Thanks for the heads up.
I’ve been using protected keepower cells for mine from Gearbest for about a year at the rate of one recharge for each 2 days. No issues whatsoever.
Both EVVA 26F & 18650GA fit in my Convoy S2+. The GA is pretty snug but no problems really. All bought from Richard at MTN.
One nice thing is that protected batteries will never rattle in the S2+.
I don’t own the s2+ but I heard people using “laptop batteries”.
You take an old laptop battery and carefully disassemble it (see youtube for instructions)
and you got some free batteries.
I was able to get between 4-10 free batteries from each laptop
I recognized a lot of them being sold on ebay, charging $4-6 each and not disclosing that they are obviously “recycled”
The Samsung 4.35v 18650D1 is incredible impressive!
4.35V cells sag pretty quick below a high drain cell if you go higher than 2A its not worth it
I had thought that was the opposite. Mountain electronics posted some tests.
My EVVA NCR18650B protected is the only battery that actually fits well in my S2+. I just got some NCR18650B protected cells from IMRbatteries, and they are too wide to fit. I also have an Anker branded Sanyo 3350mAh, and that one is too wide as well. Ugh…
very strange in my Convoy S2+ the 18650B protected fit and still ratling a little
I put additional heatshrink to my unprotected cells to eliminate bad rattling
maybe you bought a fake S2+ with other inner diameter the S2+ was about 19mm?
Is there a way to tell? I bought mine from Banggood.
Usually Banggood delivers the real ones, maybe a batch with other tube diameter
too bad its not easy to expand the tube inner diameter
I don’t have a precise measuring tool, but the inner diameter of my S2+ is closer to 18mm than it is to 19mm.
UPDATE: I removed the labels that IMR stuck on them, and now they fit (just barely) into my S2+ with a bit of a push.
Could it be possible that Convoy enlarged tube inner diameter lately as they did with C8? I measured new S2+ inner diameter 18.9mm. That is quite ok with protected batteries that has “dual wrap”, like NCR18650GA with original wrap + protection + outer wrap. Don’t know what 18650 could be thicker than that. 
I would recommend Panasonic NCR18650B.
Cheap, good, high capacity cells.
Protection is not really necessary i.m.o.
A decent charger is more important.
Like the Thrunite MCC-2, the Xtar MC1 Plus and the Liitokala Lii-100?
4.35V cells aren’t as high drain as the top imr’s so the extra voltage can be lost to sag. To my mind their best usage is in buck mode where the added voltage translates to lower initial current making the most efficient use of the power that voltage represents. Another use is in 16650 applications where the extra capacity then exceeds other chemistries in that size. For moderate drain use any of the better quality high capacity cells will be fine, exactly which one runs longer depends on how much time is spent in each mode but the difference will be small in any case. When looking at graphs it’s good to have an idea what the Vf is at your target current so you can see where it begins to drop out of regulation rather than simply looking at end points of the test. For DD drivers the discharge curve will indicate what kind of output to expect at different points. The voltage indicator in bistro can help with this if you bother to memorize the curves for discharge and led output(which I don’t).
I received my S2+ just a couple of weeks ago, yet the inner tube is rather narrow.
I just bought an S2+ two weeks ago, well four weeks ago if you count shipping.
The tube I.D. is: 19.09mm. My Nitecore batteries have an min. O.D. of: 18.40mm. The
wrap is somewhat lumpy so the max. O.D. is: 18.50mm. Regarding the narrow tube’s, I would just open up the I.D. on the tube, then anything will fit! (Provided that the wall thickness of tube
can handle stock removal without crashing through the threads.) 
I just started buying batteries and I have a Convoy S2+ and C8 coming in. How do you protect your unprotected batteries for a flash light? Do you just check with a voltmeter once in a while?? I’m a noob.