I wish I could show you a video of our production, its even more fancy 
The machines bonding and welding are very precise, we even have them pullin on each bond wire a little to make sure they’re really on there.
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Mind to share the name of the company you are working for ?
Guess what, i heated up the sucker from -2C to 33C and it started leaking:
At least now i’m 100% sure that this cell was the culprit and increasing temperature increases internal pressure. I will increase the pack temperature to check for other cells leaking.
I started a 20A(tapered to 16A) discharge and the module is at around 36C. There is a left and right side temperature diference of 3-4C because of the less than ideal current paths. Cell balancing at the bottom is good with ±0.015V difference. I know that 36C is not that much and i could go at least 45C, but until now no leaks detected.
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I don’t see any reaction to NASA’s research document , so i’ll start by pointing out what i understood:
- it seems that in some cases a bad cell will experience side wall rupture and drive adjacent cells into TR
- NASA made alu heatsinks with 0.5mm cell spacing and added mica insulation to every cell to prevent this → not possible for us
- bottom venting is an extra safety feature and doesn’t replace existing features → if we block the bottom vent with nickel it just means that we are not using this extra safety feature
- the ring cut in the bottom is 1/4 of can thicknes (Sony VC7) or less (LG M36) → avoid welding on ring at all cost
- in case of venting there should be a path for gases to escape outside the battery case → this is not very clear to me, at least i don’t know if my battery case design allows this. Maybe we will talk about this later.
NASA was making batteries which were comparable to what we are doing now in the begginings. They found out that it’s not really safe and they had to design new batteries incorporating technologies which is just impossible to DIY. Even if it was possible it would cost a ton of money and nobody would buy it.
As a conclusion, we have to draw a line concerning safety levels and incorporate everything which is feasible for us and leave out the high tech stuff, and pray to God that nothing bad will happen.
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Nice summary, thank you!
I respectfully disagree on this point though.
Bottom venting supplements top venting to help prevent side-wall ruptures and the odds of runaway happening at all. It is an important feature and the time and money spent to develop it was not done for the reason of just adding an optional feature. It was done to increase safety, to help increase the chances that a cell just vents instead of going into runaway.
IMO it’s worth considering how we can retain use of this feature whenever possible.
Before everyone stomps all over me…yes…yes…I know…it will be essentially impossible to keep this feature in play with existing build practices. Our community is not willing to create the custom cut nickel needed to open up the bottom of the nickel strip. We are not willing to make smaller packs that free up room between p-groups so the bottom vents are free to open.
I get it. 
However, this does not mean we shouldn’t strive to define best practices to help us, and other new builders, make the best decisions regarding pack design and the tradeoffs between convenience and safety. Almost no one realizes that some cells need to be handled very differently and that, in an unquantifiable way, the safety of the pack can be affected by the use of bottom vent cells.
This is not fear mongering. There no possible way that interfering with the operation of the bottom vent doesn’t increase the risks of using that pack. Even if it’s only by a tiny bit, isn’t it worth exploring best practices in case there are some packs that can take advantage of them? Things like where to weld, and not weld, what the bottom vent is for, etc.
Or…
We can all just take the easy way out and just say to never use a bottom vent cell. But if they are priced lower or other cells are not available then they will be used. Let’s make sure those who do so make an educated decision.
…rant over…nothing to see…move along…move along…
I’m willing to help out with documenting these practices. I actually enjoy technical writing, which should be obvious from the length of my posts, as it is very relaxing for me.
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As it is for me when I read your lengthy posts.
The hero we need
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Sounds like I’m just not ever going to use cells with a bottom vent.
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Awww…shucks. 
I’m just a nerd with, sometimes, way too much time on his hands.
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Absolutely insane idea of the year for bottom-vent cells…
Weld a top contact structure onto the bottom to provide a ventilated standoff. Then weld the nickel to that as is done on the positive end of the cell.
Not as a perfect solution for all the issues, just as an option to retain some bottom vent functionality for those who want it.
Whether the top contact structure can actually be reasonably welded onto the bottom of the can is a whole ‘nother issue.
