Because i lack in electricity concepts, can someone please explain to me if i can use a protected ICR18650(2.5-4.2 V) with a drop-in module for flashlight (2.7-9v)? I am planning to buy a flashlight host and then personalize it, but i don’t know about the voltage difference.
The two items are:
Looks fine to me.
Using a 2.7-9V module on one Li-Ion cell will at least work. The only issue you COULD have is poor regulation. As an example, I have a Sportac triple Nichia 219 drop-in. And it works off one cell. But brightness drops as the cell is drained. It really works best with two cells in series. With two cells in series, it maintains perfect flat regulation. Of course, individual modules differ. So you may not encounter this issue. But regardless, neither the cell nor the module will be damaged.
That drop-in will be better with two cells. A drop-in with max 4.2v will perform better using a single cell.
I wouldn’t be too concerned about it. The SF dropin has a low “dropout” voltage and should be fine. Here is another option: http://intl-outdoor.com/p60-dropin-module-cuxm2-xml2-p-740.html < This dropin should maintain regulation as low as anything on the market. It just won’t accept 2s or 3s. It’s also going to be somewhat harder driven than the SF dropin, 3A instead of 2.2A or 2.5A or something.
Thank you all for your responses. You are teaching me, so thanks!
I am planning to buy a Solar Force L2P host, then put in a module and i just realized that a P60 module can fit this flashlight (thanks to wight) which gives me more options. The more i spend time on this forum, more things i learn 
So correct me if i am wrong, if i use a 2.9V-4.5V module, 1-battery is good for that.
If i plan to use 2.7-9v module, 2-batteries would be better.
If i understand well, 2 batteries will give me more runtime, but are there differences in brightness “power” between the two mentioned modules? Do i get more brightness if i use a 2.7-9v module with 2 batteries?
Also, if using 2.7-9v module with 2 batteries, the extension tube is inevitable, which makes the flashlight much longer, right?
I am doing a runtime test of that Solarforce module vs a CUXM2 XP-L V4 3D, same cells same host. I hope to have the results soon.
1x battery (1s1p arrangment, 1 serial 1 parallel, a single cell) is appropriate for anything which is based on the Nanjg-105c / Qlite driver, which is itself based on the 7135 (aka AMC7135). That does not mean that the driver can stay in regulation (constant brightness) all the way through the discharge of the cell however!
I’ve already given my thoughts on the SF module of course. 2s would be better and would give regulation all the way until the cells were depleted.
The IOS dropin is driven at a higher current, so it has a higher output wattage. It will probably be brighter at the beginning and it will drain more power from the cell. At some point it will drop out of regulation and brightness will become lower than it was to start with. The SF dropin will also lose regulation at some point on one cell.
Shame intl-outdoor.com only does a 3a single mode XM-L2 U2 P60, I would prefer some single mode modules driven at 350ma to 700ma for a bunch of old L2 hosts i have and use them as house emergency lights.
Cool XM-L2 U2 produces 300-320 lumens 700ma and 200-220 lumens 500ma.
Anyone know of a single mode 500ma or 700ma drive P60 module, a 350ma XM-L2 U2 P60 module with a panasonic 3400mha cell would be great. I want single mode because my 80 year old mother keeps thinking the 2 mode flashlight i gave her keeps going wrong :exmark:
With or without LVP?
Can’t wait to see the results! Feel free to post them here too.
Very nice explanation. Thanks! 
It’s not an age thing. My 30yo friend has the same problem, get the light into moonlight mode and then wants me to change the battery because “light is dim” 
5-10 modes all look the same to friend, LVP warning flashes don’t phase this person one bit, but let it be in moonlight and hey, something is up with this thing! eh?
Keep in mind that is the older XP-L V4 3D and that is replaced with the more brighter XP-L V6 3D
Great! Thank you, i will go with the XP-L V6 2C
Don’t forget to look up the threads on wrapping the dropin to help with heat management.