[paul cotter (OP)]

My car holds~20kg of petrol(gasoline). This quantity of hydrocarbon, when combusted stoichiometrically with oxygen, yields the same energy as 200kg of tnt. Now taking the efficiency of the engine as 33% the quantity of fuel actually propelling the vehicle is ~6kg. To replace this with a battery one ends up with a stored energy equivalent of ~60kg of tnt. The destructive power of tnt lies in it's ability to release all it's energy virtually instantaneously and this scenario is unlikely to happen with a hydrocarbon fuel except in an extremely violent collision. A battery can in theory release a large portion of it's energy in a short time. Some of the lithium ion battery failures I have seen are extremely violent. Is this hazard being taken seriously enough, especially with the push for ever higher energy densities? 

[alancalverd]

Yes. Look at your insurance premium, and talk to your local fire brigade!

[Bored chemist]

My car holds~20kg of petrol(gasoline).

Have you tried filling the tank?

Seriously, yes, it's a recognised problem.
While a lithium battery can certainly dump it's energy in a hurry, it's not as fast as TNT (it's limited by the rate at which things can diffuse into the electrodes) so it's less dangerous than the calculation makes it look.

[alancalverd]

But, like TNT, it is impossible to extinguish as the heat generation doesn't require oxygen

[Halc]

My car holds~20kg of petrol(gasoline).

Small car indeed. I mean, I drive a 2 liter compact car and it holds over 30 kg of petrol.

The destructive power of tnt lies in it's ability to release all it's energy virtually instantaneously and this scenario is unlikely to happen with a hydrocarbon fuel except in an extremely violent collision.

Not even then. Petrol burns, and only explodes if it can be allowed to completely evaporate into an enclosed volume, much like a grain elevator explosion.

You see petrol 'bombs' in movies. Movies rarely depict realistic explosions of say armaments going off since it is all a sharp 'crack' and destruction, and not so much dramatic fireball. I digress. Point is, comparison to tnt is indeed inappropriate, but the stored energy of petrol can still kill even if not in the same way that tnt does.

A battery can in theory release a large portion of it's energy in a short time.

Still a burn I think. Super hot. Very dangerous. I suppose they can undergo 'rapid disassembly' as some put it.


Yes, it is a problem. A person in an EV is less easily rescued from a crash. There is serious electrocution hazard in a damaged EV, especially if either water or cutting (jaws of life) are involved. This hazard extends to both victim and rescuer.

And we all know that lithium batteries (phones, PCs, and yes, cars) are susceptible to spontaneous combustion much in the same way that a tank of petrol is not. And the burning cannot be extinguished by any obvious means.

There are a few advantages at least. An EV *probably* isn't going to kill the driver by oxygen deprivation or other inhalation hazards. It weighs more, so it acts as extra momentum protection in a collision with a less massive thing.


Concerning the 33% efficiency: How does an EV or a hybrid compare in terms of carbon emitted per km traveled?  This presumes that the car is charged with an efficient carbon-consuming electrical source. I did a quick lookup and it says that an oil plant is 40% efficient, which leaves little room for the EV to leave less of a footprint than my car, especially given the extra mass it has to lug around.

[Petrochemicals]

Some of the lithium ion battery failures I have seen are extremely violent. Is this hazard being taken seriously enough, especially with the push for ever higher energy densities? 

The spectacular failures of lithium batteries are those featuring a particular type of chemistry, recently Lithium Iron phosphate batteries are being used for various reasons but probably for safety and stability. Less energy dense but not atmosphericly combusting.

The old lithium batteries where like rolling bombs, or the ford pinto.

[paul cotter (OP)]

Thanks to all for your replies and I have some comments/queries. Maybe i'm stoopid or maybe i'm slightly on the autistic side but I just can't figure BC's remark "have you tried filling it". Maybe BC could elaborate.                                   Halc, the car is a Honda civic 1litre tubocharged and the fuel weight is a mental calculation. Petrol(gasoline) would need to be fully evaporated and intimately mixed with air/oxygen for a complete rapid energy release. I dispute your inappropriate description of the comparison though I could have worded it better. My real point is that with ever increasing energy density batteries get closer to the definition of a bomb- tnt was only used to give a mental picture of the energy involved. The efficiency of an ice is around 30-40%. A serial hybrid can beat a traditional vehicle by a significant efficiency margin as the engine driving the alternator is always run at it's most efficient rpm/torque. Late addition: ice efficiency and average propulsion efficiency can differ significantly.

[Bored chemist]

Thanks to all for your replies and I have some comments/queries. Maybe i'm stoopid or maybe i'm slightly on the autistic side but I just can't figure BC's remark "have you tried filling it". Maybe BC could elaborate. 

I think 20Kg is on the small side for a car fuel tank.

[Bored chemist]

Fundamentally, stored energy is always a possible hazard.
A 750 Kg car raised 3 metres above the ground has about the same stored energy as half a gram of petrol; but you wouldn't want to stand under it.

[paul cotter (OP)]

It was a quick mental calculation, I will try to find the quoted capacity.

[alancalverd]

Usual car tank is 50 - 60 liters, say 30 kg on average (only a pessimist keeps it topped up at every garage!)

[paul cotter (OP)]

Filled the tank yesterday: 50 euros  to fill with a dribble in the tank, lets say 55 euros. 55 euros at 1.8 per litre x 0.7 =21.4 kg. Alan's remark about the inability to extinguish tnt reminded me of a distant now long dead relative who witnessed a house fire being started with dynamite! The technique used was to tear flimsy strips of the dynamite and light them under the coal and repeat this rapidly until the coal ignited. Technically feasible but obviously not recommended. One could expect the mother of all headaches doing this without gloves as nitroglycerine will dilate blood vessels sufficiently to induce a migraine like condition. 

[paul cotter (OP)]

I just looked up the fuel tank specs on my car and now i'm puzzled, especially after filling the tank yesterday. The quoted capacity is 46litres, which would be ~30kg.

[alancalverd]

Often a big difference between tank capacity and "usable fuel" - and a vital distinction with airplanes! The fuel feed is never taken from the bottom of the tank, which will contain all sorts of dead flies, rust, water, and other stuff that doesn't burn nicely, so when the gauge says "empty" there will still be a gallon or so in the sump. 

[paul cotter (OP)]

Yes indeed, Alan, one would not want a gummed up fuel filter 1000ft over a major conurbation!