# My 18650 construction technology



## kennybobby (Aug 10, 2012)

That's an awesome robot welder you built, very clever!

Is there provision to inspect the welds, how is the weld checked? 

What about cell cooling and thermal control in the pack? What happens if a cell overheats or goes thermal--is there an escape path for the gases, etc.

What size pack are you building, it looks like 96p per sheet of copper, how many in series?


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## Siwastaja (Aug 1, 2012)

Quick answers.

I have manually inspected weld results by tearing them open. No other way at this point. So this is kinda cheap-ass. But with the correct settings, the weld penetration is great; a test weld of about 100 welds resulted in 100% yield, so I hope that 5 welds on the minus side, and 3 welds on the plus side (with higher power) provide ample margin. Minus side is tricky, the cell case is very thin; but at the plus side, plenty of power can be used as the terminal is thick metal and not that well thermally connected to the cell itself.

The pack is air cooled; as you can see, there are gaps between the cells. Air can be blown through. The cooling area is huge, but the fan needs to be capable of providing some pressure (blower type). On the other hand, with typical EV use in a long-range vehicle (<0.7C discharge followed by <0.2C charge), no cooling will be needed.

For single cell failures, I pretty much rely on the PTCs integrated in the cells. For "global" overcurrent events, pack fusing. But that's about it. First and foremost, I rely on the safety of the high manufacturing standards by the manufacturers; these cells are tried and tested and safest li-ion technology available.

That module is 46p7s, and with 12 modules, it's a 40 kWh 46p84s pack. The copper sheet connects two groups in series, too, that's why you are counting double the cells.

Thanks for questions, I'll try to answer them for time being, and hopefully give more information at some point of time.


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## ishiwgao (May 5, 2011)

looks great. I'm guessing that this can only be sold in a certain form factor of the rectangle blocks if you're gonna keep tooling the same right?

in any case, would like to know the final specifications when you finally release more information.


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## PStechPaul (May 1, 2012)

Looks like a pretty effective process to get an affordable lithium pack. I assume the $250/kWh is for the cells only, so one may need to add the cost of shipping, fabricating the pack, and the BMS. If I could get a 10kWh pack with at least 240 VDC (for an inverter), for $2500 I might be tempted to start a conversion almost immediately. And a 2kWh pack for $500 (for my tractor) would be easier than building the DC-DC converter to boost 4 lead-acid batteries to the 240 VDC I need. 

I will most likely proceed with that as my original plans (and I'm already partway there) but for future conversions this looks very promising. If the cells can be made available locally for low cost, it may be a viable small "cottage industry" for someone to custom-build packs like this. I wonder what a reasonable retail price would be?


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## Hollie Maea (Dec 9, 2009)

PStechPaul said:


> If the cells can be made available locally for low cost, it may be a viable small "cottage industry" for someone to custom-build packs like this. I wonder what a reasonable retail price would be?


Well, maybe...
I put out a trial balloon for some 18650 packs in the vendor section and was met mostly with skepticism if not derision. Lots of people reminding me that salvage Leaf packs are essentially free these days...

Maybe if Siwastjawa can get prices a lot lower than I was able to he could sell some. These cells really ARE amazing. I'm liking the Copper sheet; it's not easy to weld Copper (we use Nickel) but you do get a significant resistivity benefit.


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## SirNick (Jun 14, 2015)

I would love to see more with 18650s. LiFePo seems like yesterday's news now. Safe, abundant, and easy to assemble - but that's about it.


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## Jayls5 (Apr 1, 2012)

I like it. What mechanism is doing the welding? Capacitors?


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## Siwastaja (Aug 1, 2012)

Modified DC inverter TIG welder with HF start. Added a pulsing circuitry which measures and integrates weld current and stops welding once pulse energy setpoint is reached.

Weld electrode was turned from machineable ceramic; it's a small tube which presses the workpieces together tightly near the welding point and holds the tungsten electrode (minus) near, but not touching, to the copper sheet. Outside the tube, there is spring loaded copper terminal which "grounds" (or actually, '+') the workpiece. Argon flow is used inside the tube.



Re LiFePO4 safety, it's largely a myth. Sure, the chemistry itself is a lot safer than, say, LCO, NCA or NCM, and studies done on research (lab) cells show this, but there's a lot more to safety than just this. The fact that all LiFePO4 cells are produced by substandard factories (compared to world-class factories producing >90% li-ion cells) with little R&D on safety-related mechanisms quickly offset any safety advantage that comes from the chemistry itself. 

