Basic Nerf Stryfe Electrical Modifications, Part 1

Or, "How not to let the magic smoke of your blaster".


Quite frequently, a post on Reddit will ask, "Just got a new Stryfe, what mods should I do?"  Inevitably, a rapid barrage of well-meaning advice will overwhelm the newbie. Among these are:
  • Do not use IMRs, they are unsafe. You will have to rewire it and use LiPO.
  • Twist your fuse to bi-pass it.
The first, while always couched as promoting safety in our hobby, is unfortunate and mixed up with a lot of baggage.  It is also discouraging to anyone looking to take first steps in modding.  Recommending someone completely rewire their blaster with a high-energy source without proper understanding and most likely, as in the second piece of advice, without fuses, is in my opinion significantly more dangerous.  I think we should encourage beginners to take small steps and learn as they go.

Captain Xavier did a good video on rechargeable "AA-sized" batteries covering the history and why TrustFires, which are actually ICRs, not IMRs, ruined it for everyone.  My testing of various types of rechargeable batteries (other than ICR) show that they are safe in stock systems with the following caveats:
  • Stock motors only, not aftermarket which draw exponentially more current.
  • Reputable brands with capacity ratings, not cheap ebay stuff.
  • Two cells (2S) only, and with the stock fuse in place.
So, in my effort to "promote safety in our hobby", let's take a look at what actually occurs in a stock blaster when you pull the rev switch and fire a dart.  Ignoring the power source for a moment, let's concentrate on the motor, which is the primary factor in current draw.  The motor has several operating conditions:

"Total Current" is the combination of both motors, or total battery current.
  • Starting current, which occurs when power is applied but the motor hasn't started turning yet.  In our stock system with AA batteries, this may be above 2 amps, but will last less than a second.
  • Unloaded current: As the motor comes up to speed, until a dart is introduced to the freewheel, it is essentially idling.  In our stock system, this is under 0.5 A.
  • Loaded current: As the dart is introduced to the flywheels, the motor is loaded for the period it takes for the dart to clear.  At 16000 rpm, the flywheel is actually making a single revolution in under 4 ms, faster than an eye blink, so this occurs very, very, quickly, but the repercussion of the drop in flywheel speed plays out over a second or more.  This max current will be somewhat less than starting current, and will fall back to the unloaded current as the flywheels come back up to idle speed.
There is one other motor condition that is important to the discussion and that is stall current.  If a dart jams and the motors come to a stop, we basically have the starting current scenario with the exception that is does not resolve itself.  The motor will continue to draw maximum current unless a) the jam clears, b) the rev switch is released, or c) the thermal fuse comes to the rescue!  As the fuse heats up, it increases its resistance which, according to Ohm's law (I = V/R) throttles the flow of current, saving your motors.  After it cools down, usually in the same amount of time it takes to clear the jam, you are back in the game.

Stock System Stall Current and Motor Speed.
Of course, if you bypassed your fuse and fail to recognize a jam in the heat of battle, your motors become an expensive, non-resettable fuse, and the release of the acrid magic smoke occurs.  Hopefully you are carrying a secondary because your primary is literally and figuratively "toast".

Alright, enough doom-and-gloom.  We know a stock setup on AAs draws just over two amps and is anemic on dart speed.  What are the "first steps" for modding?  Change your batteries.  Let's look at data for the following:
We are primarily interested in dart speed, but in our quest for understanding the system, let's look at the startup current and flywheel speed of the various configurations.  Since this occurs on such a quick time frame, the x-axis is log scale. 


Clearly Ohm's law is still in effect as the LiPO and IMR systems, fully charged at over 8V, show significant increases in starting current and motor speed over the others.  NiMH, with the lowest voltage, is still able to provide similar starting currents as alkaline and LiFePO4, but the lower motor speed will show in lower dart speeds, relegating the venerable NiMH as merely a cost-saving option if you already have them available.


Time for a comparison of darts speed, which are the average of 10-shot series using Adventure Force waffle darts because, name brand, readily available, inexpensive, yada yada . . .

Battery
fps
% difference
AA x 4
67.5
 -
NiMH x 4
66.4
 -1.7%
LFR x 2
65.4
 -3.2%
IMR x 2
96.5
 43%
LiPO 2S
97.1
 44%

Clearly, for a first step in performance upgrades without rewiring or shell cutting, the Nitecore IMR batteries look like a winner.  Their 6.5A current rating is clearly within the draw requirements of the stock motors.  The stock fuse, while engaging in about half the time of AAs during a stall test, is still protecting the system, even with repeated rev cycles.  If power is applied while the motors are still spinning, the starting current is significantly lower, so a natural time buffer keeps the fuse from overheating.

In Part 2, I will try and convince you that if you are going to open your blaster to rewire, that MOSFET switching is the way to go and will provide a bigger increase in performance for a given configuration than removing your fuses.  Standby for more data.  And more talk of fuses . . .

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