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From: KevinJ93 <kevin_es@whitedigs.com>
Newsgroups: sci.electronics.design
Subject: Re: lithium explosion
Date: Sun, 14 Apr 2024 14:56:43 -0700
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On 4/14/24 1:11 PM, John Larkin wrote:
> On Sun, 14 Apr 2024 10:10:31 -0700, KevinJ93 <kevin_es@whitedigs.com>
> wrote:
> 
>> On 4/13/24 9:35 AM, John Larkin wrote:
>>> On Sat, 13 Apr 2024 16:14:07 +0100, liz@poppyrecords.invalid.invalid
>> ....
>>>
>>> And a defect sensor would have to constantly snoop every cell of a
>>> pack. A typical Tesla might have 7000 cells.
>>>
>>
>> Tesla (and every other EV manufacturer) does monitor the voltage of
>> every individual cell and every cell has its own fusible link in case it
>> becomes shorted.
>>
>> kw
> 
> Tesla uses many small cell in parallel, numbers like 74. I wonder how
> they could monitor the voltage of each cell.
> 

As you say many are in parallel in a module (the actual number varies 
with the particular model and revision). The cell voltage of all those 
cells are the same so can be measured with a single channel of the BMS.

> I assume "becomes shorted" means that the battery terminals are
> shorted somehow. The bigger hazard is that a cell will short
> internally, and all its paralleled friends will then dump thousands of
> amps into it.

Each cell has its own fusible link so in that case the link for that 
specific cell will blow. The maximum current for an individual cell is 
in the region of 40-50A.

In some models Tesla has an overall pyrotechnic fuse to disconnect the 
pack from the vehicle very rapidly if there is excess current (>1000A or 
so).

> Monitoring or fusing won't help a 5-second internal ignition from a
> separator failure.

Each cell is in a steel cylinder that can contain a single cell failure 
and minimize the probability of cascade failure. A fully-charged cell 
contains about 10-20 Wh of energy.

Some car manufacturers use larger format cells with significantly larger 
storage per cell.

> I'm certain that few cheap Chinese bike and scooter batteries have any
> sort of safety systems. As they age, they may get more dangerous.
> 
I believe that some of those use pouch cells (similar to those in 
cell-phones, tablets and notebook computers). There is less physical 
protection against cascade failure in that case.

kw