Ooopps! Smoke!

So its my first time rebuilding aLi-ion segway battery. I replaced about 25 bad 18650cells of the 46 in the pack.Imsoldering them all backtogether with nickel strips. I finished connecting all cells and all polarities looked ok, no major issues, howeverI didnotice a very small spark whensolderingoneof the connections fairly close to this cell. It was very minor and cant imagine it ruining the whole pack..

I couldn't leave it alone..I reflow some solder over a couple iffy spots, and I must have held my solder iron on one spot too long, because the spot I was on started to flare up...like a shortbright glow.. then 5 seconds later a narrow stream of smoke started to shoot out of the top of the cell!

I placed the pack in sand outside and it stopped after 30 seconds.
I'd take a picture but im not really feeling good about getting too close to it just yet :s

Could someone tell me if just this cell is effected or is the whole pack now bad? Would it be safe to remove the smoking cell and replace?

This Forum is probably not the best place for Segway battery issues. Most people here deal with batteries 48V and lower - Segway uses 72V. So don't expect many replies.

My guess would be that you had too much solder between the nickel strip and the (+)cell and the excess flowed outand created a short circuit. As you can see in the diagram below, the gap between the red (+) end and gray (-)casing is very narrow - maybejust one mm or so.
Another/additional cause could be the nickel strip overheating and melting through the insulation.And/Or catastrophicinternal damage of the cell due to overheating.




General industry recommendation is to not use solder on lithium ion cells, because too much heat is required to melt the solder, which could affect the cell material inside. Some acceptablemethods areelectric spot welding (industry standard), andultrasonic welding (just Tesla?).
Manypeople on this forum use quickandtiny "spot solder" methods to attach fuses, but I've never seen anyone solder nickel strips on the cells. And apparently with good reason.

My recommendation would be to safely dispose of all affected cells, including those aroundthem that may have been overheated, and cells with less than 2.5V. Drain those cells to sub 0.5V, and practice spot welding nickel strips on them. Then practice building small 12V batteries before attempting anything the size of a Segway battery.

When I was removing some cells from a pack, I shorted out one cell and it caught on fire. I had a bottle water with me and poured it on the cell to keep the surrounding cells from catching on fire. Long story short, I removed the 2 cells next to the one that caught on fire because the shrinkwrap got melted and reused all the other cells. Those cells that I reused have been working good, I wasn't going to get rid of the cells that didnt suffer too much heat stress, but I was cooling them down in seconds.

Since you said the cell vented from the top(no fire), that might have kept the heat damage to a minimun to the remaining cells, maybe just replace the ones right next to that cell. On mine I had a real fire from the side. But I would test all the remaining cells. I would be more worry that you might have damage the other cells from keeping the solder iron on them too long. That would be the real hazard.

Once the cell stops venting you can remove it. I removed the cell as soon as the flames died down. I was just very careful not to short out more cells when removing it.

I soldered nickle strip to my first several packs I built, and never had any problems, but I did have problem cells that were hard to solder. Now I use the malectrics tab welder(cost around 100 dollars) and avoid soldering as much as possible.

ajw22 said:

This Forum is probably not the best place for Segway battery issues. Most people here deal with batteries 48V and lower - Segway uses 72V. So don't expect many replies.

Click to expand...

This forum is about "all" lithium based battery DIY projects. This would include Segways, especially since he replaced the 18650 cells, which the forum predominately is on topic about.


I would agree with ajw about the cell being shorted with the solder. Smart move dumping it into sand. That probably helped to keep it from going thermal runaway. Sand is a very good heat dissipation medium.
That cell is obviously toast. But the other cells that were connected are probably fine. Though, if you really wanted to be extra leery about it, the cells that were adjacent to the hot one, remove those and put new ones in. Even though they did not burn up, they were subject to high temps. Though, this alone does not determine if they will have short lives, or sparky lives, later on.

Above all, glad you caught it in action and was able to ditch the pack quickly and safely. Only thing that may have gotten burned would be a little bit of pride.

Thanks alot guys.. yes, quite a bit of pride went up in smoke today.. so glad to know all was not for nothing and just replacing that cell, and perhaps a few others should take of it. Whheewww, its a real relief to know this. Thanks much. I'll be back!