Warning: Do not put 21700 cells in chargers not made for that cell! (pics in post #4)

Some wraps are tougher than others. Clearly it tore. Check battery voltage. Likely still good, just need to rewrap it. Edit, as others have said.

Yep. I’ve had several cells where the wrap cracked and peeled back near the positive end.

Remember plastic is made with plasticizers — chemicals used to keep it soft and flexible.

Cheap plastic uses less of the material or cheaper material.

Those evaporate eventually (they’re what makes that “new car smell” and oily film on the inside of new car windows)
Once they’re gone the plastic isn’t plastic any longer — leaving a brittle fragile material behind

Based off of the pictures you posted it clearly seen that you damaged the wrap, lightly which caused short and that damaged the wrap a little bit more
So… charger was no problem, except battery bay wasn’t long enough
No electrical issues

Because the PCB added to the 21700 is between 3 and 5mm, so that adds to the length of the 21700, and the idiots who designed the charger did not decide to add a few extra mm travel on the charger bays. I have the same charger, but fortunately the Hohm Tech School charger that cost me less than 10 bucks fits. Stupidest thing ever that my expensive Xtar, Nitecore, and Efest chargers will not allow a 21700 protected battery, but that cheap 10 buck charger works fine. It does not have the capability to measure battery capacity or internal resistance though. Why the charger manufacturers did not add an extra 10mm length to the bays is hard to understand.

I would love to see chargers with 90mm slots so my fat fingers can easily put them in and take them out. All of my battery wrapper damage happens when trying to put them into and out of chargers.

If the charger was not plugged in, it is nothing but pieces of metal. The only possible cause had to be either a torn wrap, or the pressure put on the positive post bent the positive contact and spread it enough to short under the wrap. If you look at the positive post on a battery, it is shaped like a tripod. If enough pressure is applied to the top, it can further spread the tripod shaped supports. That is perhaps even more likely than a torn wrap. In any case, it was a dead short between the positive and the outside of the battery that is negative. Lucky it did not catch fire, as they often do. Some chargers that can accept the batteries will not even list them as working because the charger was made before 21700 batteries came out. If it does not fit with a few mm to spare, do not force them enough to exert much pressure on the positive post.

You can always get one of the olight universal magnetic chargers. It consists of two magnets that you put on the positive and negative and it auto detects polarity. That will work on all the batteries except the ones with a positive and negative post on the same side like is in a bunch of olight flashlights. Go figure, they designed a charger that will work with every battery except theirs.

That is a Molicel p42a battery, and is one of the best, if not the best 21700 batteries on the market. In Mooch’s battery tests it even outperforms the samsung 40t and 30t. He can’t buy a higher quality battery than the Molicel p42a because they do not make a higher rated 21700 battery.

It seems clear that what happened was that the cell fit so tight it pinched through the plastic wrap and shorted out the cell. The charger will be fine, the cell should be gotten rid of. When you insert a cell into these style chargers, always insert the negative end first, draw back the spring, and lay the cell down, still holding back the spring yourself.

Actually some chargers like my Xtar Dragon VP4 plus can be loaded with fully charged batteries and serve as a power bank to charge cell phones and ipads and such. In any case, no charger will just drain a battery quickly when unplugged, it needs a load. If a charger not being plugged in could do that it would be too dangerous to sell. Power failures do happen as can accidental unplugging. Having looked closer, both the positive and negative are burnt between the positive post legs, so this was due to a torn wrap.

I have DLYFull T5 charger and it's very tight with 21700 and slightly better with protected 18650s.

I was always worried about wrap damage at the front in case something like this happens.

So I fixed the problem with stick on shim insulators at the front of the battery on the top of a wrap. Even if the shim gets torn (it hasn't so far) it would be much easier to see the damage than without it.

I have been afraid to jam in 21700 into the T5 otherwise like it well.

I will continue to be careful with inserting cells after reading this thread.

RC,
Your charger may be OK. Or just the one bay is cooked.
Basically, as other have said, you got a short.
But the short only passed current through the positive contact a short distance. The current didn’t pass through the charger.
So the positive contact got mucho hot and perhaps melted something internally attached to it.
Perhaps not.
Open the charger up and take a look for cooked components. Bet you won’t find any other that right at the positive terminal.
Charger being off sure didn’t cause this.
And yeah, the manufacturers could have given us a couple of more mm of length.
But heck, they save $0.004 per charger, so it’s a plus for them.

I just got the Vapcell S4+, Like the ability to throw some amps at bigger batteries.
I have the old version that turns the display off too fast. Supposedly a new version fixes that.
All the Best,
Jeff

I am now looking at HKJ's review of the Nitecore SC4:

https://lygte-info.dk/review/Review%20Charger%20Nitecore%20SC4%20UK.html

Apparently some unprotected 21700 cells fit, but not the ones that I have.

So before inserting a 21700 cell in a charger, do some research and make sure that it'll fit.

RC,
Just checked a flat top 21700, that’s 70mm long in the Vapecell.
Fits with lots of room to spare.

Oh, sorry.

(My post wasn't a reply to yours.)

I ordered a Nitecore UM4.

They will accept really long cells (up to 79.8 mm).

Just be aware that the "Olight Universal Magnetic Charger" will not work on "all the batteries" all of the time.

It will not begin charging a "protected Li-ion Battery" after the battery's protection circuit has tripped from low voltage until the battery's low voltage protection circuit has been reset by some other means.

Some chargers do not have this issue and will begin charging which resets the battery's protection circuit.

Even if you didn’t scratch the covering while putting the cell into the charger it is very possible that the small metal bump just pushed its way though the wrap due to the spring pressure.
Ideally the charger would have a spring that is soft enough when fully stretched to not cause this, but usually they are very stiff to make better electrical contact.
Plus the fact that you’re using a 70mm long battery instead of 65mm long makes it even worse.

If you had a charger that didn’t have that second bump, or if you placed the cell such that the two bumps were on the positive terminal (instead of one on the terminal and one on the edge of the body) this would not happen.
Also I still think if you used a samsung or LG cell this wouldn’t happen, those wraps are much harder to break through than this cheaper brand.

there is no electrical reason it should not work, i just think you broke through some insulation and caused a short

wle

I thought it vented, but come to think of it based on the videos I have seen of a venting battery it is unlikely he could have grabbed the battery if it vented. They usually catch fire, and even in the demonstrations they caused it to vent from a distance. That is probably just a surface burn, and the battery and charger may very well be OK. I just assumed he was saying it vented, but that battery does not really look like a battery that vented. As I post this, I have a question. Where can I find wraps that are a little longer than regular 21700 battery wraps. I have some protected 21700 that the wraps are a little short for. I need some wraps that are maybe 3mm longer to re-wrap protected batteries. I use these in some cheaper lights that have no protection built in the light, and also on some lights with stacked cells. I tried using 2 wraps, but then the battery gets stuck in the tubes of the flashlight.