Convoy S21B

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Convoy S21B review

Convoy S21B specifications

Brand/modelConvoy S21B
LEDOsram Boost HX (CULPM1.TG)
Max. OutputUnspecified
Max. beam intensityUnspecified
Max. beam distanceUnspecified
Battery config.1*21700
Onboard chargingNot available
Modes5+, with 12 different mode groups
BlinkiesStrobe, biking, battery check, SOS
Review dateFebruary 2021


Here we have a classic straight tube Convoy light with an extra throwy LED.

Convoy sell a wide range of flashlights. The “S” series are all rear-mechanical switch straight tube lights (apart from the S12). The S2, 2+ and 3 are all 18650 cell size. S21A and S21B are 21700 and S11 is 26650. This time we have the S21B, which is an updated version of the S21A.

If you’re familiar with the S21A, from what I can tell the S21B has a few changes:

  • knurling is replaced by rings
  • integrated shelf instead of brass pill
  • smooth reflector instead of orange peel
  • different LED options

I have the S21B with the super-throwy Boost HX LED.

Package quality.

Just look at this fantastic box! Convoy has spared every expense here. Not much to say, other than the box and bubble wrap protected the light and the label was useful as I forgot what LED it had.

Inside the box there is:

  • Convoy S21B flashlight
  • Lanyard

Handling of the light

The S21 lights have a straight tube from tail to head, so they fit in a pocket well. They’re 21700-sized (27.2mm wide) though, so aren’t as slim as the Convoy S2. But it could definitely be a great EDC flashlight.

The S21B has ring indents for grip instead of knurling on the body tube. This gives just as good grip and seems to be mainly an aesthetic change. The light will roll if you put it down on a sloping surface, so I’d recommend attaching a lanyard to keep it safe. 

The mechanical switch is at the end. It’s reverse clicky, so turns on and changes modes as you let go. There’s 2 lanyard holes on 1 side of the tail, which means the light can still tail stand with a lanyard attached. 

There’s no pocket clip provided. However, convoy sells one for the S2+, which fit. Alternatively, I’ve heard good things about the Olight deep carry clip.

Build Quality, and Warranty

I don’t think much has changed from Convoy’s earlier models here. Still the same quality flashlight you’d expect, with smooth, square cut threads and o-rings in all the right places. 

The S21B has a fairly shiny matte anodisation on the aluminum. Mine’s grey but they come in black too.

The tail spring has a bypass, to help the driver deliver the 6A it wants.

As far as warranty goes, Convoy doesn’t seem to have anything official but generally, if a flashlight is broken then they’ll send you replacement parts.

LED, Lens, Bezel, and Reflector

Starting from the top, the bezel is smooth stainless steel, without crenelations. Below the AR-coated glass lens sits the very smooth reflector. 

The Convoy S21B comes with an Osram KW CULPM1.TG LED. That’s the one officially called the Boost HX but also called the W2.1 or W2.2, depending on who you talk to. The Boost HX has a 2mm2 emitter in a 4040 package size. The LED has a 6500K CCT, fairly even from hotspot to corona.

The Boost HX in the light makes the S21B much throwier than it looks. In fact it throws further than the much bigger Convoy M3-C that I recently reviewed.

Convoy also sells the S21B with SST40 and XHP50.2 LEDs. Those larger LEDs would be much brighter but not throw nearly as far.


  • Length: 12.30cm / 4.84 inches
  • Head diameter: 2.72cm / 1.07 inches
  • Body diameter: 2.72cm / 1.07 inches


  • With a 21700 cell: 159g / 5.6oz
  • Without cell: 89g / 3.1oz

Size comparison

From left to right:

Convoy S21B (21700), Sofirn IF25A (21700), Sofirn SP31 (18650), Emisar D4V2 (18650), Sofirn SD05 (21700), Lumintop GT Nano (10180)

I included the GT Nano as although it’s much smaller overall, it has a similar LED (Osram W1) and reflector size.

Driver & User Interface:

The S21B (and many Convoy flashlights) come with 2 options when it comes to drivers: either a simple 4 mode driver (0.1%-3%-30%-100%) or a more configurable driver with 12 different mode groupings. I went for the configurable one, which also allows you to toggle mode memory.

With the mechanical switch, you can either tap the switch or press it until it clicks. It’s a reverse clicky, so tapping will disconnect power for a split second while it’s already on.

From off:

  • Fully click for on (mode memory)

From on:

  • Fully click for off
  • Tap and hold for momentary off (like a dead man’s switch)
  • Tap to change to the next mode

The 12 mode driver defaults to cycling between 0.1%, 1%, 10%, 35%, 100%, strobe, biking and battery check. You can change groups to disable the blinkies, pick fewer modes or even set it to just 100%.

Battery check blinks between 1 and 5 times, depending on cell voltage.


I could only detect very slight PWM with a camera and weirdly, only on the 4th mode (35%). It’s less detectable than on many other lights I have, so doubt anyone could detect it by eye. I’m not actually sure if it’s even switching between 0% and 100%.

I was expecting PWM on all the lower modes, assuming the S21B had a linear driver. From the lack of PWM (and the run times, below), it looks like the S21B is fully regulated.

