Acebeam K40M

Acebeam K40M review

This flashlight has been discontinued. Check out the list with all our Acebeam Flashlight Reviews for a great alternative.

Introduction Acebeam K40M

The Acebeam K40M comes in a nice hard carrying case. It includes the K40M flashlight, holster, straps, papers, etc., and has some cutouts to include 3 (spare) 18650 batteries. The padding is pretty stiff and hard, almost feeling like plastic, but it will do the job of protecting the light. The included holster, spare O-rings, and papers finish it off.

The holster is pretty good and fits the Acebeam K40M perfectly. Unfortunately, due to the weight of the head, the light will tilt when attached to your belt facing upwards. You can carry it head-down, but you must be careful when opening the holster.

At first inspection, I thought it included a lanyard, but it’s more of a huge neck cord. The cord can be attached to the tailcap with a clip, and you can attach any kind of (para)cord.

It attaches with a claw clip, which will damage the anodizing already after a few times using. 

The K40M is already a little dated, dating back to 2014. If you want a similar-sized but more powerful and with more throw, look at the Acebeam X75.

K40M specs
k40m set
k40m on case
k40m manual

Handling of the light

The body of the K40M is about the same diameter as a regular “soup can” light, like the SkyRay King, surefire M6, BLF Q8, etc. It can be difficult to operate with 1 hand. Using the tailcap switch (which I prefer on lights with electronic side switches/control rings because of parasitic drain) and the magnetic ring simultaneously requires two hands!

The front part of the K40M is heavier than the main part, so it feels a bit unbalanced compared to other lights.

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Flashlight Knurling

In the manual you can read: Tactical knurling for firm grip. My hand says: so so knurling for so so grip.. or in other words.. not so much knurling for not too much grip.

No, the K40M isn`t smooth and won`t slip easily out of your hands, but for this size light (and for the way of operating it), I would prefer a heavier knurling. This is especially true when you think it can be used in Law enforcement and search and rescue.

But the knurling that is done, is done very well.. No mismatch/no missing or chipped parts..

Threads print and anodizing.

The anodizing is done properly across the light. Every spot and corner is anodized, just like you would expect from any high-quality light. My first impression was very positive, except for the prints.. The Acebeam letters are printed pretty badly. (this is actually done on purpose because it`s an exact copy of their logo …oops)

Another thing that could be improved would be the brightness indicator from Low to Max. It doesn`t cover all the output levels. There is an indicator, but only for the lower levels 1-3.

Then there is an empty spot,4-6 until it reaches Standby and then Strobe. The words Standby and Strobe are so close together that someone who doesn`t read English could think they are one word.
A few dots or even symbols would have helped!!

Threads are done well, not sharp or thin.

acebeam k40m text
K40M prints

Buttons/Magnetic control ring

The power switch of the K40M can be found on the tailcap with a forward clicky switch. Which indicated it can be used as a Momentary switch. The Magnetic control ring has indents that you can feel every level of brightness. From the left to the right, it starts at minimum brightness to the highest brightness levels. After the max brightness, it comes into the Standby mode, followed by Strobe.

Operating the light and changing its brightness can be very difficult with only one hand. At the same time, it can be pretty hard to press the bottom switch without using a second hand. So, this light is best used with both hands.

Lens, bezel and reflector

The reflector is quite smooth but not ultrasmooth. If you look carefully, you can notice some machining rings, probably due to its aluminum construction. Interesting to note, though, is that plastic reflectors are more likely to be super smooth.

  • Diameter outside 70mm
  • Diameter inside 63mm
  • Depth 52mm
  • The lens is coated and is about 3.6mm thick
  • The lens is described as an ultra-clear tempered glass lens with anti-reflective coating, as described in the manual.

The bezel is not made of Stainless Steel, so be careful when handling it.

The bezel (as well as all other outer main parts) can be unscrewed and has no glue. The inner parts, on the other hand, are glued.

k40m reflector 2

Battery carrier and tailcap switch:

The battery carrier has 3 batteries in series! Because of the little nippels inside the carrier, it can be used with unprotected as well as protected cells,flat and button top.

I don`t recommend using unprotected li-ion cells in series, such as with this flashlight. The flat-top batteries sit a little loose because of the length of the carrier.

