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Does it rub off with a bit of cleaner or does it have to be scraped of.
It does not look like typical rust, its not bubbles, it looks smeared on. But you can;t be sure from a photo.
If it comes off after scraping and leaves a rough surface its more likely to be rust. If you can wipe it off its likely dirt.

Weird how the voltages vary so much…?

Maybe I’m understanding you the wrong way, but a spread of 0.4% looks fine excellent to me.
There is only one battery that is doing “worse”.

Looks acceptable to me, especially if it wiped off.

Soldering flux or some kind of varnish??

Was the glistening from a wet or greasy surface?

From the photo it looks like the residue that gets all over the inside of a vape stick.

Also the surface texture of the metal button looks like it has been painted or coated. If it were mine i would use a small round file, or fine sandpaper wrapped around a toothpick, to sand a line across the button until reaching the base metal, the feathered edges would expose any layers.

Cell cans are made of nickel coated steel, where the nickel coating provides corrosion protection; so better avoid damaging the coating. I'd use some commercial metal cleaner first, they work wonders with copper, brass and such.

Perhaps it’s the three significant places, I suppose you’re correct :+1:

Frankly i’d start with rubbing alcohol or dish soap.

The cans might be nickel steel, but what about the buttons? they could be anything, if it were mine i would cut into it with a file to see what it is. Is it attracted by a magnet?

It may not matter at all, but that’s just me.

A couple weeks ago we got three flats and eight buttons...one of the flats had this residue and I think two of the buttons had a smidge. I dabbed a paper towel on my tongue and the cells wiped clean easily. Have eight more flats arriving tomorrow or Saturday. I didn't really try to investigate but I assume it's just some oxidized lube of some sort. Wasn't hard or sticky or anything. I've only seen 18650s from the majors ever poof a little vent when shorted but none of those left a residue that I recall...that was what first crossed my mind when I saw the discoloration. They all seem well so far but none have seen much cycling yet of course.

Cells arrived, all clean with no residue. No cycles yet but after a charge up they seem to be holding steady as expected. Recent tales about various Vapcell cells have me feeling a little wary but thus far all of the ones I've bought have been excellent. Also picked up one of the rewrapped "semi button top" 50-E2 cells since they were out of stock of the regular plain factory ones. I was alarmed for a second because their Liion cell wrap says that it's made in China but I'm assuming that they must have someone apply the button and wrap them overseas before shipping here...that the actual cell is Samsung Korea like the plain ones, and like their ad copy states. They've always been a legit supplier so I'm sure that must be the case.

You can turn off the auto-off feature. I don't think you can disable just the buzzer so if you want the feature on you have to live with the alerts. Check your manual but it's probably just holding the ncv button down while you turn the unit on, if it's like other Kleins. The accuracy of this meter and most of what Klein makes isn't nearly enough for you to concern yourself with millivolts, by the way, especially for cells. Would have to look yours up but it's likely 1% +/- in the lower ranges, plus the digit count...which means that you can be off either direction by .05 or so. Meters usually gets less accurate in higher ranges but that's fine, and no concern at all for electrician/automotive work which these are made for. If you have the choice of manual range selection vs. auto ranging, choose the manual and the accuracy may increase a little bit - not sure about yours but that's common across many meters. The good news is none of this matters for flashlights really, and the electronics in our drivers are simple enough that we don't need high quality/extreme accuracy like is needed in some applications. For cells, tenths is perfectly fine.

It may be stored for a long time and caused by moisture

It is recommended to use WD-40 to remove rust , wipe it with a cleaning cloth or fine sandpaper

https://www.google.com/search?q=WD40&source=lnms&tbm=isch&sa=X&ved=2ahUKEwjAidDi3YLxAhXAJzQIHZZFDGIQ_AUoAXoECAEQAw&biw=1552&bih=729

google it

This is a good rust remover

It is better not to use sharp tools to remove rust, because there is a protective layer. If the nickel plated protective layer is damaged, it is easier to rust

Dry environment is not easy to rust