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I'm a bit stuck (Patreon)

Published:
2021-09-20 01:44:29
Imported:
2026-06
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I'm stopping on this go kart project for a bit to switch gears and work on something else. Turns out spit welding a high current battery pack together isn't quite as straightforward as I thought

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I'm stuck

Comments

Kadah

The HF spot welder might need pressure on the points squeezing something to function. Could put two or more rewound MOTs in parallel. Just need to find similar ones to use. Could make a pulsed spot welder instead if trying to use/make one with higher continuous power. Could either use large capacitors or a high current lipo battery pack (at least 50C). Or get an existing kit for such, like a kWeld.

Anonymous

100A continuous is pretty aggressive, and 300A peak seems really really high. At those currents, the nickel strip between each cell (the "parallel" strips you mention) really can't be disregarded, in other words the cells aren't truly in parallel at those current levels. Let's assume the nickel is 5 milliohms between each cell. If you connect your load to one cell through 5 milliohms, then 10milliohms to cell 2, 15 milliohms to cell 3, etc., and try to draw 300A, the cell nearest the load will attempt to deliver ~125A and the one furthest will attempt to deliver ~25A. That 125A peak current is nearly certain to make the cells unhappy, and would require cell rebalancing during charge. Also, if I did the math right, all that nickel strip will be generating almost 200W of heat, plus whatever the cells themselves will generate. Wondering if you mis-spoke and don't really mean 300A.

Anonymous

I'm more courious about the "spit" welder. I might have missed that video :) Anyhow... take your time. Great work.

Sebastian K.

As a native german speaker: „Achtung - wütende Elektronen“ is correct. If you ever need something translated or checked I‘d be happy to help.

Thomas Eriksen

just saw that bms in the off grid garage https://youtu.be/02qFIjCI820

Tuomas Haarala

I looked at one commercial battery spot welder some years ago. They had multiple contacts for the electrode that leans on the battery pole and single contact for the one that leans on the connecting strip. The device also looked like it used similar contacts as there are in the relays and switches; not so surprisingly, you can buy those separately. If you think about how spot weld is formed, your method isn't likely to behave repeatedly well, and possibly doesn't create good wide contact surface from battery to strip. One key here would be to get the current flowing from battery pole to the strip, and not so much along the strip.

József Krisztián Kovács
>>68171562

Pulsed spot welders are quite easy to make, and can weld quite thick stuff, probably the easiest solution is just a few mF mains cap that you charge and then just dump it straight to the leads

SA007

You say your spotwelder is 1500W, that's not true, that is the maximum power of the primary side of your transformer. The secondary side is made from something like 1,5m of 6mm2 wire (i don't know the actual wire) which has about 5milliohm resistance. Together with your 2.2V output voltage means the maximum current is about 450 or so amps, which is much less than a kW maximum power. Notice that 5 millohms isn't a lot, every connection you make is a lot more than that, during the actual welding you are lucky to even reach a 200 or so amps or 440W. The simple way to get that power up is more turns on the secondary side, that gives a higher voltage, find a wire with thinner insulation to get more turns in. This is effectively what connecting the mains side to 200V did, doubling the output voltage.

Anonymous

could you somehow integrate a dangerously large capacitor into your spot welder?

Kadah
>>68171562

The hard part is switching the current. Turning off is harder, though with capacitors, you can just change to the desired energy and generally not worry about having to having to switch off such a high impulse load. Mains caps wouldn't be a good idea of you are taking about running them at mains level voltages for the welding. That is very much a "kill you dead very quickly and very painfully" idea. I've gotten 4th degree burns from a photo flash cap bank only charged to around 100V. I have managed to weld a pair of pliers to the terminals of a half changed 350V large beer can sized cap, and I didn't want to try that with the full set of 6 after that happened. It'll work for spot welding, but it'd be about a safe as trying to shave with a hedge trimmer. 3-4V would be better, and mains filter caps would have very little stored energy at those voltage. Super caps rated to pulse applications would be better suited. Or just a 50C lipo.

Anonymous

Your experience with spot welding a pack is exactly why I stay FAR away from that method (there are others but that's reason #1). If I'm going with a DIY pack I will aim for a non-weld pack holder of some type as that would probably be cheaper than the welding tech tree for <5 projects (spot welders are expensive as I learned, even low-end ones). Knowing you I bet you could DIY a no-weld pack solution with springy contacts. You've got the acrylic sheeting and bolts to clamp and hold it together, you could modify it with CNC to add nickel/copper strips to the under/inside in a desired config and then once you clamp it together it does the contacting, but removable. Tis an option, though not sure how easy.

Alan Harris
>>68189200

That's what I was thinking, with thick and wide copper bus bars. Because strips won't handle the loads he's trying to control.

Anonymous

Need to upgrade yours DIY one. Swap the secondary wire to a thicker one. There is probably too much power loss in the cables. But I think your math for the strip conductivity is wrong. So your small strip, its .1mmx5mm, so .5mm2, that is about 20awg. While regular wire may not handle more than 2amp, since yours is very short you can push a lot more power. From looks of it, up to 15amp per strip. I would setup experiment and see how much actual power you can draw before the temp of the strip becomes hotter than the temp that the batteries can handle. than add safety factor that you feel comfortable with.

Chloe Surett

Had my German friend see the battery box and he found it funny. It does translate properly as well.

BrandEver

Hey I'm just an idiot but could there be some sort of safety switch that won't let that spot welder activate unless the prong thingys are clamped down?