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Comment by abalone | Hacker News Reader

You can't just "shut down" electric pathways. Not exactly. There is a manual cut that first responders need to make to sever the high voltage circuit before they can work on the car. However there's still capacitors, converters and the battery itself that remain dangerous especially if they've been compromised.

As for "fast draining" the battery, where would the power drain to? That's exactly the problem.. NOT giving the power a path to go when you're working on the car.

This is not much of an issue with most cars because the electrical systems are low voltage (12V car battery). With Tesla it's enough to cause major injury or death with one touch if you complete the circuit.

Watch the video, it explains all this. http://www.youtube.com/watch?v=ntK3rvVl2Qw

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And I guess it's unrealistic to think of a special draining device to be brought on crash sites to exhaust the car battery. All in all, are electric vehicles more 'instable' energy wise when crashed, compared to petroleum based one ? or is it different flavours of similarly sh*tty situations ?

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? ;)

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))

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.

Ah very true. Knew I was forgetting something.

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.

> 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.

> 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.

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

That's a very big question and probably only time will tell. Without getting into it too much, a very basic difference is..

Gas can explode like a bomb but (a) it needs an ignition source and (b) there can be visible signs when the fuel is leaking to let you know to take extra precautions (like spray fire retardant). You can actually get fuel on you and you'll be fine.

High voltage systems don't (usually) explode, but they have lethal amounts of energy stored that can find its way into your body if you complete a circuit. Furthermore in a mangled wreck there aren't necessarily visible signs when a component is electrified. You find out the hard way.

Basically Tesla deserves props for making an extremely strong car. That's what was tested here. But the spin is they're claiming "safest car" while completely side-stepping the high voltage safety issue.

> Gas can explode like a bomb but (a) it needs an ignition source and (b) there can be visible signs when the fuel is leaking to let you know to take extra precautions (like spray fire retardant).

And (c) if it's a diesel car (very common in europe), good luck getting it to explode.

Want to scare your friends? Put out a lit match or cigarette using Diesel fuel.

Diesel fuel requires a lot of heat to combust. Diesel engines don't contain spark plugs, they heat the fuel by compressing the air that the fuel is atomized in.

>Basically Tesla deserves props for making an extremely strong car. That's what was tested here. But the spin is they're claiming "safest car" while completely side-stepping the high voltage safety issue.

That's not a side-step. IIHS doesn't test for this, and Tesla is not beholden to do so.

Technically they only claimed "ACHIEVES BEST SAFETY RATING OF ANY CAR EVER TESTED" which is completely true.

The "unaware of any Model S or Roadster occupant fatalities in any car ever" is also a pretty strong argument for actually being the "safest car" too.

There isn't really a good way to quickly and safely dump all the energy from a battery the size of the Tesela's. Think about it: to drain in an hour, the battery would have to source 65kW continuously, converting all that energy into either heat or light somehow.

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