From: bill.sloman@ieee.org   
      
   On 21/12/2025 10:20 pm, Liz Tuddenham wrote:   
   > Bill Sloman  wrote:   
   >   
   >> On 21/12/2025 3:08 am, Phil Hobbs wrote:   
   >>> Liz Tuddenham  wrote:   
   >>>> Bill Sloman  wrote:   
   >>>>   
   >>>>> On 20/12/2025 10:02 pm, Liz Tuddenham wrote:   
   >>>>>> Bill Sloman  wrote:   
   >>>>>>   
   >>>>>>> On 19/12/2025 6:49 am, Liz Tuddenham wrote:   
   >>>>>>   
   >>>>>> [...]   
   >>>>>>>> Warning the user isn't much good, the battery technology needs to be   
   >>>>>>>> fail-safe not impending-fail-evident to the user.   
   >>>>>>>   
   >>>>>>> Fail safe would involve a big resistor into which you could start   
   >>>>>>> discharging the battery if you detected worrying warming. You'd have to   
   >>>>>>> design the system to cope with that, and it would  make the designers   
   >>>>>>> job more difficult.   
   >>>>>>   
   >>>>>> Let's do some sums:   
   >>>>>   
   >>>>> First show where you got your numbers from.   
   >>>>>   
   >>>>> I've snipped out that bit of bizarre speculation.   
   >>>>   
   >>>> In more detail: the delamination of the seperator occurs at 25 metres   
   >>>> per second but the thermal runaway reaches a peak of 600 mm/sec and then   
   >>>> falls to 80 mm/sec according to Franson, Pfaff et al. "Exploring thermal   
   >>>> runaway propagation in Li-ion batteries through high-speed X-ray imaging   
   >>>> and thermal analysis".   
   >>>>   
   >>>> For their experiment, they initiated the failure by penetration with a   
   >>>> nail, but the same propagation could equally well be started by failure   
   >>>> of a very small area of a separator.  The nail penetration was near the   
   >>>> casing and this sometimes resulted in a hole melting in the casing and   
   >>>> relieving the excess internal pressure.  A separator failure  away from   
   >>>> the casing could well result in much higher pressures and greater   
   >>>> spreading of incandescent materials.   
   >>>>   
   >>>> They measured the propagation time between the initially-failed cell and   
   >>>> an adjacent cell to be about 4 minutes but various videos of lithium   
   >>>> battery fires show cells exploding at a faster rate than this, once the   
   >>>> fire has taken hold.   
   >>>>   
   >>>> If we take the 4-minute figure as a reasonable approximation, this is   
   >>>> the time in which a 70 kWh battery must be discharged to prevent a   
   >>>> failed cell from setting off the others.  That is more than 1 megawatt   
   >>>> to be dissipated in something the size of a car.   
   >>>>   
   >>>>>   
   >>>>> In reality, the problem is picking up the increased rate of   
   >>>>> self-discharge long before you get to the point where thermal runaway is   
   >>>>> likely - the battery has to get above 120C before this can get going.   
   >>>>   
   >>>> A typical cell holds around 80 Wh of energy but less than 1 watt could   
   >>>> easily heat a small area of separator to over 120C without the   
   >>>> temperature rise or the discharge current being detectable outside the   
   >>>> cell.  if you think you know a way of reliably detecting the failure of   
   >>>> less than a square millimetre of separator in a battery containing 500g   
   >>>> of materials, including about half a square metre of separator, the car   
   >>>> industry would be glad to hear from you.   
   >>>>   
   >>>> If you don't know of such a system, your assertions that lithium   
   >>>> batteries are safe as long as the designer has done his (or her) job   
   >>>> properly, and they can be discharged before a failure become   
   >>>> catastrophic, are based on nothing more than wishful thinking.   
   >>>>   
   >>>>>   
   >>>>>> The battery capacity of cars, on average, is about 70 kWh.  This means a   
   >>>>>> resistor capable of dissipating 70 kW continuously is needed to   
   >>>>>> discharge the battery in one hour.   
   >>>>>   
   >>>>> You'd dump the excess energy slowly into the motor, letting it rock the   
   >>>>> car rapidly back and forth by about a foot or so to generate a little   
   >>>>> extra air circulation. It would take a while to discharge the battery,   
   >>>>> but it would get it done.   
   >>>>>   
   >>>>> It would be a emergency solution - the driver would get told that the   
   >>>>> battery needed attention long before this would be justifiable, and in   
   >>>>> our brave new world the battery condition monitor would probably have   
   >>>>> it's own mobile phone to rat out the inattentive owner to the local fire   
   >>>>> service.   
   >>>>>   
   >>>>>> I'm sure cars with a red-hot bedstead of resistance wire on the roof   
   >>>>>> would soon catch on.   
   >>>>>   
   >>>>> Your enthusiasm for impractical solutions is noted.   
   >>>>   
   >>>> It is probably just as practical as having a car start rocking backwards   
   >>>> and forwards for hours on end to discharge the battery.   
   >>>>   
   >>>> An even better solution (in a Brave New World) would be to have it drive   
   >>>> itself to somewhere where it can't cause any harm, as quickly as   
   >>>> possible.  Perhaps every Local Authority should have a designated place,   
   >>>> downwind of the town, where cars with faulty batteries could be   
   >>>> programmed to drive themselves and burn out in relative safety.   
   >>>>   
   >>>>   
   >>>   
   >>> Gee, maybe some small person could figure out a propulsion system where the   
   >>> oxidizer and fuel wouldn’t be in such intimate contact.  Maybe it   
   could   
   >>> even use air!   
   >>   
   >> There is one, and it's in popular use. Internal-combustion-engined cars   
   >> catch on fire rather more frequently than their electric counterparts,   
   >> if the car insurance statistics are to be believed. The fossil carbon   
   >> industry propaganda machine doesn't highlight that particular statistic.   
   >   
   >   
   >   There are a lot more internal-combustion-engined cars and they are, on   
   > average, much older.   
      
   The insurance statistics talk about the chance of single car catching   
   fire. More cars may mean more reliable statistics, but doesn't change   
   the probability of a single car catching fire.   
      
   The insurance statistics do include the age of the car. If older cars   
   did catch fire more often they would have noticed it, and jacked up the   
   premiums. The car I drive was bought fifteen years ago, and the   
   insurance premiums haven't gone up   
      
   > Most of the fires in cars originate in the   
   > electrics and most of the electrics are the same in electric cars and   
   > internal-combustion-engined cars, so the means of propulsion isn't the   
   > reason.   
      
   The fires may originate in the electrics, but most electrical faults   
   don't start a fire. The problems with internal combustion cars mostly   
   come when the fire manages to ignite the fuel tank. The batteries in   
   electric cars do seem to be harder to ignite.   
      
   > If you want a fair comparison you should separate the causes: compare   
      
   [continued in next message]   
      
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