my guess is that in putting the long cell in, you tore the wrap on the battery, and the positive contact terminal on your charger shorted the negative body to the positive terminal.
You made it sound like an electrical problem.
“I thought any 18650 charger would charge an unprotected 21700 cell.”
They technically all can, since a 21700 is the same lithium technology and voltage as an 18650.
The only issue is physical compatibility, which is what went wrong here.
If a charger can fit the extra length of a 21700, or if you use wires and an external battery holder, there is no problem.
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A wiser option would be not jamming cells into a charger so hard that you break through the wrap.
Also using higher quality batteries such as samsung or LG will also come with more durable insulation wraps.
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.
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.