Convoy S21A (B35AM) Flashlight Review

Convoy S21A (B35AM) Flashlight Review

The Convoy S21A flashlight is a big brother to the S2+, and similar in most ways. The main difference: it runs on a single 21700 cell!  And a big difference for this version is the Nichia B35AM emitter.

Official Specs and Features

Here’s a link to the Convoy S21A (B35AM) flashlight product page.


As with most Convoy flashlights, the Convoy S21A 21700 flashlight is available with many options.  The specific listing is a 4500K Nichia B35AM emitter, 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.)


When the S21A flashlight comes available again (probably soon), it’ll be $22.35.  Right now Simon and the whole Convoy Store are on break for Chinese New Year.  That should wrap up soon.  So despite these saying “out of stock” or whatever, they’ll be ready soon!  You can buy the Convoy S21A B35AM here!

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. Most importantly, this is a great way to try out the Nichia B35AM emitter!

Long Review

The Big Table

Convoy S21A B35AM Flashlight
Emitter: Nichia B35AM (4500K, High CRI)
Price in USD at publication time: $22.35
Cell: 1×21700
100% Runtime Graph 100% Runtime Graph
LVP? Warning, then off
Switch Type: Mechanical
On-Board Charging? No
Claimed Lumens (lm) 1500
Measured Lumens (at 30s) 1019 (67.9% of claim)^
Candela per Lumen 10
Claimed Throw (m)
Candela (Calculated) in cd (at 30s) 383lux @ 4.898m = 9188cd
Throw (Calculated) (m) 191.7
Claimed CCT 4500
Measured CCT Range (K) 4200-4500 Kelvin
Item provided for review by: Convoy Store
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 B35AM Flashlight what's included

  • Convoy S21A 21700 Flashlight
  • Lanyard

Package and Manual

Convoy S21A B35AM 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 B35AM Flashlight

Convoy is very highly regarded by flashlight enthusiasts for having great build quality and low prices.  At around $23, 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.

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 B35AM Flashlight tailcap threads Convoy S21A B35AM Flashlight head threads

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 B35AM Flashlight cell tube

The tail end has a nice thick spring, while the head has only a brass button.  You can see the retaining rings in both head and tail too.  Parts inside are easily accessible.  Note that this seems to be a design change from my other S21A, which had springs in both head and tail.

Convoy S21A B35AM Flashlight head and tail contacts

Size and Comps

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

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

Convoy S21A B35AM Flashlight in hand

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 B35AM Flashlight beside the TorchLAB BOSS 35 (custom engraved)

Convoy S21A B35AM Flashlight shawty

Retention and Carry

Convoy S21A B35AM Flashlight lanyard holes

The main (and only included) means for carrying the Convoy S21A 21700 flashlight is the attached lanyard.  It ships from Simon attached in this wrong way.

Convoy S21A B35AM Flashlight lanyard holes stock lanyard

Easy to fix, though.

Convoy S21A B35AM Flashlight corrected lanyard

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

Convoy S21A B35AM Flashlight lanyard installed Convoy S21A B35AM Flashlight lanyard

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 (and only) tube supports a single 21700 cell.

Convoy S21A B35AM Flashlight with 21700 cell

The cell goes into the light in the normal direction – positive end toward the head.

Convoy S21A B35AM Flashlight with 21700 cell installed

Convoy S21A B35AM Flashlight parts

Here are a few runtimes for a couple of different mode groups.

Convoy S21A B35AM Flashlight runtime graph Convoy S21A B35AM Flashlight runtime graph Convoy S21A B35AM Flashlight runtime graph

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% 1019 4.72
35% 472 1.48
10% 145 0.40
1% 10 0.04
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 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 B35AM Flashlight reverse clicky

This is the same user interface that I covered in my Convoy C8+ and S2+ reviews.  And for that matter, the other Convoy S21A before this one!

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:

convoy faux-biscotti

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

The whole point of this review and of me even being interested in another copy of this light is the emitter.  It’s a Nichia B35AM, and my copy is 4500K.

Convoy S21A B35AM Flashlight emitter

Why is this important, or awesome?  Well let me copy text from Clemence (source link here)

Near gapless configuration of 4x E21A die size on a ceramic package with phosphors deposited in between the seams. This 3,65mm x 3,65mm LED is a miniaturization of the bigger 144A (5mm x 5mm) but with color quality and CCT range of E series. Although the max output and current capabilities are less than normal 4x E21A setup, the tighter uniform “donut free” beam and robust ceramic package makes this LED easier to implement where tight beam control is required. The white reflector “fence” make sure most of the light directed upwards and reduce the color shift over angle.

The point is that it should be awesome.

Convoy S21A B35AM Flashlight emitter

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

Convoy S21A B35AM Flashlight emitter on Convoy S21A B35AM Flashlight emitter Convoy S21A B35AM Flashlight emitter on Convoy S21A B35AM Flashlight emitter on

LED Color Report (CRI and CCT)

Here’s the most important part of this whole review!  How does the Nichia B35AM fare!  Well, it’s very good.  Even without clicking the images below, you can see how tight that red circle is to the black circle – that’s good, that’s the goal.  The CRI is around 100, which means this is not just “High CRI” but “very high CRI.”  In fact, CRI of 100 is perfect.  The scale does not go higher!

There are two points other than CRI here.  First, the CCT is also fantastic.  “Fantastic” is a matter of perspective – I happen to like 4000K-5000K just fine.  The Convoy S21A B35AM 4500K clocks in just under 4500K even on the highest level.  This is great! If that’s too warm for you, grab the cooler option. Here’s a reminder though:  Just because you get a cooler (or warmer) CCT doesn’t mean the CRI will suffer!  The CRI can still be just as good as this sample even over different CCTs.

The next point (other than CRI and CCT) is the dUV.  If I was picking a downside (and also stating why this 4500K doesn’t match my 219b reference light almost exactly), it’d be that the dUV is only negative on the highest output.  There will be a slight pinkness on 100%, but all other modes, since they have positive dUV, will not be pink at all.  I think you can see this clearly in the beamshots below.


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!


What I like

  • Great way to get into a Nichia B35Am emitter option
  • Low price
  • Build quality is great, especially for such a low-cost light
  • No PWM
  • Can run 18650/20700/21700 easily

What I don’t like

  • No pocket clip
  • Positive dUV on all but highest output


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3 thoughts on “Convoy S21A (B35AM) Flashlight Review”

  1. Hi!

    I really love your runtime charts.

    But to mention “Not available to order during Chinese New Year” under “What I don’t like” is really misplaced. In a couple of days that point will just look really weird.

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