From: jl@glen--canyon.com
On Sun, 21 Dec 2025 15:54:01 -0000 (UTC), antispam@fricas.org (Waldek
Hebisch) wrote:
>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!
>
>Given trouble due to CO2 and movement to electricity, I wonder
>why nobody is talking about electrolysis of CO2? Clearly there
>are technical difficulties, starting from fact that in normal
>conditions CO2 is a gas. But is it hopeless?
We burn carbon - combine it with oxygen - to make energy.
It would take more energy - several times as much - to separate the
carbon from the oxygen.
John Larkin
Highland Tech Glen Canyon Design Center
Lunatic Fringe Electronics
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