this scared me a bit, I just ordered a bunch of Lishen LR2170LA cells with full intentions to spot weld them into a flat pack, should I not weld these cells? how else can I get them into a flat pack within my dimension constraints?
Those aren’t bottom vent, I also have some of those for a pack
Normal precaution should be taken for the negative side though avoiding 3mm circle in center where internal weld is
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is there any product has used this particular cell in spot welded fashion?
I haven’t seen one yet but that doesn’t mean much. Until I see multiple images of packs from established pack makers (not small China assemblers with no worries about liability!) using bottom vent cells, and seeing how they do it, I cannot recommend using these cells if the bottom is being welded or even just blocked.
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yea, we need to see how the big guys are doing first before we try to do the same
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I also have some bottom vent cells I’ll be using for an E-Bike pack very soon. (These are BAK 18650 2900mAh ones)
They will definitely be spot-welded and I’ve thought of something to do that safely.
Adding a fishpaper ring or maybe even better the dot from the middle on the negative end of the cell will keep the no weld area and especially the bottom vent free of welds.
This should be safe right?
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Well, i found a document about NASA’s approach to battery building and thermal runaway testing, including bottom vent cells.
I extracted some valuable information both good and bad regarding our methodolgy of assembling batteries. I don’t want to bias you by my findings so i let you read the document than we’ll discuss what changes can we make to improve our batteries.
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Haven’t finished reading it completely yet but wow this must have been a fun test to do
Very comprehensive, this is like stuff I’d see at work. Some really interesting designs in here.
Using G10 is an interesting idea
Bottom vent cells, that is intresting. Wasn’t even aware that was a thing.
Might be able to bond the cell on top side (both plus and minus) then without having to consider vent holes in the bussbar geometry. Designing a modular pack at work atm that is bricks of 14S6P, stackable to a 1000V. So this is really intresting for me 
I have to do more research.
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I think tesla is doing the fuse wire thing on the negative shoulder of the battery.
I have the same idea of modularity with modules consisting of 13s7p but meant to be wired in parallel. I made 4 of them and they can be completely wired in parallel including every cell group.
Tesla bonds on top side, both plus and minus on all new packs 
(theres a whitepaper on why thats good but can’t seem to find it atm)
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These guys do it too. Here is a screen shot from the video in the article. I feels kinda unsafe to weld on the edge of the cell, but on the other hand it makes mass producing the packs a whole lot quicker. They also seem to be using very cool nickel that is over molded with plastic, so it stays in shape and is one big sheet.
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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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