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Comment by op00to | Hacker News Reader

Will batteries still explode like LiPo batteries?

No, they’ll explode unlike LiPo batteries. LiPo batteries merely catch fire.

An electromagnet, superconducting or otherwise, has large internal forces. If the support structure lets go, it will move. And, if part of the circuit becomes non-conducting, an inductive kickback will occur, generating enough voltage to (initially) sustain the original current.

You could cut a Li-ion battery in half, and the two halves will continue to store their chemical energy, at least until they burn up. If you cut an inductor in half (which is what this type of energy storage device is), that energy will dissipate very quickly whether you like it or not.

A battery that transform all energy it stored into heat sounds more like a bomb instead. Imagine someone accidentally heat up the 'super-conductive' battery to the point that it is no longer super-conductive.

That goes for any sufficiently dense energy storage system, including most batteries. Imagine someone accidentally shorted out a car battery with something that can withstand a few thousand amps. (Come to think of it, a room temperature superconductor would do nicely for that purpose...).

I did something similar once with a high power lowish voltage battery array - my soldering iron's shroud literally vaporized with a deafening bang before even triggering the fuse.

Ri of those is typically 1 Ohm or thereabouts, which at 500 A develops 500W in the battery itself and a large multiple of that in your 'load' (in this case your soldering iron :) ).

When working with large battery arrays I use tools that are taped in all the way except for the business end, just in case. All you need to do is drop a wrench in the wrong spot and it's party time.

Not 1Ohm, in my case the array had an internal resistance of 15 milliOhms, which is way more fun. Though I imagine the BMS and fuse were relatively effective resistors in that situation too.

Agreed about taped tools, I figured that one out right after replacing the fuse :)

Oh that's really danger territory.

A BMS typically has a small shunt that helps to figure out the state of charge as well as a large transistor in series with the current to allow switching the battery in and out of circuit.

Yes, I had an additional fuse in series, what I was referring to is that the BMS's own internal resistance is larger than that of the battery pack. Indeed in an overcurrent situation you can't rely on the MOSFET not to fail short.

I've seen fets in BMS in Bosch packs cracked in half, top blown off. Quite impressive.

Well, for starters, batteries have their own internal resistance. Not a whole lot, but enough that whether you short out with copper wire or superconductor, the effect would be the same.

> Well, for starters, batteries have their own internal resistance.

Indeed they do.

> Not a whole lot, but enough that whether you short out with copper wire or superconductor, the effect would be the same.

Not necessarily, assuming a charged battery in many cases with a copper wire the wire will simply heat up to the point of evaporation and then break the circuit as it sprays molten copper bits all over the place. Some heat will be generated in the battery as well. Watch people mess up with starter cables for some ideas on how this tends to go (and do so from a distance...).

Using a massive copper connector that you some how instantly put across the terminal and manage to keep there would indeed make the balance of the resistance shift to the guts of the battery, which would heat up faster than that that energy can be shed and hence in all likelihood (violently) explode. Besides bits of molten lead and zinc for a car battery you now also have the joy of having to deal with spraying acid. Which depending on the state of charge of the battery can be really nasty stuff.

With a superconductor there would be no chance of the conductor evaporating first, there isn't any work done in the superconductor so it will stay cold, an explosion of the battery would be all but guaranteed.

Keep in mind that a superconducting energy storage device is kind of the opposite of a battery. An idle battery at full charge has some voltage and zero current, and it’s perfectly happy to stay like that.

An idle superconducting energy storage at full “charge” is not carrying a charge at all — it’s carrying a current. If you cut the wire (or blow a fuse), V = L dI/dt will generate an arbitrarily high voltage to keep that current flowing.

I imagine one would need some spark gaps and/or capacitors to limit the voltage.

It's a coil. You can just place another inductor around it and increase / decrease that current at will. This is how Kamerlingh Onnes injected current into his superconducting media during the original experiments and that is a two-way street.

https://www.lorentz.leidenuniv.nl/history/cold/DelftKes_HKO_...

When an MRI machine has a quenching event, it's a very serious deal. All the energy from the electromagnet gets dumped as heat, which boils off the coolant very quickly. MRI machines have to have a quench pipe which allows the now-gaseous helium to escape to the outside just in case of such a thing happening.

The LHC had a quench event in 2008, which explosively vapourised about 6 tonnes of helium, resulting in considerable damage, and it took more than a year for the accelerator to come back online.

I haven't taken a science class since I was 16 but could the battery be grounded to prevent an explosion?

How do you intend to keep the grounded battery charged ?

Well there wouldn't be fires that produce toxic fumes and burn even under water, but if the coil fractured in an accident all that energy would be released in a tiny fraction of a second instead of a combustion event that takes time. That seems bad...

> all that energy would be released in a tiny fraction of a second

Probably along with a large electromagnetic pulse...

Necessarily with a large EMP, as the energy is stored in the magnetic field.

Fortunately the energy density[0] is comparable with rubber bands[1] and supercapacitors[1], so while the EMP is large, it's not terrifyingly large.

[0] at least according to wikipedia, 4-40 kJ/kg: https://en.wikipedia.org/wiki/Superconducting_magnetic_energ...

[1] 1.6-6.6 kJ/kg and 10-30 kJ/kg respectively: https://en.wikipedia.org/wiki/Energy_density

The energy density isn't the scary bit, the power density is. The referenced paper in that Wikipedia article indicates ~10^5 higher power density than li-ion. So while it stores 10x less energy than li-ion it dumps it so much faster. The difference between a combustion and a detonation if you will. I'm guessing the vicinity of the thing would get turned to plasma.

A very small and localised plasma near the normal conductors that suddenly experience a rapidly decaying magnetic field; but the total energy is still (relatively) low compared to, say, the thermal heat capacity of the air of about 1.3 kJ/K/m^3.

That energy density number likely includes the cooling system.

That is what I thought too, but reading the paper referenced it seemed like they only included the coils, not even the support structure to keep them from collapsing. But it wasn't that clear.

Edit: they were clear that the limiting factor on energy and power density was the forces exerted on the coils.

A colleague recently showed me the coil of a broken superconducting magnet after it quenched. There was an easily visible rip in the structure, even though the whole thing was enclosed. It must have at least had enough kinetic energy to tear the metal apart.

I've had a relatively small (about 2" x 2" x 1") ceramic magnet explode on me. The shrapnel was embedded deep into the adjacent wall.

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