Convoy S21A 21700 Flashlight Review

Convoy S21A 21700 Flashlight Review

The Convoy S21A 21700 flashlight is a big brother to the S2+, and similar in most ways. The main difference: it runs on a single 21700 cell!

Official Specs and Features

Here’s a link to the Convoy S21A 21700 flashlight product page.

Versions of the Convoy S21A 21700 Flashlight

As with most Convoy flashlights, the Convoy S21A 21700 flashlight is available with many options.  The specific listing I purchased this item from is only Luminus SST-40, but there are two CCTs available: 6500K and 5000K (seen here).  Many other emitter options are available (and probably custom options if you ask.)

There are also two user interface options: 12 groups (seen here) and 4 modes.  Also available are many body colors, but not Convoy’s full range.  Just Green, Orange (seen here), Purple, and Red (at the time of writing.)


The Convoy S21A 21700 flashlight sells for $17.40 regardless of the options picked on this aliexpress listing.

Short Review

There are a few reasons to buy this light (that is, instead of something like the S2+, which everyone should have anyway).  The larger mass should deal with heat better, and give a higher output for at least a little longer.  There are “bigger” emitter options for the S21A (like the XHP50.2, which I don’t think is usually offered in the S2+).  And you might just have a bunch of 21700 cells laying around that need a good home – this is a great low-cost option for that!

Long Review

The Big Table

Convoy S21A
Emitter: Luminus SST-40 (5000K)
Price in USD at publication time: $17.40
Cell: 1×21700
100% Runtime Graph 50% Runtime Graph
LVP? Warning
Switch Type: Mechanical
On-Board Charging? No
Claimed Lumens (lm)
Measured Lumens (at 30s) 1632
Candela per Lumen 11.2
Claimed Throw (m)
Candela (Calculated) in cd (at 30s) 807lux @ 4.866m = 19108cd
Throw (Calculated) (m) 276.5
All my Convoy reviews!

^ Measurement disclaimer:  I am an amateur flashlight reviewer.  I don’t have $10,000 or even $1,000 worth of testing equipment.  I test output and such in PVC tubes!!  Please consider claims within 10% of what I measure to be perfectly reasonable (accurate, even).

What’s Included

Convoy S21A 21700 Flashlight what's included

  • Convoy S21A 21700 Flashlight
  • Lanyard

Package and Manual

Convoy S21A 21700 Flashlight box

The Convoy S21A 21700 Flashlight has the “old style” lower quality box.  Newer S2+ lights, for example, have a harder side and are more crush-resistant.

There is no manual.

Build Quality and Disassembly

Convoy S21A 21700 Flashlight

Convoy is very highly regarded by flashlight enthusiasts for having great build quality and low prices.  At under $20, and available with tons of options, this is a very high-value flashlight.

Very minimal branding.  The shorty does not have any branding at all.

Both head and tail come off the light.

Convoy S21A 21700 Flashlight

The cell tube is not reversible.  One end is anodized, and one is unanodized.  The anodized end is the tail end.  Also, both ends have their own o-rings.

Convoy S21A 21700 Flashlight Convoy S21A 21700 Flashlight

The finish of this orange light is really spectacular.  The knurling is right on point.  Lights costing orders of magnitude more have had worse knurling!!

Convoy S21A 21700 Flashlight

Convoy S21A 21700 Flashlight Convoy S21A 21700 Flashlight

I failed to get photos into the head and tail, but both have springs much like the Convoy S2+.

Size and Comps

Length / Diameter : 123mm / 26.6mm
Weight: 88g    

If a light will headstand, I’ll show it here (usually the third photo).  If a light will tailstand, I’ll show that here, too (usually the fourth photo).

Convoy S21A 21700 Flashlight

Here’s the test light with the venerable Convoy S2+.  Mine’s a custom “baked” edition Nichia 219b triple.  A very nice 18650 light.

And here’s the light beside my custom engraved TorchLAB BOSS 35, an 18350 light.  I reviewed the aluminum version of that light in both 35 and 70 formats.

Convoy S21A 21700 Flashlight

Ooooh if there was a shorty of this one!!!

Convoy S21A 21700 Flashlight

Retention and Carry

The main (and only included) means for carrying the Convoy S21A 21700 flashlight is the attached lanyard.

Convoy S21A 21700 Flashlight

It’s a standard lanyard from Convoy and attaches through two holes in the tailcap.  This setup allows tailstanding fairly easily, too.

Convoy S21A 21700 Flashlight

No pocket clip is included.  The diameter of the S21A is likely too much to fit a standard S2+ friction clip, but I believe the screw-in-type pocket clip would work fine.

Power and Runtime

The Convoy S21A 21700 flashlight is powered by a single lithium-ion cell.  The default tube supports a single 21700 cell.  The cell goes into the light in the normal direction – positive end toward the head.

