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Comment by derefr | Hacker News Reader

Resistance is what makes things hot, and heat is what makes dumping huge amounts of charge current into batteries a bad idea. No resistance → no heat → no need to charge with low current†.

Another way to say it is that, with a superconducting wire, you can make the wire as thin as you want and still pass the same amount of current through it, without melting the wire. Picture using a USB-C cable to charge your car.

† (There'd still be a current limit due to the heat generated by the chemical reaction that rebuilds the battery's voltage potential... if said reaction is exothermic. Some battery chemistries are endothermic when charging!)

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I’m not an expert on this, but I think superconducting wires have an current limit, as a current flowing creates a magnetic field which the superconductor has to repel. I read that the paper states a very low current limit for LK-99, meaning it loses superconductivity once a very modest amount of current is passed through it.

That is.....kind of a huge limitation of the technology lol. Still very cool but less hype

I believe the implication is that LK-99 is basically a demonstration of an entire class of materials which should have room-temp superconduction properties. IE we can enumerate through the entire class and find the ones with the properties we want.

There is no reason to assume that even if it's real.

In fact, the tight tolerances of this seem to indicate the opposite.

At least according to this wikipedia chart on superconductor discovery timelines[1], it seems like most discoveries aren't one-off.

I have no knowledge in this area though.

[1] https://en.wikipedia.org/wiki/History_of_superconductivity#/...

A limitation...at ambient temperature and pressure.

Usually this is an optimization frontier, where something that has tetchy critical current/field at high temperature is going to have very good critical current/field at the same temperature as a lower-Tc superconductor.

If it superconducts at all at room temp, cooling it down even to 200K (about dry ice temp - quite cheap to do) could get you something very usable.

It's hard to tell what the critical current density of LK-99 is, because their sample is porous and probably very impure. They measured the critical current they could pass through a sample, but the conducting cross-section is somewhat unknown. Its high critical temperature suggests that it should probably have a higher current capacity than other superconductors. That said, in the extremes, current density is also limited by tensile strength, because electromagnetic coils repel themselves.

Couldn’t you bind it in resin?

Is there any indication of what the breakdown voltage of LK-99 might be?

> you can make the wire as thin as you want

No, superconductors have a specific current above which they stop superconducting so you will want to stay away from that limit. This particular superconductor has been presented with a very low Ic (150 mA in the original paper0 which would not make it particularly useful in such applications but future iterations (assuming it is all true) may improve on that (they should otherwise we have the equivalent of a superconducting straw).

I assume this would rule out things like fusion reactors, MRIs, and other high energy stuff. Would it still be revolutionary tech with a 150 mA limit?

Yes, it would be upending just about everything because the race would be on to improve on that. Think of it this way: once you show that something is possible at all there will be substantial funding available to improve on it. As long as you can't show that it is possible at all you're on your own. So if it works and that 150 mA is the limit then you can expect a ton of effort to be expended to improve on that and I fully expect those improvements not to take decades to show up. The more interesting question is if it really is that low of a limit what the reason is for that and I don't recall seeing any explanation so far.

On another note: a superconductor that can only do 150 mA / cm^2 seems intuitively strange, as though that figure is somehow off, it's a gigantic cross section for such a small current. It is very well possible that this is somehow an error in the reporting or an actual measurement on a thin sample with small cross section. So there are many explanations possible and only one of those is a true limit of the material.

The current hypothesis is that most likely whatever they made is not a pure sample of the material which actually superconducts - this is expected, since when you make YBCO superconductors you also tend to get low yields (i.e. ~20%) that actually superconduct.

So it could be the whole sample, or it could one micron-sized link of grains of whatever the "real" material is running through the sample.

> The current hypothesis is that most likely whatever they made is not a pure sample of the material which actually superconducts

That has been the hypothesis from day #1.

Superconductors also have an inherent current limit above which they go back to having a resistance.

There are many things that seem like electrical resistance but are different phenomena. Capacitive reactance, inductance, "radiation resistance", etc. Superconductors don't prevent any of these effects. But, these effects are usually smaller than ordinary resistance.

Wouldn't the battery itself still have resistance? Or is the superconducting material itself a battery?

Depends. A single battery cell would have nontrivial resistance, yes.

But a big bank of batteries, like are in an EV? Very hard to give them enough current to heat them up. Most of the "heat problem" is from the bottlenecked current path into the car; once you fan out across all the individual cells, each individual cell isn't receiving much current.

And a bank of supercapacitors? You could charge it effectively instantly.

The current is limited by what the battery chemistry can take, not by the cables. This is why the first 80% can be charged quite fast in modern EVs, and the last 20% are really slow.

Additionally you need to have the current to deliver in the first place. Having a grid that can dump 25-100 kwh into any given car in a couple of minutes is no small task if everyone is doing it.

The utilization factor would obviously be much lower than it would be if everybody charges at a lower rate so if the total amount of energy is equal that just means that individual vehicles will spend less time charging, and the grid will see - roughly - identical utilization on average but the peaks may be higher.

Probably worth pointing out that the peaks and troughs are what are challenging to deal with. Generators aren't generally great at changing output super fast.

I keep hearing battery tech is getting good, and the research I've seen suggests that more storage on the grid would improve efficiency by a lot, so I don't know if it would even pose a particular challenge if that sort of demand arose.. but overall utilization isn't really the limiting reagent.

Heat from power transfer is not the problem with current battery tech. We are already capable of delivering 350kW worth of power into EV batteries. The limiting factor is not the power cable delivering that power.

Thick cable, high voltage (900V typically) and everything is fairly manageable. Assuming we could consistently charge at that 350kW we could fully (0->100%) charge an 80kWh ev battery in 13 minutes. That's not slow.

The limiting factor is the battery chemistry, not the wire chemistry.

Sure, it would make the wiring smaller and more efficient. But I also don't see how it would help in the chemical energy transfer to charge the battery.

What I was trying to say is, with some battery chemistries, the current (heh) limiting step for charging speed is the wiring into, and of, the battery, rather than the safe reaction speed of the battery chemistry itself. We could safely "crank the chemistry" by an order-of-magnitude or more if we could get the desired current into the battery without the wires+electrodes conducting undue amounts of heat into the electrolyte.

No, it’s not. It’s the chemical reaction the limit. In li-ion for example you will create dendrites when charging/discharging too fast or too deep. This is the cause of the relatively short cycle life.

According to the paper, this material stops superconducting at about 150mA per cm^2 of diameter, meaning that a 1cm-thick cable made of this material could conduct up to 150mA before the current is too much and it stops superconducting.

If my math is correct, then for a basic 500mA USB device, that would mean a cable a bit over 3 cm^2 in cross-sectional area, or about 2 cm across (for each of the power and ground leads, at least).

Alternately, a cable of just over 1/2cm in diameter (for power and ground, each) could charge a rechargable Ni-MH AA battery in about 12 hours and 40 minutes.

Tl;DR this is absolutely revolutionary science, if true, but we're definitely Not There Yet.

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