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Just inspected my battery, no dd smells or any pool of liquid, I assume the liquor would absorb into the fishpaper pretty fast no? So I should see wet spots. Anyhow, this is all good to know, will avoid bottom vent cells from now on, this battery however, I will likely keep using and watching, see if at any point bad things start happening
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My take from it is that the best simple way is to leave a generous gap between cells, possible for use but also not possible since we usually want to cram as much cells as possible
I think nickel welded to the bottom/vent isnt going to seal it. It needs to pop outward just a tiny distance and not obstructed by the nickel.
I think it depends on the cell and it’s vent. these A123s have a very large circle/vent on the bottom and youre pretty much forced to weld it. They’re recycled cells and had been welded on the vent too. The vent is very thick material on these and take like 3x the current to weld.
I think the 2 vented because I had welded with high current doing copper
My attempt to blow up/test a vented cell by charging at 4 amps resulted in a bit of warmth and the voltage wouldn’t go over 1v and dropped to nothing shortly off the charge. But these are lifepo4 so safer
Buy my cells for a buck each!!!
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Derail jail…
What current settings do you find work well?
I’m having trouble getting consistent welds…
what welder? what nickel? what cells?
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I have some missing data regarding these 2 events: venting and thermal runaway (TR).
From my understanding TR will force a cell to vent, which could be on top, side or bottom. Best case scenario is on top or bottom because it won’t trigger nearby cells into TR. If a vent occurs this means that the cell already suffers TR. Is everybody agreeing ?
I think the bottom vent can function properly with nickel covering half the cell and spot welding in the inner circle. If a vent occurs nickel shouldn’t be stiff enough to prevent opening. I want to implement this feature in the following revision and make 4 spots instead of 6 → less chance to weld on the circle. This would be nice to test though, as NASA did.
One inconvenience is on the - side of the battery, where my battery design takes 3-4layers of stacked nickel covering the whole cell. I am thinking to change this: put half tabs on cell then link tabs with a busbar.
My batteries are pretty closed anyway and i dont think a TR or vent won’t trigger nearby cells into a chain TR. The case is plastic and most probably a vent will make a hole in the 2mm thick case. The case is not fully enclosed, there are gaps which are covered with shrink foil.
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I probably should have sent @hummieee a dm instead of posting here. I’m trying to weld .15mm nickel (passed the salty saltwater test) to the a123 cells i bought from him. I’ve got a sunko 788h spot welder.
Definitely not.
The two venting disks are designed to blow at internal pressure levels well below that seen during runaway. This is done in an effort to stop the cell from ever going into runaway or bursting its top crimp.
Lots of cells have cracked/popped their venting disks and simply leaked electrolyte due to excess gas pressure from being used too hard, well before they would enter thermal runaway.
A problem with that though is you must avoid the center couple of millimeters to not affect the internal spot weld and you must also avoid the venting ring. That leaves very little room.
IMO if someone is using narrow nickel strip like your setup does then they should just weld outside the venting ring and let the vent disk push up the loose end of the strip if the vent activates. The pivot point for the nickel strip would be the partial ring of welds at the outside of the cell’s bottom.
This is all guesswork though until we can see what the good pack assemblers do with cells like these. Use whatever method you feel is best and I’m looking forward to seeing how it turns out!
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hi didnt know you were asking me or had these cells but even then i dont remember and im using a different welder. id start low and work your way up and do some rip tests. the positive (bottom) of those A123 cells is much thicker strangely than any other cell I’ve welded and takes about 3x as much current.
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Thanks for the suggestions. And apologies if that was unclear.
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So these bottom vent cells are maybe meant for spring terminal use cases? Heh cases
About venting when not in TR… would a compromised vent leak electrolyte a lot under vacuum? Not sure when vacuum would ever happen but idk.
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I would think that the vacuum would actually help the vent open and would actively suck out some electrolyte. How much more, compared to a standard air pressure situation, I do not know.
A pack used in space would experience this I would imagine. NASA has put a lot of thought and testing into making better packs for space use.
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