Hey guys, I am interested in modifying some C and D cells to accept 18650 batteries.
I ordered a Stansport Tactical flashlight running 4 D cells(more rigid than Maglite )
and own the Costco 1000 lumen running on 4 C cells.
I would to know best way to modify to accept 18650 batteries.
If too expensive then, I will forget project and just use as is and buy higher grade lights.
BTW, It is just a hobby of mine. Thanks in advance.
I am doing something on my TK70 too, it should work fine with 8.4v.
You need to know your flashlight working voltage. And then make dummy battery contact to extend 18650 or 26650 batteries length. I will use neodymium magnet to be dummy contact.
Three 18650s physically fit pretty close to the length of four C batteries. However, the voltage will easily be double that of four C’s.
Knowing the voltage range is crucial.
Then you will know how many 3.7V 18650s you will need to power it.
You cannot go much over the range or you will burn it up.
Three C/D equals around the range of one 18650 voltage wise.
Others will know more!
Yeah if you want to save yourself headaches, the simplest way would be to get a light that uses 3 C or D batteries and replace it with a single 18650 or 26650.
With a 4 cell light, you’ll end up either lower or higher than the intended voltage although it’s possible you may be within the voltage range tolerance.
I have no idea about voltage on either flashlight. I am new to flashlight collecting so I wanted to do a project for myself. All I do know is the 4 C cell is from Costco in store right now. The 4 D cell is the Stansport Tactical one.
Any detailed help would be appreciated. Thanks for all replies.
If they are connected in series (are they?), replacing 4 C or D cells with a single Li-Ion cell is going to be challenging - you will not have enough voltage.
Yes, all are connected in session. Similar to Maglite. The only reason I am doing the mods is to transform to recharged batteries. The recharge C and D (regular ) (recharge ) cells take to long to recharge and much harder to find than 18650 batteries.
Thanks for reply.
there should be no problem converting the light to work with 18650, you will need a new driver. i have a feeling your light is same as defiant lights home depot sells. they rely on alkalines cells voltage sag, and use either linear driver, or no driver at all, the board is PWM dimmer.
Yes, they use Alkalines for sure. Will new driver cost much? The reason I ask is because it expensive to mod I will scratch project. I would rather use money to get better or higher grade flashlight. Thanks for reply.
BTW, Just a side project for me for fun.
Not sure if that’s of any help, but you can buy C to AAA as well as D to AA adapters. This way you can use standard AA/AAA Nimh rechargeable cells in place of C and D cells.
Good idea. But will output be just as good or better than 2 18650’s? Any leads on those adapters?
BTW, I am going to buy the Nitecore Tiny Monster, Olight, Klarus or equivalent thrower and I know those use 18650 hence I wanted to mod these lights to 18650 as well.
Thanks in advance.
This mod probable will not save you money. A mod like this would be for the fun of the project only. You most definitely can find a suitable new flashligt that is smaller, cheaper and more practical than your proposed mod. But hey, guys do these mods all the time as they are a fun change, give experience, and gain knowledge. So if you are interested, by all means, Go For It! 
Output will be the same as on C or D rechargeables, but at least you don’t have to deal with changes in voltage and installing new drivers.
Here are some examples of various adapters:
Yes, true. I only wanted to mod these lights to 18650 because I will buy Nitecore Tiny Monster, Klarus, Olight or compatible. I know these use 18650 so it would be nice to have all batteries the same time.
Nice, I will check them out.
to answer that i’d have to see how the light is made, if you could take it apart, as much as you can, and post pics we’d know better what driver can fit, and work. it could be anywhere from 5 bucks, depending where you are, to up to 50
I do not have the Stansport 1 in hand yet. Still waiting on it to ship to me.
The Costco one seems like one solid unit. Minus the tail cap removal for battery.
But at $50 I would rather spend that on a decent tactical name brand like Nitecore, Fenix, Olight as such.
I though the mod would be simple and decently price but seems more difficult and expensive.
Thanks for reply.
well 50 is what very good drivers cost. you may be able to get away with simple linear driver for 5, but you’ll have to use either 1 cell, (with will give you shorter run time than 4 d cells. 4d cells 6v 10ah, 1 18650 4v 3,4ah, i’m not even gonna convert to watts hour, we can see already D cells have more energy), or make a holder that would take 2 or 3 cells and have them connected in parallel.
but unless you really want to do the mod for the sake of doing it, get another light, get convoy c8 for 25-30 bucks. quality wise they are no worst than brands you mentioned, convoys may not have as many bells and whistles, but they are quality build lights that follow KISS concept
Have you found a source for the 18650 battery holder that will have the external diameter of the D cells?
Gene Malkoff makes some LED upgrades for maglite, and VestureofBlood/ Adventure Sport Flashlights offers kits and modding parts.