I use protected Panasonic NCR-B 18650 with 3400mAh and they fit tightly. These batteries are used in most of my 18650 flashlight reviews. During testing with Sanyo unprotected batteries I made a short, and it boiled the springs, and because of that the short directly stopped. The batteries didnt make good contact with the springs anymore which saved my light! The Positive contact touches a beefy coiled spring on the driver’s side.

The Tailcap can be unscrewed and has no glue.

Dimensions and weight of Acebeam K40M:

  • Length 185mm
  • Diameter head 76mm
  • Diameter body 49mm
  • Weight empty 497 grams
  • Weight loaded with 4 batteries 642 grams

The K40M consists of 2 main pieces..
The Head and the body.

  • The head weighs 320 grams
  • The body + batteries weighs 322 grams

User Interface

The K40M has 6 main modes within the rotating magnetic ring, plus a Standby mode and a Strobe mode.
Starting from the left of the control ring, you`ll have
Levels 1-6, a Standby mode, and at last a Strobe mode.
No hidden modes ofcourse. So it is very easy to understand and use!

From the Standby mode, you have instant access to:

  1. Turbo (dial left)
  2. Strobe (dial right)
  3. Low, if you dial it counter clockwise quickly  (it will skip all the brightness levels)
  4. Strobe


  • Nope… nothing to worry about.
Lock out
  • It doesn`t need one, because you can use the tail switch.


Measured with 3x NCR18650B panasonic 3400mAh protected.

Ceiling bounce in lux

Overview power level K40M

The numbers are taken as if 100% is 3000 lumens
From my experience, the lowest level is not 0.9 lumen as is written in the manual, but a few lumens.

Current Draw

When trying to measure the Amps, I made a short. The batteries started to heat up, and the springs disformed. Fortunately, I had flat top unprotected cells, which minimized the spings, so the short finished by itself. Now the springs are shorter, so I will only use protected batteries.

Afterwards, I heard that you can read the amps by touching the probes to the threads/body of the light instead of the battery carrier

So I have no numbers!! sorry


The run-time test is done with a set of Panasonic 3400mAh protected 18650 batteries.

Level 6: max:

The runtime was 65 minutes, and 2 small laptop fans were used to cool the light down. Just before 60 minutes, the K40M started to blink with the Low Voltage warning. And at 1 hour and 5 minutes the light turned off automatically. (65 minutes) All batteries were slightly below 3Volts.

My Opus BT C3100 charger didn’t accept the batteries. So I did a Discharge Refresh cycle and then it worked..

Next: Special close-up of the first 9 minutes without cooling

And this is on Level 5… (maximum is level 6)

Runtime is 115 minutes, which equals 1 hour and 55 minutes


Measured at 5 meters

  • 62500cd within 5 seconds = 62kcd
  • 57250cd after 30+ seconds= 57kcd

Beam shots:

Fujifilm camera F200 EXR on Manual setting with ISO 400, F3 , 0.2 seconds

Control shot:

Level 1

Level 2:

Level 3:

Level 4:

Level 5:

Level 6:


Positive points:

  1. Great value for the money
  2. 6 output modes
  3. Magentic control ring 
  4. The minimum brightness is low enough on this type of light and not too useless dim.
  5. Not glued
  6. Tailcap switch for permanent Off
  7. Standby mode, so a 1 handed operation is doable (not that great)
  8. Low voltage warning
  9. Throws a fair amount of light
  10. Can use flat and button top batteries

Negative points

  • A drop in output of 20% within the first 10 minutes 
  • There is not so much grip; the knurling is well done, but not very aggressive
  • You need to use both hands to operate the light.
  • No stainless steel bezel, so the bezel damaged rather easily
  • The smooth reflector is not completely smooth, but has some sort of rings (spots)
  • Front heavy.
  • Text and indicators for some brightness levels are missing, plus Standby and Strobe are too closely printed and therefore look like 1 word.

Final verdict

The Acebeam K40M is a great flashlight. Very easy to use and still a good performer.

Acebeam K40M for sale

It has been discontinued. So, if you are interested in this kind of flashlight, check out the list with the best flashlights.