, a theoretical unrealistic engineer guy, a hardass, an elitist... all over this electrical stuff which I think is ridiculous - but you know, I don't really give a damn. No one ever had any vision or introduced any new disruptive idea without stepping on some toes. The status quo fights back. And fight back it has. I expected it. So that's that. If you think I was a jerk, don't take it personally, and keep reading because I have a feeling it will seem far from an insult when you understand the logic of it.

It has a new name: Goodwrench 280!

The situation with nerf electrical, and the prompt for me to have an active interest in its improvement, is that we are lagging far behind other hobbies in our standards and common sense and our results DO in fact suffer from it. Seriously, the number one problem here is lack of perspective.

I have seen many posts and comments along the lines of "*Fires work, quit hating" and many, MANY "elitism" comments about people advocating the use of i.e. battery packs. Especially directed at me.

So let's look at some of our fellow hobbies, shall we?

RC has small electric motors of considerable performance requirements powered by batteries. Just like us. Airsoft, also has small electric motors, and batteries, as well. Granted, these are not nerf, and no nerf gun has precisely the same requirements as anything done in RC or airsoft - but the general nature of the hardware and the requirements are similar, especially with respect to current handling, the issue of contention in the battery wars, and the big problem with people using subpar hardware. I don't want some discussion of some "voodoo thing" that makes RC and nerf so radically different because such an argument is bullshit and missing the basics and confusing the issue. It really can be boiled down to exactly two situations for a power system to operate under.

1. Supplying continuous average currents to motors in steady-state operation.
2. Supplying transient currents to motors under transient conditions, such as startup, acceleration or sudden load.

The magnitudes of the currents encountered in nerf may not be the same as those in other cases - but the magnitudes of the currents are known. And that leads us to the nitty-gritty of what can be called the Reference Battery Example.

A SMC FN130A-2080 motor (OEM in the Nerf Barricade and extremely similar to the Stryfe motor) has a nameplate rating (from the manufacturer's datasheet) for stall current of 6.94A at 7.5V.

This means that for a flywheel system with 2 of these, theoretical stall current (a logical design point for transient conditions, such as the instant the switch is closed on stationary motors) is a little under 14 amps with a 7.4V supply voltage. Since stall is a purely resistive condition and V=IR, you can extrapolate 20.5A for a 3S lipo.

The minimum continuous current can be measured in the real world with the actual Nerf flywheels. With a set of smooth Rayven flywheels, the motors draw a little over 3 amps while freewheeling at full speed on a 11.1V battery that holds its voltage well under said load.

#blades4life

For flywheel guns, these are excellent example figures because these motors are a best-case scenario for minimizing the current demands placed on the batteries and wiring harness. Excepting lame and slow-revving green Rayven motors, everything we could ever use is wound more aggressively, with a lower winding resistance, thus a greater stall current, and is less efficient (shoot your Blades and feel the heat!). Our current requirements to make flywheel motors give up their reasonable spproximation of their full performance are thus, on average, higher than 3A continuous, 20.5A burst. AEG motors are of course much larger than tiny 130 flywheel motors, and typically of greater current requirements.






Now let's look at trustfires.

These are a ~500-700mAh (real capacity) cell, a classic low-drain LiCoO₂-cathode cylindrical lithium-ion. The manufacturer (and manufacturers of reputable, name-brand equivalents of this cell) state the maximum continuous discharge rate as variously 1C, 1.5C and 2C, the latter of which works out to about an amp; or if you use the nameplate rating for capacity (which is a lying "900mAh") you get 1.8A. This is backed up by what very experienced and trusted, and well-equipped, independent battery reviewers say in the flashlight world - one such test, of an average-performing Fire-brand 14500 cell, revealing an internal impedance of 200 milliohms. Not twenty, 200. Zero point two ohms. Remember that.

Now we add, to that, a battery holder having spring contacts made of steel. We will be generous, and say that the total length of the spring wire is one inch per spring (uncoiled) and the wire gage of the spring wire is similar to 24AWG electrical wire. The resistivity of steel (specifically the music wire typically used to make springs, for completeness) is approximately 6.941 times greater than standard values for copper. Thus, versus the copper wire with 25.67mOhms/foot, the steel would have 178.18mOhms/foot, or 59.4 extra milliohms for all the springs in the typical "Radioshack tray" - again, conservative on the spring length and gage, and completely throwing away the interconnects and steel rivets in the tray, and the contacts of the cell terminals with the tray which are not exactly low-resistance.

Ouch

Then we have, in the average mod, about 2 feet of 24AWG copper wire - that's 51.34 more milliohms.

Let's once again ignore anything else, like stock switches, thermistors, and unsoldered joints which all have an associated resistance.

So we have, for the 3 Trustfire cells in a Radioshack (or, typical, including stock) AA holder wired to a flywheel gun with stock or 'cade motors:

• Cells: 200mOhm/cell x 3 = 600mOhms.
• Holder: 59.4mOhms.
• Wire: 51.34 mOhms.

In total we have 710.74mOhms - a substantial part of one ohm - of undesired resistance in a best case NIC-typical TrustFiring of a nerf gun.

Our nominal battery voltage is 11.1V. Note that if we allow 15.64 amps to flow, these parasitic resistances have consumed the full available battery voltage. In other words, the TrustFire setup can supply 15.64A into a short circuit - a load with an effective resistance of zero, to which zero power is delivered as a result.