Such mechanisms include: extreme quality control and inspection (to avoid internal shorting), high-tech shutdown separators (to protect against internal and external shorting), overpressure shutdown devices, PTC devices (which is the primary safety mechanism in 18650 that works before the aforementioned take action).

And indeed, I have read a study where they compared _actual_ commercial LiFePO4 25650 and LCO 18650 cells by performing 12V overcharge and short circuit; all the LCO samples were fine whereas the LiFePO4 samples vented and some of them flamed, IIRC. In fact, I have connected a laptop 18650 cell (made by Sony) to a 30V lab supply for hours, and not much happened, the cell basically was a PTC thermostat controlled heater which stayed below the thermal runaway temperature (about 150 degC for LCO). But don't repeat this experiment; I'm not saying it's safe. It's not! Just talking about relative safety. Li-ion safety has come a loooong way in 20 years by introducing new safety mechanisms, but it's been a lot of R&D, and small manufacturers can't use them; they just use the stuff from the 90's, combined with the "safer" LiFePO4 chemistry, to get relatively safe (safe enough) products.

LiFePO4 had high promises when the numbers were 130 Wh/kg vs. 160 Wh/kg, but now the competition is at 270 Wh/kg and LiFePO4 hasn't increased at all.


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## AntronX (Feb 23, 2009)

You are TIG welding them and the cells survive? That's scary.


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## Jayls5 (Apr 1, 2012)

AntronX said:


> You are TIG welding them and the cells survive? That's scary.


What's scary about it? Welding batteries in general?

TIG literally just means that he's using an inert gas with a tungsten electrode. The inert gas makes it a cleaner weld, nothing more. He's got a mechanism to regulate exactly how much current (thus heat) given before instantly cutting off... so it sounds ideal to me.


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## AntronX (Feb 23, 2009)

TIG is slow and creates a hotspot inside the battery case during welding that could melt the edges of separator membrane or break down some of the electrolyte. There is a reason why Li cells are spot welded using capacitor power source. Spot welding is very fast and does not create enough heat to do any damage to the cell.


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## Siwastaja (Aug 1, 2012)

The welding time is about 50-100 milliseconds, so it's not slow. The cell doesn't heat up at all. Similar technology is sold at ridiculous prices when called "micro arc".

This is spot welding, but not resistive spot welding, which is a very primitive technology and has severe limitations (requires resistive material) and often poor weld quality compared to arc spot welding -- which is why it's not usually used in large 18650 pack construction.

It's OK to admit not knowing something instead of making false claims based on limited knowledge.

Melting the separator or boiling the electrolyte cannot simply happen, because this arc method would blow a hole through the cell case a lot earlier; I have seen that when finding appropriate weld energy setting. The energy is delivered in a very short pulse - much shorter than resistive spot welding - and is even more localized.

<-- Short time -------------- long time -->
<-- localized energy --- energy spreads -->
Arc spot -- resistive spot -- soldering


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## kennybobby (Aug 10, 2012)

Thanks for the explanations, this is a great learning experience.

Do you have a car in which to use this pack, or what is the intended target application? i suppose it could be built up as a replacement or alternative tesla pack.

What sort of fixtures and handling equipment are necessary to move the modules around?

How are modules connected to each other?


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## AntronX (Feb 23, 2009)

Siwastaja said:


> It's OK to admit not knowing something instead of making false claims based on limited knowledge.


You're right, I should not have been so quick to dismiss this method. Do you know if anyone else in the industry uses microtig for battery tab welding? Quick google search only found this article.


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## Hollie Maea (Dec 9, 2009)

I recently used these cells (Samsung INR18650-29E) as a control group in a high current test. Hooked them up to discharge at 25 Amps and let it go...It crapped out after 4.5 minutes, but I did discover that it's possible to go up to 120 degrees C without thermal runaway...not so good for the cell health though


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## AEM (Sep 12, 2014)

Have there been any updates lately on your Construction Technology since it simplifies 18650 pack construction so much. Personally I'd be interested in a welder like that.


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## runcyclexcski (Mar 21, 2016)

the weld is so fast, and the heat dissipation in the copper sheet would prevent any damage to the cell


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