My battery pack turned into fireworks


I got some Samsung 35E cells. 3,5Ah with 10A current. I made an battery of 10S3P.

I installed cardboard covers for the positive terminals to protect. And i spot welded the batteries: Here i’m going to fold those into what’s in next picture. (Quit making assumptions that this is folded from middle! It is two-way fold, like a book) Folded. Installed the wires and connectors. Made an enclosure with some paddings and made sure that wires are loose and that nickel stripes won’t cut the wires. Then i wrapped it on duct tape so it’s sealed. I have charged it every now and then with 42V and 2A. It started sparkling like an rocket and i tossed it outside. Here’s what i managed to salvage :smiley:

What do you think could have caused it? All the cells were oriented the right way, and connections were triple checked. This was the very first time those cells were under a charge. series charging without balancing on 2A current. So it’s 0,2C rate.


Fake cells?

Dude, cover the cells with some tape from next time, cells might have touched and caused this, you are lucky that there is no major damage beside of burning up your cells.

After Soldiering all the wires make sure there is no contact point for anything to touch the +&- parts of the Battery


Looks like poor insulation to me, and who knows maybe even a solder short

Ok looks like no solder used? Question why are the spot welds so discoloured? This should not be so…anode damage?


Post up what spot welder you used and the settings.

Glad you and the rest of your board are ok.

Not likely fake cells. They are from here:

Hmm. I’m going to shrink-wrap the next cells from now on. But i don’t think that was the case here of those rubbing to each other. But good point. And i’m going to use double cardboards on positive terminals also.

Discolored? Hmm… Googling a little… yea they are discolored, meaning there is big currents going in spot-welder.

My spot welder is based on this: (Appears that it has LCD screen nowadays. My circuit has only selectable main pulse.)

I have adjusted the settings, so that it welds that nickel stripe i have, not any thicker. Settings are 50ms pulse, 500ms wait, 400ms pulse. The current is that i remember, around 1000A-1300A

was this the fold line?

the cells dont look insulated in the middle, i may be wrong…


haha that’s hilarious. you folded the cells into each other. of course they shorted.


Wait, did you not insulate them in the middle? I’m surprised it lasted till the first charge, the one time i made that mistake my whole pack instantly started to short and a nickel strip fused to save the day, and it happened because i had opened and uninsulated the whole thing for maintenance and my hand slipped while doing it!

Yeah i was gonna say: How did you manage to just fold them onto each other without insulation?? :smiley:

my understanding was that he folded between row 1/2 and again between 3/4.

I don’t think that was the fold line. I think the fold was here and here :slight_smile:

Then the bottom (or top when folded) was done. Also, if you short the cells out, you don’t really have to wait for a charge process to see a fire… :slight_smile:


OMG. You need a good isolation man.

If you folded them down the middle without putting insulation there I’m pretty sure you didn’t even get to plug in your charger…


Oh this is awesome, thanks for the lesson dude, many people will learn from this…complacency will get you…

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Yes i think you are right.

So this fold looks okay, right?

I’m thinking balance wires shorted, but this was first charge on bench, not a lot of movement. Maybe cells torched by diy welder?


All those folds are not needed That’s the weirdest config I’ve seen :thinking:

I clicked the photos, the discoloration is the sort that happens from lotta lotta heat. Like when cutting tools are burnt, and lose their temper and edgeholding ability. Very consistent amount of it on each weld.

I’m voting for damaged cells.

Only 2A charge current…

@Quezacotl if you have a spare cell, maybe weld nickel to the terminals (with same heat discoloration) and try to charge that single cell, see if flames shoot out? Make sure camera is running this time. :wink:

1 cell is probably not going to be conclusive, but it should give you more information on whether you need to rethink your welder.


may i ask how you charged it without a balance charger?