Batteries & Charging

The S21B didn’t come with a cell but Convoy are including one in some listings. The driver outputs 6A, so I’d recommend a cell that can sustain 10A to get the most out of the light.

Both 21700 and 18650 cells fit fine. 18650 cells will rattle a bit and there’s no adaptor provided, so 21700 cells are best. Even a flat top 18650 cell worked fine, without disconnecting when shaking the light.

So, make sure you get a great 21700 battery charger accompanying the S21B


Lux meter: All lux and lumen measurements are from my home made integrating sphere, calibrated with a range of factory specced lights. Measurements are done with a UNI-T UT383S lux meter and Adafruit TSL2591 connected to a Raspberry Pi (using RuTiTe by bmengineer). Expect them to be within +/-10%.

DMM: Current readings were taken with a Precision Gold PG10B DMM, all with the cell charged to 4.20V.

Cell: I used a Sofirn branded 4000mAh 21700 cell, which states 40A. Charged up to 4.16V

Amp measurement  

  • 0.1% – 0.01A
  • 1% – 0.03A
  • 10% – 0.60A
  • 35% – 2.19A
  • 100% – 6.13A

Runtime graph

This was a nice surprise.

On the highest mode, the S21B just hits 1600lm at turn on. At room temperature, 30 seconds in and it’s still above 1400lm. It continues to ramp down slowly and after 4 minutes is down to 650lm. On my 4000mAh cell it manages to stay above 600lm for just over 2 hours! With a 5000mAh cell you should be able to get 2.5 hours.

The driver then blinks a bit to signify low voltage, then shuts off, leaving the cell at 2.97V.

The next level down (35%) is very similar aside from the first 5 minutes. It starts off at 750lm and barely drops 100lm over 2 hours. It lasts an extra 5 minutes compared to 100%.

There aren’t any reviews of the S21B with a SST40 LED that I can see but looking at some S21A reviews, they seem to show similar lumens and efficiency. That means the trade off between Osram Boost HX and Luminus SST40 is really only about throw vs tint.

Lumen measurements (for each mode)

ModeOutput @ 0 sec30 seconds
0.1%0 lm0 lm
1% (low)7 lm7 lm
10% (medium)251 lm251 lm
35% (high)766 lm717 lm
100% (turbo)1600 lm1439 lm

Throw numbers: 

Throw measured at 5m:

ModeMeasured CandelasThrow distance
1% (low)34637m
10% (medium)9,406194m
35% (high)29,136341m
100% (turbo)57,500 cd480m at 0s / 455m at 30s

Yes, 480m throw from a 27mm wide bezel! I measured it 3 times, just to check.

The Boost HX LED and smooth reflector results in 38cd/lm from something not much more than an inch wide. With the great run time on the 35% mode, the S21B gives you 341m of throw for 2 hours.

For comparison, the Noctigon KR1 has a much bigger 35mm bezel and throws 632m with the W2. The Lumintop FW1A is about 26mm wide and throws a measly 370m with the XP-L HI. The smallest thrower I have is the Manker MC12, which has a W1 in a 40mm bezel, throwing 650m.

There aren’t any LEP flashlights this small either yet. The Acebeam W10 is the thinnest LEP I know of and while it may throw twice as far, it’s also 5mm wider and won’t fit in so many pockets.


Distance to the far building is 55m. Photos were taken with a Pixel 3a, set to 1/2s shutter speed and ISO 400, F1.8. 

Disclaimer: This flashlight was sent to me for review at no cost by Convoy. I have not been paid to review, nor have I been holding back on problems or defects.

Final Verdict


  1. Pocketable 480m throw!
  2. Sustains 600lm for 2 hours.


  1. No pocket clip included (you can get a pocket clip for the S2+ for a couple bucks more)

Explanation on star ratings:

1: Avoid: my phone flashlight would be a better choice – 2: Poor: significant defect or issues; almost unusable – 3: Average: some defects or issues; but still usable 4: Good: recommended (minor issues) – 5: Great: highly recommended

Reviewer Dave
Author: Dave

5 stars: ★★★★★

While our star rating provides a reliable indicator, we encourage you to read the full review to make an informed decision based on your own needs and preferences.

When you see a Convoy S series lights, you don’t expect it to throw. But this one does. With the S21B, Convoy have combined their great host with a great LED. The Boost HX LED is a great choice here, giving 480m throw, whilst still providing 1600lm at turn on.

For a bit more throw you could ask Simon for a Boost HL instead, though I’d expect this to be around 510m with 700lm throw, so not a great trade off. There’s also the narrower Convoy S2+ that takes 18650 cells. Convoy sell the S2+ with a W1 LED at the moment and I wouldn’t be surprised if they started putting Boost HL LEDs in soon.

The Convoy S21B easily gets 5 stars. If you want a slim thrower then the S21B is probably your best bet right now. There’s nothing anywhere close to 500m throw under 30mm that I know of.

Buy your Convoy S21B here

1lumen selects and reviews products personally. We may earn affiliate commissions through our links, which help support our testing.