Can i add three more extension tubes to the Trustfire 3T6 so it would run of 6 3,7V batteries for a double runtime? (i know, it would be like walking with a telephonepole in you hands, but hypotehtically) 
No, you couldn’t. DX wrote: 3~12V. With 6x 3,7V you will burn the LED or driver or both.
Oh, so 6 batteries would be like 22,2V? Isn’t there any way to prolong the runtime?
With 3 batteries it is feeling like too long for me, imagine 6….
What with changing batteries when empty?
hehe, i guess 
But if im going to my cabin in the woods… Is it ok for the batteries to stay empty for a couple of days? Just heard that they could get defects or something if they stay drained too long…
My TR-3T6, 6 feet staff… I was thinking the same thing.
I was wondering, the new Fenix TK75 can run with 12 3,7V batteries, would’nt that be too much voltage for it?
I would think the holders are wired in parallel, unlike the batteries in your flashlight.
Not to mention that you could run 10 x 18650 @ 42v in a light IF it was designed to handle 42v. The TR-3T6 is made for 3 cells max. Comparing it to another light that can use more is just silly.
Aha 
So the the 3T6 is wired in series? What is the different with parallel and series wiring when we’re talking about flashlights? With parallell the flashlight just take as much Volt and Current it needs to run in the different modes decided by the driver, while in series the flashlight gets as much Volt as the batteries give together?
I’m kind of a noob, but i want to learn
you can have 100 of battaries im paralel, and they will still give 4,2V max all together, you just get more capacity.
if you put them in series capacity is the same as of one battery, just gives more voltage and current P=U*I
correct me if i’m wrong
Aha. So, just for fun, is it much job to make the 3t6 run parallel?
I mean, when i see the need to go for a more powerful torch, i could make this a project to convert the 3t6 to run parallel and make the runtime huge
Running numerous cells in series is asking for trouble; you're creating a pipe bomb!
-Garry
Which batteries do you have?
With better batteries runtime increases too.
Hehe, yes, but if I made the current go parallel 
I’ve ordered the 3000mAh (2500/2600mAh) batteries. The best for its price. The runtime on High wont be much more than 1,5 hours with the best batteries i think, but if i could make the torch run parallel, i could add like 3 more batteries and make the torch go for 3 hours on high Just hypothetically speaking though…
Which batteries? Trustfire flames…?
I am sure you don’t know what is best, I unsubscribe now.
Why don't you look into a different light designed for long runtime? Does it have to be a triple XM-L? How about this one at FastTech with 3 18650's in parallel? Get some good high capacity Panasonic 3400mAh cells (or back down to 3100mAh for cost savings) and you've got some serious runtime! You also have the added benefit of using it as a USB charger and being able to charge cells in the light. A much safer alternative to a TR-3T6. Mini-review found here.
-Garry
Wow, that is a nice light! Absolutely something to think about. Thank you
Trustfire flames yes. I dont mean those batteries are the best, but at that price they were the best i could find.
Of course, if want even better batteries i have to double my costs