Electronics > Projects, Designs, and Technical Stuff

Panasonic NKY467B2 36V 15AH 540Wh. Ebike Li ion upgrade, burning my father'ass?

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Spikee:

--- Quote from: nctnico on October 12, 2018, 03:57:49 pm ---It is not clear from the video but I don't see a slot in the copper plate to force the weld current through the top of the battery. Nickel strips always have these. When welding batteries you are doing a series spot weld which makes two weld in one go. Without the slot the current can go directly from one electrode to the other electrode of the welding machine making the welds vary in quality. I'm also not sure whether welding copper to nickel is a very good idea due to the metals being different.

--- End quote ---

This is correct. Recently I worked on a 10S li-on battery pack in an e-mobility situation and these are some of the standard design practices. I worked with the two big German firms in this field.
Regarding bms the TI BQ763x is widely used and easy to implement. A main fuse is always used for a worst case scenario.

Zucca:
Thanks Siwastaja for correcting me. I smoked something more dangerous than a lithium cell before doing the math.

BTW what do you think about lifepo4 battery for energy storage? I was doing some goggle on that topic and that chemistry type was popping out quite often...

Zucca:
So I proceeded to take the unknow BMS out, first I took out the 30A fuse screws..



then I removed the two T-couple from the potting







and voila done after disconnecting the cells voltages monitor connector...



I tried to resurrect the BMW with a power cycle...



no no no, that guy did not respond at all. Probably design meh...  :horse:

This is what I am talking about sizes old vs new... you can just imagine the weight difference.



Oh wow I can chose between the glass stock fuse 30A F30A, or that automotive (?) flat 30A 58V. I will go probably with the glass one... everything is already there....



Now the hard part, it's a mechanical problem. The battery is almost 8mm too high.... I will think about a nice solution.
 


This is the battery bike attachment..

Siwastaja:

--- Quote from: nctnico on October 12, 2018, 03:57:49 pm ---It is not clear from the video but I don't see a slot in the copper plate to force the weld current through the top of the battery. Nickel strips always have these. When welding batteries you are doing a series spot weld which makes two weld in one go. Without the slot the current can go directly from one electrode to the other electrode of the welding machine making the welds vary in quality. I'm also not sure whether welding copper to nickel is a very good idea due to the metals being different.

--- End quote ---

Again, you are making the wrong initial assumptions :). You clearly see a system which, according to your knowledge about spot welding, is impossible, but instead of questioning your world view, you question the system, even though you clearly can see it doing the impossible.

What you know as the only "spot welder" is actually just a resistive spot welder, which is based on heating up the material-to-be-welded by running current through it by using two contacts.

Now back to your initial assumption:

This is not a resistive spot welder at all!

If your assumption was right, you would be spot on - it wouldn't work like that! Resistive spot welding would be nearly impossible to achieve with copper strip, anyway - nickel is used exactly because it has more electrical resistance, and lower thermal conduction, allowing enough local resistive heating compared to the exiting heat flow, making the weld possible. Still, slots are very important to shape the current.

I wanted to question all that complexity (needing a more expensive, inferior material (nickel), and requiring it to be die-cut to exact shapes). So, looking at the market, I saw you can weld copper directly to the cells, with some very specific high-end tools. This is called "micro TIG" or "micro arc" welder in the industry. Sunstone makes some battery CNC solutions capable of this:
https://sunstonewelders.com/product/250i2-ev-cnc-battery-welding-system/

Yes, actual battery manufacturers are using these. If you are only seeing nickel being "traditionally" spot welded to the battery, you are not looking around properly. Modern ways to do the same without nickel and without cutouts is normal business practice as of now,  but it's still considered novel high-tech. Manufacturers show off their direct-copper welding robots at every possible e-mobility expo. My system is just a crappy DIY attempt for the same, but it works nevertheless.

The cheapest quotes I was able to get started from $30k (for non-CNC tools), which is why I made my own.

It's basically a well tuned el-cheapo TIG with a custom pulsing add-on. The electrode has no physical contact to the workpiece; it's about a millimeter above. HF strike starts the arc, and it's the arc making a local melted pool of the copper, welding it to the cell directly. This was nontrivial to get right, the electrode geometry is important; if it goes wrong, it just burns through the copper, leaving a hole and not touching the cell much. I have a custom ceramic piece I made on a lathe from machineable ceramic. It needs to hold the tungsten electrode correctly centered and offset, and implement small channels for the Argon gas. Spring-loaded copper ring around the head is for "grounding" (actually +) the workpiece.

This makes one spot weld (not two) at once, the size is about 1.5mm. They are really strong, I quality control it by making dummy welds to scrap cells and tearing them apart. While strong as is, to prevent rotational forces from tearing that weld spot (and to add redundancy), my software does multiple spots. Higher welding energy is used on the plus side of the cell, since it has thicker tab material. Nowadays I do three small-energy dots per minus side, two higher-energy dots per plus side. In the beginning, I did more spots for added redundancy.

I haven't actually measured the welding time, except by looking at video still frames, it's below 30ms. If you touch the resulting weld of three dots a second after welding, you'll feel that the cell and copper stay cold.

Ultrasonic welding is an another approach for the same end result - spot welding copper directly to the cell.

Internally, the cell has aluminium and copper electrode sheets. These are ultrasonic welded to the cell case, internally. I'd expect the same metal choices work outside the cells, as well. And I don't see anyone reporting metallurgic issues with direct copper usage using the commercial micro-arc or ultrasonic technologies.