Now let's get back to our motors. The datasheet tells us for stall current:

6.94A @ 7.5V

This is assuming a terminal voltage of 7.5V - or worded another way, an infinitely current-capable 7.5V supply, which has an apparent internal resistance of zero and does not sag at all under load. From this we can back out the winding resistance from Ohm's law as 1.081 ohms. We have two of these motors in parallel, so we will cut this in half to 541 milliohms. Now we plug that back into the circuit.

Not SMCs... But ya get the point

Our total resistance is now:

• Rest of the power system: 710.74mOhms.
• Stationary FN130A-2080 motors: 541 milliohms.

All totaling 1.252 ohms. Now what current can our 11.1V battery voltage drive through the 1.252 ohms? V=IR... 11.1=I*1.252 so I=8.87...

8.87 amps.

Now, this is still super conservative. When batteries are heavily overloaded, they do not act purely resistive - the current/voltage relation becomes curved and they really do "fall on their face". If you look at HKJ's wonderful archive of flashlight-oriented lithium ion data here you will see some examples in which even with his reasonable test currents he manages to overload a weak cell into nonlinearity (1, 2; the "protection test" curves). But we ignore any possibility of this happening, which in reality is there with these Chinese and old used cells - we want to give the TrustFires the benefit of the doubt, right?

Remember that we calculated the stall current of one flywheel cage of these motors at 11.1V to be 20.5 amps earlier! So we now have cut that more than in half, to 8.87 amps for the motor pair. Again, conservative.

Well, current and torque are best friends. You can't have one without the other. Current IS torque. It's a long story, but the two are, approximately, linearly related. See: http://www.farnell.com/datasheets/62656.pdf http://www.me.umn.edu/courses/me2011/arduino/technotes/dcmotors/motor-tutorial/ http://en.wikipedia.org/wiki/Brushed_DC_electric_motor.

As a result, by throttling the current through those motors so severely, you just took the "mountain" of your torque curve and hacked the top off of it. Turned it into a bit of a molehill, so to speak. Now you can make far less peak torque, which is exactly what you need as you come around that corner, run into that zombie, mash your rev trigger, and... oh god, please accelerate! It's also not just a "hard limit" scenario, in which the batteries bang into a ceiling and below that you get "optimal" operation. That extra parasitic resistance (whether it's just harness resistance or the IR of your batteries) will be there, choking your system, at any motor speed and any current draw.

The alternative way to look at this is with the term "voltage sag". At any given current, the high-resistance battery/harness combo will reduce the available voltage i.e. you have less speed with a given torque - or less torque at a given speed. This better explains what I mean by "not a hard limit, but an always-on effect". At idle, your motors will rev slower with the SkipFire/24AWG harness, because the idle current sags the voltage proportionately to that resistance.

Is it a bit clearer now the magnitude of the shortcomings and why I push so hard for them to be corrected? Even in such a mild case, the benefits to be had from upgrading this electrical stuff are significant.

LOL... This mess was actually removed from a donor gun. WTH?

Now back to the other hobbies. Once again, perspective. Doing things like running 200mOhm cells on a multi-amp motor load, using 24AWG to wire multi-amp motor loads, and the like, are completely and utterly laughable everywhere EXCEPT nerf. You do not see RCers running packs of TrustFires on their cars and planes. You don't see airsofters duct-tape PVC pipes full of half-dead alkalines to their guns. Everyone knows the value of a good honest battery pack, nice thick wire, a quality connector - performance, reliability, safety. These people have been over this, years ago, decades ago, and they figured out that it was logical to use the sorts of hardware that me and other modders are advocating we use.


How about this: Would you use TrustFires on your cordless drill? You will have, like, no torque (I know for real, as I tried some of my old shitty NiMH packs, some AA cell cheapo Radioshack jobs I have since sold, that are still superior in IR to whatever-fires on a fairly average-spec'd B&D drill of mine, and compared to my newer Li-ion packs and my good NiMH they have zero grunt!) What would you haters SAY about that? That having 1/4 the torque is not a problem? "Performs fine"? "Good enough"?

Well... I don't think that is what people would say. Why? BECAUSE IT'S NOT NERF! So you give "other stuff" what it deserves and works well, and cut all the corners on nerf? Bullshit!! This is what makes me so damn angry. This right here.

So let's stop with the "elitist" comments. If I wanted to be elitist... I would declare all flywheels not powered by brushless 540 motors to be subpar, and insist on the use of 7500mAh 75-130C ThunderPower lipos and 8 AWG welding cable, or you aren't meetin spec! Now THAT is overkill and disconnected with reality! Perspective, people!

Care to explain what's elitist?

The things being advocated - something like using a 3S 1300mAh 25C lipo, 16 AWG wire, and XT60 connectors to power your Blade motors - are far from overkill. They are just a minimal to moderate level of "doing it right" in my opinion - and on paper, from a design standpoint. Wanting to get at least 80% of the performance I paid for when I bought my motors (whether the ones that came with the gun, or $2.50 Blades, or $10 Xtremes, or $20 Tamiyas...) is not elitist, overkill, or tantamount to nerf Monster Cable!

Despite persistent myth, entry-level battery packs are not uncommon, exotic, or expensive. In fact they can have better economics, especially in the long run, than WhateverFires. They are used by LOTS and LOTS of people for lots of things. Same for the other parts. Fairly pedestrian.

Yes, including my expensive overkill RC-specific connector... that costs pocket change and is robust and durable as all hell.

So hopefully that explains the logic behind this side of the argument.

I welcome comments, questions and flames.