From: bill.sloman@ieee.org
On 21/12/2025 3:12 am, john larkin wrote:
> On Sat, 20 Dec 2025 14:39:28 +0000, liz@poppyrecords.invalid.invalid
> (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.
>
> I wish you wouldn't use mathematics here. It confuses some people.
>
>>
>> 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.
>
> They would have to drive like Indy drivers, out of garages and parking
> lots and parallel parking spots on streets, to get to the designated
> burn zone in a minute or so. Ignore the baby buggies. Well, if the
> sensors provided a minute of warning.
>
> Better idea: sense a cell runaway (somehow) and disassemble the
> battery pack. Use explosives.
>
> Or put water-cooled steel separators between cells to limit the flame
> spread. Replace flammible tires with tank treads.
>
> I took a taxi yesterday. They are almost all hybrids here. A few are
> electric, but have to be recharged mid-day to manage a full shift.
>
> I wonder what kind of electric car Sloman has.
You know perfectly well that I've got the same Mercedes 180B that we
bought in 2011. It burns gasoline. If I needed to replace it I would
probably buy an electric car, but I don't drive much and the little Merc
is perfectly adequate. My wife bought it largely because it has a
four-way adjustable passenger seat. She'd got European Lyme disease
around 2005, and needed that sort of adjustment to get comfortable.
--
Bill Sloman, Sydney
--- SoupGate-Win32 v1.05
* Origin: you cannot sedate... all the things you hate (1:229/2)
|