What I do think about is that I'd like like to still have some cutouts, not for making the weld possible, but for strain relief due to possibly dissimilar thermal expansion within the system when finally installed in varying conditions.

Siwastaja:

--- Quote from: zucca on October 12, 2018, 05:15:02 pm ---BTW what do you think about lifepo4 battery for energy storage? I was doing some goggle on that topic and that chemistry type was popping out quite often...

--- End quote ---

Since you asked - IMHO, LFP serves quite a niche purpose. It's best for replacing 12V lead acid system, as it's the only li-ion chemistry that happens to have compatible voltage range so that a 4s pack can almost directly replace a 12V lead pack. For other li-ion chemistries, 3½ cells would be required :). The voltage curve is more flat, too. It's really close to lead acid. It's case-by-case whether it needs some tweak in the product voltage setpoints, or some management, but chances are it is completely a drop-in, even without a BMS, and people do that anyway.

LFP was a really biggie 15 years ago, academically, and for small battery startups, which picked it up for manufacturability reasons AFAIK. Possibly some patent reasons as well, I'm not sure.

LFP was touted as the next big thing. Back then, the only commercial li-ion chemistry was LCO (originally commercialized by Sony), with energy density around 160Wh/kg back then. LFP was supposedly 130Wh/kg, a fair compromise, with supposedly radically lower manufacturing costs (due to abundance of iron and phoshpor, compared to the price of cobalt!); and supposedly radically better safety.

Safety-wise, the thermal runaway onset temperature of LFP is somewhere well over 300 degC (IIRC) compared to the frightening ~150 degC of LCO, and even then, the thermal energy release in the runaway is more benign. But, this ends up as a fallacy; the batteries are complete products, and the safety is the sum of all chemical and physical design within the cell. Even with the imminent danger of the LCO cathode material, the bare cathode chemistry is just one thing. LFP cathode is still not completely safe, and can run away thermally, producing nasty amount of energy release. The electrolyte is still the same, flammable liquid, which shoots burning out of the cell due to the internal pressure, because no one has come up with a better non-flammable electrolyte.

And then, it comes to R&D and engineering:

Because the LCO, and upcoming LMO, NCA, NMC, were more marketable with their higher energy density, they received the actual R&D budget, going through safety improvements, such as advanced shutdown separators (meaning the plastic or ceramic layer in the cell melts "shut" and works as an insulator, stopping the ion transfer, in overheating parts of the cell), or physical cell design things, such as embedded fuses, current-interrupting rupture valves...

As a result, what do we have now, available on the real market, for putting into real products?

Very few LFP products. I have seen numerous safety tests where A123 and K2 cells failed more dramatically than our contemporary high-energy-density cells. Why? They are made by small players, with limited resources in safety engineering. They believe they have chosen a "safe" chemistry, giving the classical "false sense of security". Or, they are some Chinese players (like the absolute classic Winston Chung) not too interested about the actual safety. Happens in China, not saying there isn't good engineering there as well. And maybe they are right, maybe we are too fixated on safety here?

And, in the end, what do we have? We have:

* LFP cells are still at around 130Wh/kg (actually many Chinese LFP plastic boxes at below 100Wh/kg), which cost around $300-$400/kWh,

while rest of the world has gone forward, and so,

* the modern NCA/NMC cells are at around 250Wh/kg, and cost around $200-$300/kWh!

Especially for mobile anything, this energy density difference is baffling. It makes a real difference whether an EV can drive for 150km or 300km on a single charge! Or, if you can play your "sponsored by NSA" Candy Crunch whatever app for baffling 2 hours straight instead of just one!

What's left after this, are fairly empty promises that an LFP cell lasts for 2000-3000 full cycles while an NCA cell lasts for only 500 full cycles. The point is moot if the NCA cell can be derated to, say, 70% capacity for the same price (yet much lighter weight), increasing the cycle rating manyfolds, or who cares about a promise of 2000-3000 cycle promise if the manufacturer is either just on the brink of bankrupt, or a Shenzen special? Who knows all the failure modes and aging modes for those cells without extensive testing? I did quite a lot of such testing and found out that:
1) The reason the Samsung NCA cell is "only" specified for 500 cycles is that they actually test them, guarantee them, and add a generous safety margin. They actually tend to last approx. 1000 cycles on their own conditions,
2) Number 1 way to increase cycle life in NCA cells is to reduce charging current near full state-of-charge. That's where the cycling damage occurs. Want to fast charge at 1C? Do it, but taper it off after 4.0V.

We saw the same discussion, only on stereoids, with lithium titanate cells, which is even inferior per energy density and price, but supposedly even better per safety and storage and cycle life. Charge from zero to full in just 10 minutes! OK, true, but... what do you do with this rating, when you can get, with the same money, and with the same weight, and NCA pack which charges equal amount of energy in the same 10 minutes, but then still has 5 times more capacity left you can still go on charging! Or, who cares with claims of 10000 cycle lifetime, if you need to do 5 times more cycles because of the miniscule capacity of the pack, and after just 5000 cycles, it's already swelling and leaking electrolyte despite manufacturer promises (disclaimer: this last part is industry hearsay, but it's better than internet forum hearsay.)

So yeah, I don't see use for LFP anywhere except 12V replacement, but YMMV, and maybe I'm not 100% correct in all this. I'll change my opinion for energy storage as soon someone starts making LFP cells for considerably lower price (per kWh) than NCA right now. That would require over 50% price drop, however; I'm not holding my breath. OTOH, the market right now is price-fixed by the Chinese manufacturers. Price fixing is not dictated by laws of physics, so it can suddenly stop, given right conditions.

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