From: liz@poppyrecords.invalid.invalid   
      
   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:   
      
   Assume the fault propagates at 10mm per second and begins at one end of   
   a typical cell.  The cell is 70mm long so the entire cell will be   
   involved in 7 seconds.  Videos of vehicle battery fires show one cell   
   exploding every 10 seconds, which is roughly in agreement with this   
   figure.  The maximum discharge rate for a lithium battery is around 1C,   
   so it would take an hour to discharge the battery.   
      
   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.   
      
   I'm sure cars with a red-hot bedstead of resistance wire on the roof   
   would soon catch on.   
      
      
   --   
   ~ Liz Tuddenham ~   
   (Remove the ".invalid"s and add ".co.uk" to reply)   
   www.poppyrecords.co.uk   
      
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