[ausev] meeting

[Chris Robison]

Some points to consider.  Mainly, realize that TS's cells are actually 
made up of multiple smaller cells inside the plastic boxes. They combine 
the smaller cells to make assembly of a large scale vehicular pack more 
convenient. There are a few manufacturers who do make large-scale truly 
monolithic cells, and as far as I've seen they claim that they should 
not be used for EVs due to very poor power density.

I'll respond to your points individually below:

Willie McKemie wrote:
> I enjoyed my second AustinEV meeting today.
>
> I was chatting with someone about the advantages of using large LFP 
> cells and did not come up with the points I wished to make.  
> Befuddlement, be gone.  Here are the points:
> 1) large (>=200ah) TS LFP cells have an advertised life of 3000 cycles 
> at 80% discharge and 4000 cycles at 70%.  Smaller cells are 2000 and 
> 3000 cycles.
>   
This may be just marketing puffery, to get you to buy larger cells, or 
it may be an honest guess that larger cells will tend to be discharged 
less deeply and therefore should last longer, assuming by extension that 
you're comparing packs of similar voltage (and therefore very dissimilar 
total capacity). 


> 2) For a given application, the large cells can be discharged and 
> charged at lower relative rates.  I infrequently demand more than .5C 
> and never more than 2C.  The low rates are supposed to lead to 
> longevity.
>   
Again, this can be achieved by having more small cells in parallel, 
similar to how the large TS cells themselves are constructed.

> 3) For a given voltage and given average trip length, the large cells 
> will be discharged to a higher SOC than smaller cells.  That, also, 
> leads to longevity.
> So, that is the "triple whammy" that makes large cells more attractive 
> with respect to longevity.
>
>   
In fact, voltage and the size of individual cells don't really matter 
here -- what you're really getting at is total pack capacity in 
watt-hours.  The more watt-hours you have, the less deeply the pack will 
be discharged on any given trip, and the lower the current will be for 
any given cell. This applies to any voltage, any cell configuration, and 
any chemistry.


  --chris