Today I got a single 18500, first I was impressed that it came in a card box with additional kaptontape over both battery ends.
But after removing the tape the positive side looks a bit corroded…?
What do you think is this kind of brown water from something else just ran under the tape or is the cell damaged?
I measured 3.5V and it is charging with 0.5A just right now….
Personally I’d return it from where it came from.
Might just be a corrosion preventative. Hard to say.
Looks like rust underneath the insulating ring, that is “bleeding” through a bit. I bet if you strip the wrap off that, you will see a lot more rust under the ring. No good!
I could be wrong, but the wrap and ring don’t look very good either (the wrap is “loose” on the ring, and ring looks too big).
I’ve also never seen any genuine factory-new cells ship with Kapton tape on the ends.
Can you tell us where you got this from?
I will recycle it probably just to be safe but I am also very curious so I will strip the battery tomorrow after making a discharge test and inspect further if I can see where it came from.
I got it from banggood and they put the kaptontape tape over the battery before shipping, which is kind of a good thing to prevent shorts… The wrap and battery itself looks fine to me.
From the colour it could be iron oxide.
It looks like it vented.
Thanks for the info. I should have clarified my original post: I have never seen factory new cells with Kapton tape UNLESS they come with tabs (which these do not).
Also, I know that those particular cells were (are?) a bit hard to find for a while, and as such there have been numerous fakes reported. I would just be suspicious.
Before unwrapping it, you might weigh it as well. 
I have had a few 18650 cells do that I’d like to know exactly what it is too. I think it happens if they get wet
Or if you feed them after midnight. 
Looks like it was caused by heat to me, perhaps the cell was shorted. Odd considering they used Kapton tape to prevent such a thing. Maybe that’s why they started using the tape… IDK, just my first thought.
I see cells that look like that and worse in the drill packs. Makes me really nervous.
Here’s some pics of some Sony VTC4’s I had to strip off the wrapper, all these cells test out at about 2100mAh. Better than the 2000 they are rated for.
I’d unwrap it and look under the disc for clues. Perhaps a bit of oil or grease from machinery during manufacture? Any stains on the box? It shouldn’t be overheat or a short since the cell had correct ‘shipping and storage’ voltage on arrival.
Phil
So i did some discharge tests and at 1A I get around 1850mAh to 3V and I can squeeze out 100mAh if I do a 0.3A discharge to 2.8V.
A Sanyo 18500 gives around 1570mAh at 1A down to 3V.
So the battery has a nice capacity, the highest on a 18500 I have seen.
I also did a charge up to 4.5V @1A to cook it a bit and nothing obvious wrong happened… will unwrap it after another discharge to 2.5V…
Edit:
I charged it with 2A, then let it rest and recharged it with 0.5A just to be sure it is full. After that I did a discharge down to 2.5V with 1A and got 1970mAh.
I unwrapped and inspected the Panasonic 18500:
There was a very small dent on one side and the bottom of the cell is a bit convex. Around the top is a lot of dry rusty stuff.
So the cell seems to be the Panasonic 2000mAh but the one I got is damaged.