Ok, with the torch(Flashlight 4000lm 3x CREE XM-L2 Flash Torch) came 2 UltraFire 26650 5000mAH batteries. I tested them and they only last about 45 minutes. My torch works with 18650 also. I want a pair of batteries that would last longer, say about 1.5 to 2 hours. Any help?
First, WELCOME to the forums.
Those figures are Chinese figures, exaggerated.
4000lm out of 3 XM-L2’s would mean putting well over 3A through each one.
3A to each would be 3368lm total (from Cree’s data sheet).
1.5~2hours run time is asking quite a bit on high, lets say 2A to each emitter, 6A draw, so you need at least 12Ah (9 for 1.5h)
I think 26550’s come in 4000mah max, someone correct me if I am wrong, that would give you about 40 minutes, just about what you are getting
Edit:- sheesh can’t even do the math now, a 6A draw from a total of 8A = 1hr 20minutes :bigsmile:
The only thing I can think of to lengthen your runtime is to lower the current being drawn, is medium to dim at the depth you are at ?
Most medium’s are 50% or thereabout so that would give you 80~90 minutes or maybe a bit more as the lower the drain the more the batteries hold up.
Don’t be shy about asking questions 
Cheers David
Edit No2:- The 26550’s linked to below are 4500mah, 2 = 1hr 30 min at 6A draw in theory :~
Pommie is right about the exaggerated battery capacity rating & Lumen output.
I guess this is your light? (eBay). What's your tailcap current reading with fully charged cells? Here's a pic of a light measuring 1.5 amps with digital multimeter...
(source HKJ here)
Just buy authentic 26650 KingKongs. These are cheap and great batteries. (buy: CNQG, non-protected FT)
you will want non flat top. 
Raised top vs. Flat top
And protected cells are best/safest when putting them in series configuration.
If you have lots of money... one of the best 26650s are the EnerPower (HKJ test, eBay, EnerDan) - But I don't know if the cell top is raised enough to make a good connection when in series.
Also, BLF has a diving light section now, if you want to talk about your dive light in more detail. 
Thank you David, 
May be 1.5-2 hours is a bit more but usually I stay underwater for an hour minimum. Like you said, dimming the light at some points would make it last longer. I have seen a few 26650s on ebay saying 7300mAH, I don’t know if it is true or not. If it could be extended to 90mins, thats big amount too. I was wondering if there would be other batteries that could give me longer time with full luminosity on the torch. Your calculations are right, although Im never good at it.
hello MRTdiver,
Yes, that is my light there on eBay link. I just checked tailcap current and it comes at 2.45Amps and it is slowly decreasing…I read some other posts in the BLF and noticed that KingKong have good power. The EnerPower seems a bit expensive for me yet.
also note that my torch work with 18650s with battery covers.
Does any 26650s have any advantage over 18650s?
Thank you.
Well, 26650 are bigger and hold more capacity. They also tend to be able to discharge faster (higher amperage) with less voltage drop.
Trustfire Flame 26650s are actually pretty decent from a good vendor, not too sure about ultrafire ones. The last time Ultrafire claimed 6000mAh on their batts, they were only good for 3000mAh max. Same as a Panasonic NCR18650A.
You can work out what you need now you know the current draw :bigsmile:
The current decreasing straight away will probably be the nasty xxxx-fire cells, 40min at 2.5A means actual capacity of about 1.7Ah not 5Ah, please replace them as soon as you can.
At a 2.45Amp draw decent 4000mah cells will give you about 90~95 minutes on high, the short end of what you want, using medium when you can could give you the 2 hours you are after.
2.45A means about 4.5A at the led’s, so about 1.5A for each, assuming a T6 that would be about 600lm (led junction temp=40c when underwater) so 1800lm total, yes you can easily get more, but, there is always a butt, it will mean less runtime 
Cheers David
Yeah, true. The Ultrafires don’t come close to the capacity which is said on the label. They are much less. Trustfire also may not be enough for what I am after, but they are much better than Ultrafire ones.
*Yeah, these ultrafire cells are crappy. I have purchased 26650 KingKong batteries from CNQG on 27th Dec and am waiting for them. They haven’t shipped it yet though, still says ‘preparing’ and this is my first time purchasing from CNQG since its cheaper than ebay there.
I am hoping the KingKongs would do better and I would be happy with 90~95 minutes on high. There will be times when I use the torch at medium though, so the run time will be far longer than the 45 minutes I am getting now. Thanks David :)*
26650s have a few advantages over 18650s. One is capacity. On paper, the difference is not huge - at least with ICR (it’s MUCH bigger with IMR and IFR). However, neither a Panasonic 3400 or 3600 actually delivers a full 3400 or 3600mAH. Both fall short by about 100mAH. On the other hand, a 4000mAH King Kong delivers closer to 4200-4400mAH. This means that you actually get closer to 1000mAH more capacity from a King Kong vs a Panasonic 3400.
Another advantage is the discharge curve. By nature, most Li-Ion chemistries (IFR being the exception) have poor discharge curves - they just don’t maintain voltage as they discharge the way, say, NiMH does. ICR 26650s are no exception here. But at least they have a flatter discharge curve than ICR 18650s.
Higher current draw is another. This is kind of a no-brainer, since the capacity is higher. So the recommended ICR discharge rate of 2C will also be higher. And, of course, IMR and IFR discharge rates are ALOT higher in a 26650.