Strange bumps on 18650 batteries

Hey guys, I was wondering if anyone in the community who knows their batteries knows what these bumps could be/caused from?

I’ve had these batteries in a 3S2P pack for a portable speaker, taken them out for a new project and noticed all the little bumps/speckles. The pack was protected, so I highly doubt it’s down to abuse. Never experienced extreme temperatures. When I was assembling it I did a pretty poor job at rincing off excess flux, so as you can see here it’s began to rust a little. I doubt that that’s related to the bumps? The bumps are apparent on 2 out of the 6 cells. Cells are at 3.8v right now. Any thoughts would be appreciated! Really curious…

The rust may have spread along the cell under the plastic coating… this could expose the internals and be potentially VERY dangerous if it exposes the lithium. I would recommend properly disposing of it ASAP

Looks a bit like water damage to me… not a pro… anyways… perhaps there was some sort of ‘‘chemical’’ that reacted with the outer casing/wrap of the cell?

Though taking a closer look… it does look like the ‘‘reaction’’ started from the inside… mhh

Maybe remove the heatshrink and see how it looks underneath?

Remove the heatshrink and look at the casing. I’d guess it’s nothing

Looks like moisture or flux is underneath the insulation… I’d stop using em.

Took a look under the wrapping, sure does look like rust… I didn’t know 18650’s could rust? These cells were inside an airtight speaker enclosure that was left outside overnight one summer’s night. Been indoors ever since. I can’t imagine how that could cause rust, especially to this degree. Very puzzling…

Imho, play it safe, toss those cells out. You don’t want to take any chances.

@Baz_L I’m pretty risk-averse when it comes to batteries. Even if it is possible to rehab these by scrubbing off the rust and rewrapping them, I wouldn’t. Toss them and get some new ones. Thanks for sharing the images, now I know what a rusted 18650 looks like!

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Disposeeeeeeee it won’t explode but it could have weakened the casing which is unsafe

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Thanks for the advice guys, I’m sort of thinking the same… not very safe. But I am really curious about it since I’ve never seen anything like it on 18650’s. I think I’ll put them into storage somewhere dry and out of the house, to see if they will deteriorate further over time. If anyone has any speculations I’d love to hear them, as moisture exposure doesn’t seem like a definitive conclusion to me

It’s just a steel casing. If you get any kind of moisture exposure, it will rust. It’s completely uncoated, just raw steel.

Yeh im also bending towards the theory that some moisture got inside and made the cell’s casing to rust…

At least im not 100% sure can electrolyte induce rust… and is it any sort of electrolyte that got leaked or it’s something else besides moisture what could have done this

I need to check how one of the bad cells look. Im sure it wasnt rusty just a bit ‘leaked’. Involving color change of the plastic wrap

Casing is nickel not steel if I remember correctly from my conversations with a Panasonic engineer