I have three 18650 lights and 2 of them act weird with protected cells. I took the “protection” part off and they both work great. I don’t know if its the length or what. They were flickering and not turning on. With an unprotected cell they work great. Am I the only one having this issue? Seems odd on 2 out of 3 lights.
probably length.
ken
Could be the over-current or over-discharge protect cutting in.
some of the smaller lights and twisties dont like protected cells from what ive heard. too long and it presses the driver too hard i think
Thanks both are c8 lights that are having the issues. Different manufactures. One is a XinTD and the other is a Citipower
I unscrewed the light with a protected cell and it does work with the tailcap unscrewed so it must be the length issue in atleast one.
Don't forget to isolate the poles when you measure them 
What type of batteries do you use?
Just measured and its 68.84mm. The cell is a trustfire flame 3000 mah protected. Will try posting a video later.
I use an XTAR 18700 in an Ultrafire C8, no problems. Maybe the PCBs on your Trustfire cells aren't attached properly and give a bad connection under pressure...
I’ve only had that issue with one UF “3500 mah” cheapie 18650 and RCR123 rechargeables when powering up a KD drop-in. Something kept tripping protection. I have not experienced much of that.
Just checked my XinTD SC-82 with an Xtar 18700. Works no problem
Sounds to me like you have protected cells that have a high internal resistance (cheap or worn out).
A fully charged cell with high internal resistance will suffer a significant voltage drop under high load, perhaps a drop low enough to kick in the under voltage protection.
I have had problems with brand-new xtars plain not working on high or at all in my hd-2010 (with conversion tubes), ke-5 and a couple other lights. Just tried an xtar in a xin for you and it worked fine, but that means nothing. Yours may have different protection circuits. Some TFFs will work in some of my lights, and won’t work in others.
I have also had xtars where the turbo is dimmer than the turbo on the same light with a different battery, acting like a mini rheostat. There is a thread from me somewhere on BLF about this problem from back in late summer/early fall.
I think with me it’s the protection circuit because in some lights, the high/turbo mode won’t work. But in others, it may be the length pushing against something, causing something to ground. I theorize in the dead turbo scenario that the protection circuit views the high-demand for electricity as a short and shuts down.
I do NOT suggest this for everyone, but that’s one of the reasons I use almost all unprotected batteries now, except in multi-cell lights. ALL my *fire batteries have been retired permanently. I only use name-brand new (NO CHINESE) or pack-pull cells. How safe can a protection circuit be if it’s doing weird stuff like this?
Once again, I am not on a campaign to convince anyone else to go the unprotected route, so don’t flame me.