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Years ago, when Tesla was a sports car, my uncle had a friend who was called by fire departments to help when a Tesla crashed. They would not touch the vehicle

Years ago, when Tesla was a sports car, my uncle had a friend who was called by fire departments to help when a Tesla crashed. They would not touch the vehicle or anyone inside until he came and did his work. He got several 55 gallon drums and filled them with water. Then he gets a heating element and hooks it to the battery. He drops the element in the water. The water starts to boil within seconds. Once it is at boiling point he takes the hearing element and puts it in another 55 gallon drum. He repeats this until the battery is drained. It sounds like fire fighters are still scared today. Perhaps it is because there is not only a risk of fire, but also electrocution.

martin_a 3y ago
> It sounds like fire fighters are still scared today. Perhaps it is because there is not only a risk of fire, but also electrocution. Yes, as mentioned there's not enough training happening. Every firefighter should be trained for handling fires in electrical installations. If you can't shut it off, there are safety distances you'll have to maintain, these are even defined in industrial standards. For "low voltage" (up to 1.500 V DC) the safety distance is 1 meter for a wide beam of water and 5 meters for a narrow beam of water. This can be well kept (don't lay down in the water, though) with appropriate tactics.
mkmk 3y ago
Could someone who took EE please remind me... how many drums of water could a fully-charged Tesla boil?
- gary_0 3y ago
  That's not EE, that's basic math/physics. The largest Telsa Model S battery is 100kWh, which is 360MJ. Boiling 1L of water from room temperature takes 330kJ. 55 US gallons is about 200L water. 200 * 330kJ = 66MJ to boil the whole drum. 360MJ per battery / 66MJ per drum = 5.46 drums of water.
  - mkmk 3y ago
    Nice, that's a pretty manageable amount of water to imagine a firefighter having access to. Are there better ways of discharging EV batteries?
  - tzs 3y ago
    That's how many drums it could take from room temperature (20℃) to 100℃, but it would not be boiling yet. You need to put in more energy at 100℃ to make it boil. To go from 1 L @ 100℃ to steam at 100 ℃ would take in additional 2.26 MJ. A 200 L drum of room temperature water then could be used to tank 200 x (2.26 + 0.33) MJ = 518 MJ. They would only need one such drum, and one fully charged Tesla battery would be able to boil away 70% of the water. That actually works out nicely. Assuming a standard sized 55 gallon drum when the battery runs out juice there would still be about 25 cm of water in the drum. You want enough water to remain so that the heating element stay immersed because it is often bad for them to run in air.
    - prometheus76 3y ago
      But which method would take longer? Heating five 55-gallon drums to 100C each, or boiling one barrel of water until the battery is dead? Which is more efficient, time-wise?
      - tzs 3y ago
        My first guess is that heating multiple drums to 100℃ would be faster than boiling one drum. This is based on the rate of heat exchange by contact when two things of different temperature touch is proportional to the temperature difference. When the water is at 100℃ the difference between it and the heating element should be less than when the water is colder and so it should be transferring heat slower. By switching drums you are spending more time with the heating element in colder water so can transfer more energy. However there may be other factors. First, dH/dt = k ΔT, but k depends on the particular two things that are exchanging heat. I don't know if the k for the heating element and liquid water is the same as the k for the heating element and water that is undergoing a phase transition to steam. Someone who remembers their college thermodynamics a lot better than I do will be needed here. Second, the battery is going to have some limit on how fast it can transfer energy. That may be low enough that whether you use one drum or multiple may not matter.
HeyLaughingBoy 3y ago
And no one considered how dangerous 55 gallons of boiling water is?
tzs 3y ago
I wonder if that could be built into new fire trucks. For a fire truck with 1500 gallons of room temperature water (20℃ or 68℉) the energy from a full charged Tesla would raise that to 35℃ or 95℉ which is not so hot that it would be dangerous when used to put out the fire. For a smaller fire trucks with only 1000 gallons that would raise to around 42℃ or 108℉ which might be too hot to use. For fire trucks with only 500 gallons this approach it would definitely be too hot to use. The water would end up at around 65℃ or 150℉. That scalds people in under 2 seconds.

martin_a 3y ago

> It sounds like fire fighters are still scared today. Perhaps it is because there is not only a risk of fire, but also electrocution. Yes, as mentioned there's not enough training happening. Every firefighter should be trained for handling fires in electrical installations. If you can't shut it off, there are safety distances you'll have to maintain, these are even defined in industrial standards. For "low voltage" (up to 1.500 V DC) the safety distance is 1 meter for a wide beam of water and 5 meters for a narrow beam of water. This can be well kept (don't lay down in the water, though) with appropriate tactics.

mkmk 3y ago

Could someone who took EE please remind me... how many drums of water could a fully-charged Tesla boil?

gary_0 3y ago
That's not EE, that's basic math/physics. The largest Telsa Model S battery is 100kWh, which is 360MJ. Boiling 1L of water from room temperature takes 330kJ. 55 US gallons is about 200L water. 200 * 330kJ = 66MJ to boil the whole drum. 360MJ per battery / 66MJ per drum = 5.46 drums of water.
- mkmk 3y ago
  Nice, that's a pretty manageable amount of water to imagine a firefighter having access to. Are there better ways of discharging EV batteries?
- tzs 3y ago
  That's how many drums it could take from room temperature (20℃) to 100℃, but it would not be boiling yet. You need to put in more energy at 100℃ to make it boil. To go from 1 L @ 100℃ to steam at 100 ℃ would take in additional 2.26 MJ. A 200 L drum of room temperature water then could be used to tank 200 x (2.26 + 0.33) MJ = 518 MJ. They would only need one such drum, and one fully charged Tesla battery would be able to boil away 70% of the water. That actually works out nicely. Assuming a standard sized 55 gallon drum when the battery runs out juice there would still be about 25 cm of water in the drum. You want enough water to remain so that the heating element stay immersed because it is often bad for them to run in air.
  - prometheus76 3y ago
    But which method would take longer? Heating five 55-gallon drums to 100C each, or boiling one barrel of water until the battery is dead? Which is more efficient, time-wise?
    - tzs 3y ago
      My first guess is that heating multiple drums to 100℃ would be faster than boiling one drum. This is based on the rate of heat exchange by contact when two things of different temperature touch is proportional to the temperature difference. When the water is at 100℃ the difference between it and the heating element should be less than when the water is colder and so it should be transferring heat slower. By switching drums you are spending more time with the heating element in colder water so can transfer more energy. However there may be other factors. First, dH/dt = k ΔT, but k depends on the particular two things that are exchanging heat. I don't know if the k for the heating element and liquid water is the same as the k for the heating element and water that is undergoing a phase transition to steam. Someone who remembers their college thermodynamics a lot better than I do will be needed here. Second, the battery is going to have some limit on how fast it can transfer energy. That may be low enough that whether you use one drum or multiple may not matter.

HeyLaughingBoy 3y ago

And no one considered how dangerous 55 gallons of boiling water is?

tzs 3y ago

I wonder if that could be built into new fire trucks. For a fire truck with 1500 gallons of room temperature water (20℃ or 68℉) the energy from a full charged Tesla would raise that to 35℃ or 95℉ which is not so hot that it would be dangerous when used to put out the fire. For a smaller fire trucks with only 1000 gallons that would raise to around 42℃ or 108℉ which might be too hot to use. For fire trucks with only 500 gallons this approach it would definitely be too hot to use. The water would end up at around 65℃ or 150℉. That scalds people in under 2 seconds.