From: pcdhSpamMeSenseless@electrooptical.net   
      
   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!   
      
   Cheers   
      
   Phil Hobbs   
      
   --   
   Dr Philip C D Hobbs  Principal Consultant  ElectroOptical Innovations LLC /   
   Hobbs ElectroOptics  Optics, Electro-optics, Photonics, Analog Electronics   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)