I did xD
realized now it also works the other way around, think i overshot a step when planing it out last time
1 Like
I always draw 2 wiring diagrams per battery bc its easy for my brain to understand. 1) All the main serial connections, terminals etc⌠2) bms wiring. Makes your life a lot easier
my brain often makes thinking errors and builds upon this, so it actually doesnât help me because i think itâs right like i drew it
It only works the way you drew it if the numbers of the cells work the opposite way. This is how it should look
B- (- cell 1 +) (- cell 2 +) (- cell 3 +) ⌠etc ⌠(- cell 12 +) B+
2 Likes
but this is now an S group, right?
aka you connect cell 1 to 12 and connect 3 of those in parallel?
Your drawing above was also in series, you just had your numbers backwards
so the difference is that you made it
next P group[+ cell 1 -][+ cell 2 -][+ cell 3 -]previous P group
?
But I would also get a 3S pack unless I connect 4 of those in serial
Your numbers are still backwards 
What does it matter that I numbered my cells from the other end?
Because if youâre joining cells in series, you join the positive of cell 1 to the negative of cell 2. The way you have it labelled doesnt make sense
Every bms i have ever used counts the series groups starting from the negative end, so thats just what makes sense to me
1 Like
Itâd probably make a big difference if you confused the numbers and plugged a bms in backwards
2 Likes
Youâd prob do better drawing a more traditional wiring diagram. Itâs visually easier to understand⌠for me at least. This is what I drew up for my last 2 flat packs:
8 Likes
I plan on using 50mm wide nickel to weld across a double stack 21700, what is the strategy for welding the positive terminal? Its hard to see it when its under the sheet of nickel, and the last thing I want is to accidentally short the cell. I though about 3d printing a jig but it seems like there should be a better way.
1 Like
Use a blunt and soft object to push the nickel down. Itâll make an outline of the positive terminal. Or stick insulator rings on the nickel so you can tell where the center is
5 Likes
This is my first time spot welding so bear with me, a good weld when pulled off of the cell will leave a little bit of nickel behind correct? It should not be a clean removal?
1 Like
correct
2 Likes
Maybe this already exists somewhere and I just canât easily find it, but I feel like maybe we should have a better description and/or references for what a good pull test is and what you expect for different thicknesses. I didnât realize for quite a while that you probably wonât get the long shards ripping out with 0.2 mm nickel even on a âgoodâ pull test, but recently I read someone saying that. In retrospect, I probably overcooked my welds on my first battery I recently built a bit because of this. They still took about as much to get off as some that were less hot, but because doing them hotter left more material behind and I thought I was looking for big shards to rip off I ended up going with higher than optimal weld time I think. I realize now itâs probably more about the force it takes than how much is left behind even though at least some should be left behind. I think I had also read/seen something before that said you should get big shards that pull off with the cell. It may be better to emphasize that they should be very hard to pull off assuming Iâm correct that is the more important factor. A lot of people seem to concentrate on nickel being left behind.
1 Like
I find this as well with 0.2mm nickel, I never get shards left behind even on settings that blow through the strip, but I can get it to a point where I need to pry off the nickel with pliers to get it off and even leave holes in it, but no shards left on the cell, just dots.
The point behind the âmake sure you leave nickel shardsâ advice is that your welds should be as strong or stronger than the sheet metal you are welding. You can have a weld which is strong, but if the weld is failing before the sheet metal, then your weld has room to improve.
You may be welding too hot, and/or your probes are not sharp enough, and/or your technique is off.
At the end of the day, you are the only person in charge of deciding whatâs âgood enoughâ for you. But when folks tell you what the best practices are, in my opinion itâs best to keep learning until you can achieve them.
7 Likes
This is often not discussed but is quite important. When cleaning your tips, makes sure to be mindful of this. Blunted tips really affect the strength of the welds.
1 Like