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

Or the wrap is faulty from shopping and you don’t see… it happen
Before put battery check always technical spec on original site.
For example, I don’t know why but Xtar Vp4 Drugon Plus don’t charge 21700 protected
but charge 21700 unprotected.

The wraps can be pretty durable, but catching it just right can tear it. All its takes is a tiny little hole…

Probably what happened. Either the insulation was already damage or it got damaged by inserting it in the charger. (not saying that you were careless or something like that!)

Either way, this is a great lesson to all of us to really be careful when handling a cell. And to not put any other cells in a charger that's not designed for them.

Thank god it didn't get out of hand and that you are fine!

Well, it does not suddenly make a short.
Are you 100% sure your charger is ok?
Are you 100% sure your cell was ok?

If so, it must be a human error as described above.
Probably by damaging the wrapping by trying to fit it in a charger which is not suited for that size cell.

Someone correct me if I’m wrong but aren’t you supposed to have a charger powered up before putting in batteries to charge?

Yes, I'm always very careful when handling cells, and this is the first time I have tried to put a cell in a charger that doesn't fit.

I think I ruined that one cell, and I won't be using that slot in that charger any more, so I did get lucky.

Good question.

I always put the cells in first, then plug in the charger.

In today's incident, I didn't even plug in the charger.

The ring on top of the cell is were most wraps tear up first, IMO. those tears can be really thin and invisible; when the cell was inserted the charging contact accidentally connected BAT+ and the cell body.

I’m not an expert on battery chargers nor an electrical engineer but how is the charger and all its built in protections supposed to work if it’s not plugged in?

Some people put the cells in first and would then connect the charger. In fact, in the RC hobby many people do it this way. hobby chargers can be finicky at times…

I would rather be elsewhere when switching on the charger rather than put the cell in when already powered on… I mean… normally nothing happens… but what if?! I like my face the way it is.

Like I said I’m no expert but I’m pretty sure the chip in your charger is not powered by putting your batteries in to charge so how can it work? It’s not like a flashlight where the chip is powered by the battery. So I’m assuming here but you basically put in a very powerful battery in and completed the circuit with no control and it dumped all that power with no regulation and nowhere to go, unlike when it’s plugged in.

With sprung bay chargers like the one used, if there is no clearance of the cell positive and the charger positive when inserting the cell then there is a risk of damaging the wrap when inserting.

You should use the cell to push the sprung charger contact down, lay the cell flat in it’s bay then it should slide up to make contact.
Don’t push them directly in flat as you will catch the cell wrapper on the charger contact and wear or a direct rip may occur in time.

Cell wrappers are very thin plastic and easily damaged especially on corners when pressing over other hard objects like metal with protrusions.

If you do need to cram one of the larger cells into a charger then use a thin sheet of tough plastic etc to hold in the bay over the charger positive contact while inserting the cell to prevent wrapper damage. Once the cell is flat in the bay pull the plastic sheet out while holding the cell flat.

Also check your cells wrap from time to time to make sure it is intact, chargers with dimple protrusions on the contacts like the one pictured are better at poking through any holes and welding, I prefer contacts that are flat.

I use several Molicel INR-21700-P42A. I have found their wrappers to be brittle and easier to damage than the others. Great cell, weak wrapper.

rewrap the cell and move on.
that charger is too tight for a 21700.
it tore the wrapper with the 2 contacts due to the pressure of forcing it in.
at least you got it out.
super high drain cells like the p42 get pissed off very quickly when shorted.

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.