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

I feel really stupid.

I put this 21700 cell:


in a Nitecore SC4.

The charger was not plugged in, and there appeared smoke!

I quickly took the cell out of the charger, and am airing out my room.

The cell and charger are both damaged.


I am now in the market for a charger that charges 21700 cells.

I don't need recommendations yet as I need to do some research first.

I thought any 18650 charger would charge an unprotected 21700 cell.

I was dead wrong.

(I hope my health isn't negatively affected.)

What do you think happened? Where did the smoke appear? Any pics?

The charger wasn’t even plugged in. Did it short the battery somehow?

I don't know much about batteries or chargers, but I think the cell did short.

I'll some take photos later and post them up.

The damage to the cell is obvious.

The damage to the charger is subtle.

I put a red circle around the damaged area of the charger.

The charger is a little dusty, and it shows.

That does look bad. Was it difficult to put the cell in the charger, or did it appear to fit properly?

The cell would not fit in the charger.

The cell was crooked, and was easy to remove when I saw the smoke.

Also, I was wrong about which Nitecore charger I have.

(I have a Nitecore SC4, and I have changed the OP.)

I have a ’14 I4 Nitecore charger, but I’ve never been a fan of the brand.

Xtar for me.


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.

I didn't tear the wrap on the battery.

I think the wrap caught on fire when the cell shorted.


I don't blame Nitecore.

I tried to put in a cell that is too long for the charger.

I take full blame for the problem.

I think I'll get a Nitecore UM4.

They will accept really long cells (up to 79.8 mm) and they're pretty cheap.

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.
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.

I didn't jam the cell into the charger hard.

I just tried to put the cell in the charger, quite carefully might I add.

I didn't break the wrap, either.

I think when the battery shorted, the wrap caught on fire.

Also, this is a very high quality cell.

I don't know how good the insulation wrap is, but it's a great 21700 cell.

I never made it sound like it was an electrical problem, and I never believed that.

I just figured since a protected 18650 is about 70mm long and an unprotected 21700 cell is about 70mm long that it would work.

(Obviously it didn't because my 21700 cell is too long, hence the title of this thread.)

I think 21700 it’s defective in wrap and the negative side of battery touch the positive side (nitecore) when you plugged in

I've been thinking...

How easy is it to tear an insulation wrap?

If it doesn't take much pressure, I might have torn the wrap without even knowing it.

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?