How many C-cell batteries do you want to fit?
No particular number, but I am curious of how they match up with various numbers of extensions.
4 C cells (200mm) are about the same length as 3 unprotected 18650/26650's (195mm). So 2 extensions on a Y3 should hold 4 C cells. Three more extensions to hold 8 C cells.
Thanks, that gives me an excuse to pick up some C rechargeables.
Ordered one. Thanks the!
Thank you for this post. I purchased two extension tubes with this light, and am excited to learn that I can now also run 4 rechargeable C cells in this Y3 when it arrives. I do not own many 18650 cells.
Your very welcome. 4 cells should work pretty good since the driver is designed to work with the voltage of one Li-ion cell.
EDIT: I just remembered that 3 unprotected 26650's need a spacer in my Y3 to work. I checked with 4C's and a short spacer will be needed for them too. The light seems to work fine on 4C's.
What kind of spacer?
Is the run time better or worse on C batteries?
I just taped together a about 4 pennies for the time being. When I get around to modding the light, I plan on replacing the driver spring with a short piece of copper pipe to fix the length issue permanently.
Don't know the answer to your second question. I don't have any Ni-mh C cells. I guess it would depend on what C cells you uses and what Li-ion cell you would have used.
If a 26650 is about 70mm and a C battery about 50mm, that isn’t a good match up is it?
The new KeepPower 26650 3.6V/3.7V 5200mAh is 71.1mm, does anyone see three of them being too long for 2 extensions?
He’ll probably need a short dummy cell. Considering that ImA4Wheelr says that 3* unprotected 26650’s need a spacer and that he used 4 pennies (~6mm)… actually yeah, good question 
3:4
Three 26650's @65-70mm are roughly the same length as 4 C cells @ 50mm.. The springs can usually accommodate the difference.
I guess that three of the protected Panasonic 16650 3400s might be the way to go, until I buy three of the Keeppower 26650s.
Does anyone see a problem combining one protected EVVA 4000mAh 26650, with two protected 16650s with sleeves?
Heh. You’re opening a can of worms there. My personal position is to politely caution anyone who has to ask against doing that sort of thing. I always recommend against relying on “protection” circuits on individual cells. Since the 4000mAh 26650 will run much longer than the other two and you’ve got a ton of voltage delta above the output voltage, the driver will probably just let you keep overdischarging those 16650’s a long ways. If the protection circuit either cuts out at a low voltage or fails to cut out….
That said, doing it as a test to ensure that the light works - go for it.
I should have pointed out that I meant just for playing around with the light, not for full time use.
For full time use where I might run down the batteries, I am very cautious and wouldn’t mix types.
Yeah, I see no problem there.
brad wrote:
If a 26650 is about 70mm and a C battery about 50mm, that isn’t a good match up is it?
The new KeepPower 26650 3.6V/3.7V 5200mAh is 71.1mm, does anyone see three of them being too long for 2 extensions?
My guess is they will be fine. With 4 pennies, my cells are not tight in the least. I would prefer tighter but I don't want too tall a stack of pennies in the tube.
I haven't seen pictures posted yet and as this group buy was still on thought I'd post the Y3 extension on the HD2010. I assume there is no difference between shops. My HD2010 was an unbranded grey one. The tubes do indeed screw on perfectly though the tailcap feels a little rough to screw on. Lube should help this. With two Trustfire 26650 batteries installed they were about the perfect length. Needless to say the standard driver will last about a billionth of a second if you switch it on.
Does output at all change with use of two or more cells?
I have just received my Yezl Y3 from the previous GB and for the sub $30 price tag this thing is a real winner 
There are slight mismatches on the anodizing between the body and the reflector but other than that the light arrived in near perfect condition. With the exception of imperfections on the reflector, the beam doesn’t appear to be affected by the imperfections.
I will say however that this light is in desperate need of a serious bump in amperage. Stock, its doing 2.24A on high and this thing is destined to do at least triple what its currently offering 
The side switch is… meh. It works but it is extremely mushy and offers no definitive click.
The extension tube looks and works flawlessly and the driver does indeed handle 2S batteries. If you’re looking for a $25 thrower this is without a doubt the best option around.
No, Current draw with two cells is 1.2A. So assuming efficiency is relatively the same with different voltages,
the current to the LED remains almost unchanged no matter how many cells are used.