I think I know what you mean but just for others who are reading this…
The potential (no pun intended) for a short circuit won’t change based on the cell’s voltage but the amount of energy a cell can dump into a short circuit changes a lot depending on how much charge is in the cell (its voltage).
This can be the difference between the cell just sitting there getting a bit warm and the cell bursting and tossing flaming debris across the room.
There might be other concerns with welding fully charged cells but right now I can’t think of any. LOL…the short-circuit energy issue is such a big one that any others probably pale in comparison.
Now that we talk about it, I did fully charge the cells that caught fire last week. Man I was surprised by the amount of smoke and flames those little Fuckers can spit.
Absolutely.
A couple of research papers I read mentioned that a li-ion cell can create up to 2 liters of gas for every Ah of capacity the cell has. It made it so clear to me why the cells need a venting disk and can burst when in thermal runaway.
Does the pressure inside the cell go up and down with heat? seems like a small amount at room temperature. Oh ventilating disk does that mean preasure is equalized or only when burst?
I would expect that it would a little due to any gas expanding when heated. But the real pressure doesn’t start rising until the electrolyte starts decomposing at, IIRC, about 75°C internally. Other compounds start decomposing and creating gas at higher temps.
I don’t think there will be any pressure at room temp unless the cell has aged a lot and the electrolyte or electrode materials have broken down some.
The venting disk is a one time emergency device to get rid of excess pressure that could eventually burst the cell if not released. Under the top contact of the cell (and sometimes on the bottom) the metal is pre-scored to create weakened lines that split open at a certain internal pressure level. It’s the same setup that beverage cans use except that the beverage can’s lifted tab applies pressure on the score lines to split them.
A cell’s venting disk can never be used to just equalize pressure and close again since the loss of any electrolyte vapor/liquid during the opening would affect the performance of the cell. Any oxygen or water vapor that got in would also affect the cell.
It’s all about where the gas is trapped. Considering that a cell is almost completely filled with stuff that means the (potentially) up to 8 liters of gas from a, let’s say, Molicel P42A is compressed into less than a cubic centimeter of space. That would tend to up the internal pressure a bit.
This exact thing has happened to me as well. Put a nice little melty spot directly in the center of my vision on my left eye. The annoying blur in my FOV was an excellent daily reminder that I would probably be down an eye if I hadn’t been wearing my glasses.
Need some advice on connecting two P groups. The nickel strip Im using is 2mm, I ok to use the braided wire on the back like in the picture. I want to avoid saldering it on top of the cells after they’re spot welder.
Is that braid for current collection on the most negative group? (Edit: I can see its not on 1 group, but the point I made still applies)
It’ll work well, but you can actually solder before you weld, so 0 heat will go to the cells. Its also WAY easier because the nickel sheet has basically no thermal mass so the solder will flow much more easily.
In saying that, i usually just use a couple 16AWG wires to collect current. Only needs a few solder points, and does the job. The nickel you have there can already do about 80 amps or so, so you’d only need 4 16awg bridges to make it to 200A output.
Yep the braided wire is for current collection on that specific P group only. The others are all front to back although for one its side by side. I wasn’t sure if there would be heat build up between the first cells if it were configured this way.
The length of the “cable” (the nickel and braid in this case) is not long enough to have a real difference in resistance between the path from the further cells and the closer cells. If you have a 200A capable connection and then only run 50A, it’ll be fine (Most batteries run well below 50A 99% of the time, and only burst higher for such short times it won’t ever heat the nickel)
If you breathe once you cycle almost double the amount.
