Illumination Supply has a warning about fitment problems with protected batteries in the S6.
Which spring is on the driver? With the tailcap off, where does the tail of the cell come to, how much of it sticks out?
Are protected batteries to long or too fat?
I measure 18.97mm ID on the S6 (vs. only 18.46mm on the S4), so the ID shouldn't be an issue.
S6 battery tube, end to end, is right at 70mm long, and the pill at the front and the switch retainer at the rear seat against the ends of the tube when assembled. Then add in a spring on the driver and you're well under 70mm available length.
The cells are too long. At work for a couple of more hours, will check everything else when I get home. If I remember correctly, the spring is the same size all the way up, doesn’t gradually get smaller.
It was noted in the review by mhanlen and then commented on again later by racoon city, also there were a few other threads where I seem to recall someone might have said there was a variation that did take them. Mine also does not take them, but as I never use them in single cell lights it has not been an issue, what has been an issue is I simply don’t care for it anyway.
I just realized this is a XIAOZHI copy, which I do have. If I remember correctly there is a steal washer behind the switch. You might be able to grind down the washer a few thousandths to gain some clearance. Before you do that, look and see if the switch pedestal could also be trimmed a little. If the washer becomes smaller the switch sticks further in the switch boot. If the switch can shed a little off the button pedestal and still work this might gain you a little. Then tighten the switch threaded retainer down tight. The further you can screw it down the more clearance for the battery. Just don’t put the breaker bar on it or it will break. 
This model may not even have the washer.
You might could remove the spring from the driver and add a solder blob to gain a little, you might all so cut the spring shorter or solder on a shorter cone type spring.
Nope, it doesn't work like that. The tailcap threads & tube threads are anodized, it only works when the brass retainer contacts flush against the rear face of the battery tube. Nothing you can change in the tailcap will change that, it has to make contact or you get a light that's no worky.
Yep, you got me, I hade forgot all about that. Mine if you swap ends with the tube it will not light up, cant make contact on one end or the other. What if you added a small washer same diameter as the outside of the tube, with most of the center removed placing it in the driver pill, maybe something in brass or copper that could be soldered on.
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I don’t know how much you need to gain to make protected batteries work. My XIAOZHI is tight but works with protected 3400mah batteries.
There are a few tricks that might work, but it depends on how much of the cell hangs out the back. Shorter/no spring on the driver, spacer with the same wall thickness of the battery tube (that'll depend on how much space needs to be gained, it can be extended either/both ends only so far before the o-rings are uncovered).
(Actually, the correct answer here is 'use a good quality unprotected cell in single cell lights especially if the driver has its own low voltage protection', which this one does)
If you thread the light completely together at the head and then put the protected battery in and then thread the tail cap on. How much space difference, in the tail cap end is there from when there is no battery inserted or just take a pic with the protected battery in it and screwed down as far as it will go, with light force. Might help figure out how much space is needed to work.
OK, got home and tested all my protected batteries in the S6. Got the light just before going to work and only tested 3 different batteries. The ones that won’t work are Fastteck/Wallbuys/Banggood Panasonic 3400, 3100, and Sanyo 2600. The one that will work are Keeppower Panasonic 3400, 3100, Xtar Sanyo 2600, and Trustfire Flames 2400. With the tailcap removed the ones that won’t work stick out at least 2mm without compression of the driver spring and 1mm with compression of the driver spring. The ones that will work are 1mm uncompressed and flush compressed. The ones that will work are very close to not working, zero extra room, some with the same light and batteries may find they don’t work.
I have about a half-gallon of those reflectors leftover from various stuff, I can shorten one by, what, .080"? at the front end and send it to you. That'll let the pill screw in farther by the amount cut off, should get it working without messing with anything else.
Thanks confychair but I can just use it with the protected batteries that will work or use unprotected batteries, not really that big of a deal to me. I might try the short reflector trick if I get any S6’s for nonflashaholic friends.
If its a amc7135 driver with the spring in the middle, you could unsolder the spring, cut it in half. Then use the top half of the spring, solder it back to the driver. Looking at mine, that should give you enough clearance.
Did those lights ship with the Nanjg 105C driver? I can’t remember, don’t those drivers have low voltage shut-off anyway, or is it just a low voltage warning blink? Could there be any negative consequences to shortening the center Pos spring, such as allowing the edge of the battery to short on the driver and thus blowing your protection circuit? If that’s possible, then, as it is a single cell light, would it not be safer to use an unprotected cell?
Bought from FT, driver AMC7135*8, 2800ma, 4 gold stars on the driver, 4 chips visible on battery side of driver.
Nope it can’t touch a thing besides the spring, even at half the length it still taller then all the other components. Even a solder blob cant cause problems as long as it taller than the amc7135 legs, which isn’t much. No matter how wide or small the battery button, as long as its taller than the amc7135’s legs it cant touch a thing.
Borrowed Brted’s pic.
With button-top cells you can replace the spring with a solder blob shorter than the surrounding components. It just has to be tall enough that the button touches before the body of the cell hits anything.
But, no spring at the front can cause problems. If you drop the light and it lands nose-down, the impact can break the driver. And if you bump the light while it's on enough to compress the tail spring, it will break the connection at the driver and change modes on you unexpectedly. Even the stiff short spring is usually enough to avoid both those things.
I agree with you on the fact that the spring is already so compressed it could break if dropped pretty hard, I have never broke one, so I don’t know how hard it would have to drop. Given the fact that almost all lights only have one spring in the tail cap, I cant see much difference besides the tail cap spring being tightly compressed but then again, its that way in almost all single cell lights with the longer protected cell inserted.