Good news Simon. If also M21C-U will be updated with new driver with fixed ramping?
What could be the reason for the step down at 3.8V? It’s almost winter right now so I assume Thermal stepdown would be a non issue right?
Plus 1
Simon, someone is asking about the driver size in the Convoy S21B. Others would also like to know. This lack of information in the S21B host advertisement is a discouraging nuisance. 
S21B driver is 22mm. It was described somewhere but i forgot where.
Anyhow, seeing model running CULPM1.TG @8A and looking at Convoy site will tell you that only 22mm is working with 8A. Smaller drivers are 6 and 5A
Now with a bit of detective work I’m sure the S21B uses 22mm drivers not 20mm, check these images I pulled from AliExpress:
Customer picture of S21B
22mm SST40 driver
20mm SST40
I still want Simon to confirm compatibility with S21B and the new 22mm 8A driver, though, since for one thing the new driver uses a pin contact instead of a spring and there might be other things I haven’t considered.
I think when I reviewed the M21C-U it had actual low voltage protection. To quote myself:
I won’t say the flashlight is perfect but it is one of my favorite flashlights in quite a while. The driver is very efficient and well regulated.
Reason for the pin contact in the 8A “buck” (hope it were boost-buck) driver is conductivity, as even high conductivity springs could have problems at 8A or close (with just 10mΩ of resistance a spring dissipates 0.64W as heat when 8A crosses it). Simon and/or the driver manufacturer played it safe this way, but it has drawbacks, namely for people who really need damping or shock absorbing contacts (i.e. hunters for example).
The solution, of course, are really high conductivity springs. A bypassed spring can do, but a durable spring bypass requires careful work, giving the wire a coiled shape. Problem is the contact surface under the pin of the driver, it just has the size of the pin contact and is too small, unnecessarily forcing spring modders to remove the solder mask around it.
Let me also say that, in all honesty, I think 8A is probably a bit over the top. We know the CSLPM1.TG peaked at 8A or just a hair above, and imho it's over the top for it. The wrong thing about this is we don't have any CULPM1.TG tests. So actually we still don't know how much higher is the 4040 CULPM1.TG going to peak over the 3030 CSLPM1.TG. And honestly, it's the same led, only the footprint changes. Nothing else to say, except that being able to configure a driver's output current in firmware would be a godsend. 
P.S.: low voltage cut-off is overall set too high in drivers. Bear in mind that drivers don't see the actual battery voltage, they see it after it goes through springs, contacts and switch. All of this causes voltage drops, the higher the current the larger the drops; therefore, the actual cut-off ends up being at a higher voltage, namely if running in high current modes. In essence, I think driver cut-off voltages should at least drop down to 2.7 or 2.5V. After all, I've yet to see a battery complain about a low cut-off, namely when datasheets specify 2.5V (and I've seen lower).
Fri, 11/20/2020 - 04:55
gchart. Thank you for the detailed response.
I also think it is almost a perfect flashlight.
What do you think what could be the reason for these random step downs at 3.8V?
The driver currently in use is not a new batch of drivers
22mm driver for S21B
There are the following possibilities for this problem,
- The internal resistance of the tail spring is too large.
- The internal resistance of the battery is too large or the voltage drops too fast under high current.
- Poor contact somewhere.
- The driver is defective.
Hi Simon, any update on 3x21700 light? Thank you.
Sorry, I’m not familiar with that. The only stepdown I encountered was when thermal protection kicked in around 4 minutes on turbo.
Okay, my new finding is the M21C-U is doing fine, in fact very well, after the light has warmed up. It step down from high(100) to moon about 3.3V. This time I’m using a Vapcell T50, previously on P42A and 40T. Taking into account all factors and voltage sag. It’s about right that it step down about 3.3V on high(100) to moon.
I will test again with the P42A and 40T. Will also test with Vapcell G50 and Vapcell Red 4500mah(20A) and update here again.
Even before this finding. I love the M21C-U. It’s my most carried light for all my night walks.
Hi Simon, I can relate your points. I have some findings mentioned prior to this post.
The weird part is the step down happened when the light is cool(below 30 degC-batt below 3.8V) . Again, as above post finding, it’s all fine and good after the light has warmed up.
Anyone care to share if there’s a theory to this effect that I missed?
There is similar complaint on TLF
Looks like a similar point on all fine after light has warmed up.
Has anyone measured the throw of the Z1 with CSLPM1.TG or compared it to any other lights?
I’ve got one on the way from Simon but I’m not sure if it’s going to throw enough to scratch that itch.
Thanks.
That is very interesting, I will try to verify it ans warm up the head to 30C before using,
That is also odd, I thoght the M26C has a different driver 12 mode vs Ramping