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Comment by jlgreco | Hacker News Reader

85KWh is a lot of energy. Back of the envelope calculations puts that at enough to boil around one metric ton of water from 25C.[1] If you wanted to rapidly drain a Tesla you would have to dump that energy somewhere.

That might actually be plausible for firefighters to do[2], but I doubt the battery itself would withstand being drained anywhere near fast enough for an emergency situation. It would probably find some interesting way to fail if you forced it to discharge that fast (bypassing all the safeguards).

[1] 3.06e8 J / (4.181(J/(g * k)) * 75K) = 976,000g ...I probably got that wrong, it's been a long time.

[2] I mean, their main job is getting rid of large concentrations of heat, right? ;)

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That's the amount of energy needed to bring nearly 1000 kg of water from 25°C to 100°C, but not enough to begin the phase transition from liquid to gas (i.e. start boiling).

The heat of vaporization of water (i.e. the amount of energy required to convert liquid water at 100°C to vapor at the same temperature) is 2260 J/g. So to completely boil your metric ton of water at 100°C would require an additional 1000 kg * (2260 J/g) = 2.3 GJ, which is 627 kWh!

At 85 kWh, the Tesla's battery contains enough energy to completely boil off 119 kg of water starting at 25°C [0], which is still pretty impressive!

[0] 85 kWh / ((4.181(J/(g * K)) * 75 K) + (2260 J/g))

Ah very true. Knew I was forgetting something.

While that's true, I don't think it's what the parent meant. Colloquially, when we say to boil water we don't mean until the vessel is dry, just until a roiling boil is reached (this is typically how we cook pasta).

If the 1000 kg is heated uniformly, this is not enough energy to bring it to a "roiling boil". There will be no bubbles.

Right. It's a good thing stoves/ranges heat from the bottom!

You're forgetting convection.

Also remember we're talking about an accident scenario. There's no way emergency responders would intentionally run a high voltage circuit like that where there's a risk of short circuit in a physically compromised vehicle.

They're going to be focused on containing the energy not draining it.

It is fun to do back of the envelope calcs though.. You could run 50 space heaters for an hour off that sucker. Insane.

> They're going to be focused on containing the energy not draining it.

In an emergency that involves extracting passengers from an electric car, the most likely outcome is that the battery will get shorted to the car's chassis, at which point the emergency links in the battery will open up, isolating the battery from the rest of the car and its passengers.

As the video explains, even when the battery has been isolated there are other components that remain electrically hazardous such as capacitors and the converter.

Also there is the possibility that the battery itself may be physically compromised in an accident, e.g. it could protrude into the cabin (it sits right under it). Not much you can do to isolate it then.

Caps hold energy, sure, but if you're cutting through one it will self-short, which will be momentarily unpleasant but should not be close enough to anyone to actually injure them. What would be more worrying would be to cut through a circuit without completing a current flow path for the capacitor as that would leave open the possibility of someone accidentally completing the circuit themselves. But even this is not a large risk if you use conductive tools (which will absorb the brief current surge) or wear anything insulating.

The concern with the inverter-converter is mostly due to the fact that it's hooked up the battery. If the battery is already open-circuited then the inverter-converter should just be a dumb block, modulo any residual energy left over in a rectification circuit or something.

> It is fun to do back of the envelope calcs though.. You could run 50 space heaters for an hour off that sucker. Insane.

Wolfram Alpha puts it at 1/5th the energy in a typical lightning bolt... Absolutely nuts.

And it won't get easier since "we" want battery capacity to increase.

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