I am curious…i now see info about 26650 batteries…are they REALLY any good an if so…do i need to mod anything or just use and…do they really make more power or just longer usage. Thanks
Wait till you see the 32650 cells. 
If your flashlight has a 32650 or 26650 sized tube AND there is room lengthwise you’re good to go. They function similar to 18650 cells, just with more capacity.
Go check the discharge curves:
26650: http://lygte-info.dk/review/batteries2012/Common26650comparator.php
18650 (and 26650): http://lygte-info.dk/review/batteries2012/Common18650comparator.php
26650s are going to give you more milliamp hours (longer runtimes) typically and some of them can give higher currents than 18650s depending on the cell.
I’m not sure exactly what you mean by mod to use them. You’ll need a light designed to use them.
Lithium ion batteries are named by their size, so a 26650 is a 26mm wide 65mm long cylindrical ( 0 )cell.
I’d rather stick with a common battery size (18650) and carry a few spares, but that’s just my 2 cents.
Edit: Forgot to mention cost. Laptop 18650s are way cheaper per mAh if you don’t need the extra performance.
I believe the theory is the extra capacity means they can supply the right voltage for longer, ie: less voltage sag, or they will take longer to sag is more accurate.
But none of the larger batteries can compete in amps with the likes of the 20 and 25R, HEII, VTC4 or 5, or Efest 35A for example which are 18650s.
As outright amp suppliers 18650 reigns supreme, but for maintaining voltage for longer the larger capacity are good. It depends on your need.
I could be wrong, or half wrong but Im unlikely to be completely right.
Ejected Filament, I just had to. It was too good not to.
Great…thank you all for info…i gonna stick with my usual then…i sure appreciate the help.
“Best” in what respect?
What are you going to use it in?
Why 3400mah?
We need to know to advise you.
Thanks,
-Chuck