Convoy S21A 21700 Flashlight

Here are a few runtimes for a couple of different mode groups.  Since the highest three modes (100%, 50%, and 35%) are broken up across two mode groups, please note that these three aren’t really all accessible “on the fly” from just one group.


Modes and Currents

These output levels are not all available in a single-mode group.  But this represents all the steady output levels.

Mode Mode Claimed Output (lm) Claimed Runtime Measured Lumens Tailcap Amps
100% 1623 6.13
50% 3.18
35% 914 2.22
20% 1.28
10% 0.63
1% 0.05
0.1% 0.01

Pulse Width Modulation

These PWM graphs are representative – this is mode group 2 (0.1%, 1%, 10%, 35%, 100%).

For reference, here’s a baseline shot, with all the room lights off and almost nothing hitting the sensor.  Also, here’s the light with the worst PWM I could find.  I’m adding multiple timescales, so it’ll be easier to compare to the test light.  Unfortunately, the PWM on this light is so bad that it doesn’t even work with my normal scale, with is 50 microseconds (50us).  10ms5ms2ms1ms0.5ms0.2ms.  In a display faster than 0.2ms or so, the on/off cycle is more than one screen, so it’d just (very incorrectly) look like a flat line.  I wrote more about this Ultrafire WF-602C flashlight and explained a little about PWM too.

User Interface and Operation

There’s a single switch on the Convoy S21A 21700 flashlight.  This is a reverse mechanical clicky.  A reverse clicky has the benefit of allowing mode changes while the light is on.  But this also means that the switch does nothing until it is on – no momentary action whatsoever.

Convoy S21A 21700 Flashlight

This is the same user interface that I covered in my Convoy C8+ and S2+ reviews.

This driver is known as “Biscotti” and has a bunch of mode groups (or faux-Biscotti – I’m not sure Simon runs an official version of Biscotti).  The two big differences are the number of taps required to enter programming (20+ instead of 10+) and that this firmware has thermal protection (with the max temperature being 55 degrees C, per Convoy.)

Mode memory can be turned on or off (yay!), and programming is easy!  But there are simply too many possibilities for me to list the UI in a table as I usually do. Here is Simon’s flow chart for the UI.

Here’s the official guide for the Biscotti firmware:


From the mode group selection above, the light ships in mode group 1.  You’ll want to switch it to mode group 2 quickly of course.  🙂  And I always turn off memory if possible (and it’s possible here!).  I made a first pass at my own flow chart, which you can see below.

zeroair reviews biscotti flow chart revision 1

If something’s wrong (or even just “unclear”) in there, please let me know!

LED and Beam

I opted for the 5000K CCT version of the Luminus SST-40.  The reflector here is orange peel.

Convoy S21A 21700 Flashlight

Convoy S21A 21700 Flashlight

You can also see that there’s an AR coating on the lens, which is a nice feature.

Convoy S21A 21700 Flashlight


Convoy S21A 21700 Flashlight

LED Color Report (CRI and CCT)

The 5000K claim seems about right.  Definitely more so for the higher output levels.  The CRI is not impressive, though.  These CRI/CCT measurements are representative – this is mode group 2 (0.1%, 1%, 10%, 35%, 100%).


These beamshots are always with the following settings:  f8, ISO100, 0.3s shutter, and manual 5000K exposure.  These beamshots are representative – this is mode group 2 (0.1%, 1%, 10%, 35%, 100%).

Tint vs BLF-348 ( 219b version) (affiliate link)

I keep the test flashlight on the left, and the BLF-348 reference flashlight on the right.

I compare everything to the 219b BLF-348 because it’s inexpensive and has the best tint!

Conclusion on the Convoy S21A 21700 Flashlight

What I like

  • Low price
  • Great way to get many emitter options
  • Build quality is great, especially for such a low-cost light
  • Highly modifiable (drivers, emitters, etc).
  • No PWM
  • Can run 18650/20700/21700 easily

What I don’t like

  • This specific Luminus SST-40 at 5000K is too low CRI and slightly green
  • No pocket clip


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2 thoughts on “Convoy S21A 21700 Flashlight Review”

  1. Looks like I’ll be adding that to my list. I’m at three S2+ and counting. I also found that emitter to be too green for my taste. Have you ever tried using a minus green filter? Could be a game changer for these. I need to source a sheet.

  2. I bought the S21B when an XHP50.2 version came out (tried 4000K). The same for the S21A (got an orange one in 3000K). Worked out that the S21A with CSLPM would likely have similar performance to the S21B with CULPM so the cheaper price enticed me to buy. One reason I rarely used throwers was their bulk.
    Apparently the S21B has an integrated shelf, it’s not just a cosmetic difference.

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