Shorted 18650 cells safety question

[k1kk0_1t4]

Hello. Tonight I was spotwelding a battery pack 16S5P Panasonic pf cells but I dropped a piece of nickel on two groups of cell connected on the other side shorting them.
The nickel burned and one plus side of a cell became burned. Two nearby cell were phisicaly damaged too: the cell that I shorted and another near the plus side one.
The three damaged cells were put aside but I don't know if I can still use the other cells in that group.
Are the cells still safe?
How should I discard the damaged cells?
( Should I try to make them explode charging them with a car battery?)
Thank you very much.
By the way that was sad but funny... The nickel popped and burned in a second!

 

[alexis57]

I made a shortcut with my test leads, a big flash and one of the test lead was melted, like "soldered" haha
Not really funny since I know how much current they can deliver...
Picture ?

 

[k1kk0_1t4]

Here is the photo:

ThE other cells seems fine but I'm not sure if I can still use them... In theory the short didn't affect them current passes from the minor resistance path and that path was on the two cells that shorted and the other one that came in contact with the burning nickel, right??

 

[alexis57]

Maybe the small plastic inside was metled... You could get problems in some weeks.
I wouldnt use these cells, just for safety reason, you dont know what can happened, and then your whole battery pack could disappear.

 

[Nobuo]

what you have on the cells is probably part of disintegrated nickel and part of smoked insulation wrap because the burned nickel.

Not all the cells could survive a "torture test" like that :lol: , but these cells are NCA, and indeed the most safest NCA ones.

As your short was limited by your nickel strip and those strips really won't conduct more than an average of 10A - 15A during a few milliseconds of exponential decrease of current (if strip < 0.15mm x 7mm) until the strip will burn and kill almost instantly its conductivity capability. You just made a burst peak discharge.

These cells are capable of a 5 seconds of 20A bursts, and 10A continuously. So the cells, none of them, despite the fireworks, should has suffer any appreciable damage in their inner chemistry. However, the melted nickel could has damage their anode surrounding protection, so the cathode (the whole cell is the cathode except the small rounded part of the anode, in this case the closet part to the anode) would be exposed now, or barely protected.

You should do a charge / discharge test with an analyzer to check if their capacity shows any differences than non damage same cells. I bet that are perfectly right

* Just for safety I would apart those cells, re-insulate again and use for power bank applications

 

[999zip999]

Thanks for the heads up as a lot of people don't understand all the stored energy of those pretty green wrapped cells. Building packs is a lot of prep and set up.
One time I had a 36v A123 20 ah pack I carefully set down on a linoleum style tile floor with the pos. ~ neg wires 8 in apart and spark as the end conducted thru the floor tile for a lighting bolt white spark. Floor tile conductive ? Oh luck too.

 

[k1kk0_1t4]

Thanks everyone for the replies.
I am aware of the the stored energy in those things... At least I am aware aware now!
This hobby is teaching me a lot of new things some by reading forums like that some by doing and some by errors... Like that one.
Thank everyone
Hope to finish my battery (last piece of the new ebike) soon..

 

[Syonyk]

What are the voltages on the cells involved vs what they shipped at?

If they haven't dropped below the minimum rated voltage, they're probably fine, but cells can lose an awful lot of state of charge in a hurry through a short like that.

You should recharge them so they're at the same voltage as the rest of the cells before you put them into the pack, or you'll have some serious current when you hook the pack up as everything tries to equalize.

 

[k1kk0_1t4]

Thank you everyone. The cells maintained the same voltage. I discarded the externally damaged ones and maintained the rest. The only cell that showed some drop was the one that get damaged directly and that shows some burning on the top.

By the way I finished the pack last week and seems strong and OK. for now...