So i guess to be on the safe side I should not put 26650 batts in my J18?
better not… unless you enjoy half bright medium mode 
it should be the driver problem.
when i tested 3pcs J18 at high mode. it suddenly turned dim like only 1 LED after a few minutes.
no matter how i change batteries,it didn't work.so i sent them back to TF factory for a repair of the driver.
DayLighter i think you can ask the seller for a resend of new driver.
how long since you bought it?
they did better than that… they gave me RMA# and send me a new one… can’t ask more better than that!!
i’m hooked on fast tech… the service and the good price and of course their fast shipping…
i wish they have more flashlights… but i’m pretty sure they will someday 
i only had it for about 8 days
Sounds like a driver problem. 26650’s wouldn’t cause the issue anymore than 18650’s. They both put out the same voltage.
Hey,
I too have the same issue. I have two TRJ18’s so was able to simply swap the tail section on the brighter one to the head of the dimmer one and there was no change in brightness - still dim.
Reading on all these forums it seems to be a fairly common problem with this model. I can’t seem to get the driver board out - but don’t think there’s much point asd I don’t know what to look for, have no soldering iron and no replacement board.
I just emailed Trustfire explaining my situation and the fact I have several of their torches. Hopefully they will be kind and offer a replacement head if I pay for postage - considering it is only 4 months old.
Pretty disappointed to be honest. It was a great torch when it was working normally.
I will keep you posted on their response.
Edit:
I had a very similar problem with my ultrafire C8, it went dim and checking it on a multi-meter there was very little current. I ended up replacing the driver thinking it was a driver issue, however it was actually the LED that had gone bad.
If I understand it right, the J18 has a single positive and negative wire from the driver connected to the LEDs, the LEDs are wired together and complete a circuit/loop. If one of those LEDs go bad, then the circuit won’t complete or barely complete, causing the entire flashlight to go dim.
Problem is I don’t know an easy way to determine this. Looking at my LED and starboard it gave no clues that it was bad, solder points were good, however it was sitting loose in the pill so had poor thermal transfer.
One thing that should be easy to check is to remove the reflector and check the LED and LED starboards are firmly seated and don’t move around, if one is sitting loose then that could be the problem. If it is an LED problem and they all look fine, then perhaps someone with better knowledge may be able to suggest how to isolate the problem LED.
You could try turning it on and carefully checking each emitter to see if any seem to be performing differently, presuming you don’t blind your self, however considering the entire circuit would be effected it may be impossible to tell.
Of course it may just be a driver issue, however considering the odds, 1 driver vs 7 emitters……………
I emailed Trustfire factory re: dimming in my TR-J18 - I received this response.
“Tks for choose our trustfire flashlight.
Fo tr-j18, i suggest you can repair it by yourself.
You can buy j18 circuit board or j18 led light it.”
I can’t get the driver board out very easily due to it’s design and the size of the hole.
Damnit,
This is the trustfire manufacturer your contacting? What about the dealer you purchased from, presuming you didn’t go through the manufacturer direct. There was a 6 month warrenty I recall when I got mine, it should be honored. Are you past the 45 day paypal claim limit?
Yea it’s been 4 months, it was some random ebay seller. Bad luck on the item I guess. Emailed them but no reply. Had a go at trying to feed a screw driver past the drjver to release the reflector and led’s but no luck. Need an extra thin tool… I was using electical type screwdriver but still not thin enough. I can’t figure out how they got it in…
Sounds like upgrading might be your only option, and not a bad option. Aside from all the bad words you’ll probably mouth on your first mod I’m sure it will be quite a rewarding and learning experience, might as well go the for OMG lumens option, plenty of threads on the forum. You may want to pm some of the members that have upgraded their J18 for their advice, and adding some pics of your troubles would be a good idea.
i don’t have the problem anymore, returned it fasttech and got a new one… and then swapped it with the one that “Dr light” Tom E modded with this driver
and now it is “OMG, look at that, where did you get that light? ” 5000+ lumen
For sure, no real other option than fixing it myself. Trying to find a thread that details removal of the board… look like I may the first. Seems my model has a bizarre board that can’t be removed without releasing the reflector.
Sounds excellent. I want one!