I can teach you that too…
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its a required course at my college. whether or not I wanna learn how to weld, its coming 
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Also doesn’t look like the person who built this pack avoided welding dead center on the negative terminals which can damage the cells internally right?
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It can technically but I don’t think we’ve seen a pack being affected by it yet
Better to avoid them anyway to completely skip the risk
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With what those welds look like I really wouldn’t be worried about it. But in a normal scenario with a proper welder, yes you do want to avoid the center on the negative terminal
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Hm, I’ve got a cell from a crappy prebuilt pack that conked out after hitting a larger than normal crack in the sidewalk. It was the only one in the pack with a dead center negative weld so I thought that might have been what did it, but who knows. All I know is one second the board was working, the next it wasn’t, and when I took it apart that cell was reading basically 0 volts.
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Yeah hard to say exactly what it was, but most likely a weld popped off somewhere
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The big problem is, how would we know? 
Agreed. This is where “best practices” are great to have. We can’t always quantify the different risks so all we can do is strive to make the best packs we can to help lower the odds of anything bad happening.
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I made a woopsie on one of my welds for my first pack by sliding a few mm last second and ended up with this. How bad do you guys think this is?
It’s fine, it’s just a messy weld. I’d add an additional weld pair on that cell for safekeeping
It could have been worse if you did that on the pos side
welds look hot though
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Hm, I’m afraid to turn it down because it seemed like I was in a narrow window between blowing holes and actually getting things to stick. I should have bought more cells to test with though. I had some junk ones I messed around with, but not a whole lot of extras of the ones I’m using to test with.
Also, yeah I was glad it was on the negative side. I’m much more worried about the positive side, but I’ve just been doing those tightly in the center.
Ah I see. What are you welding with?
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One of the cheapo red ebay spotwelders with a mod so it doesn’t blow itself up. I know it’s not as good as the more expensive ones, but I didn’t want to spend 200$ on a welder for my first pack and it seems to work.
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Ah that makes sense. Well just be careful then, I think you’ll be okay
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Actually, now that I’m looking at it closer I think I am still blowing small holes, but they do seem to be sticking very well. I think I’ll turn it down just a bit, but I don’t know if I should use that one.
Put some iso on it and check for bubbles. There’s a possibility you could have burned through the can.
Electrolyte smells sweet.
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Hi! I am building my first battery pack and I would like to share my progress here. I would appreciate any feedback, especially any major flaws I might have made.
Basic pack info:
- 12s5p
- Samsung 40T cells (was aiming for P42As, but were out of stock at purchase time)
- P packs are in 3D printed (PETG) cases (credit: @janpom)
- parallel connection: 0.15mm nickel
- series connections: 3x 12 AWG
- main discharge leads are 8 AWG
- charging leads are 16 AWG (except the positive lead which is smaller - I was able to get inline fuse case with this wire only)
- LLT Power smart BMS (charge only)
- Charge and discharge lines will both be fused (thinking about 10A and 150A). I am aware of the downsides of the fuse on discharge line, but I am really paranoid about the pack going kabooom while at rest so I decided to accept this.
- Balancing lines are fused with 500mA
After series connections:
After balancing leads:
I am planning to put temperature sensors on the bottom side.
Thanks!
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Looks good. Familiar as well. 
3x12 AWG is an overkill. 2x12 AWG would have been more than enough. It of course doesn’t hurt to overbuild though.
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Quick question, how many times is it safe to Dremel off shitty spot welds and do them again? I’ve done it 3x to a cell without any indication of failure but I’d like to know a safe threshold to where grinding it down will make it too thin.
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