[ausev] meeting

[Willie McKemie]

On Sun, Mar 15, 2009 at 04:39:43PM -0500, Chris Robison wrote:
> 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 thought the TS-LFPs are monolithic.  Since I'm not yet willing to 
take one apart, can you refer me to a dissection?

> > 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). 

I agree it may be market puffery or dishonesty.  Note that I said 
"advertised life".  However, TS states life (number of cycles) as 
dependent on depth of discharge and not less discharge for larger 
cells.
> 
> 
> > 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.

I agree that you can make your own large "cells" by paralleling smaller 
cells.  But at a great complexity penalty to the converter.
> 
> > 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.

Well.... yes.  My point is that you can maximize longevity (based on 
information that we have right now) and minimize complexity (and 
probably cost) by using large cells.  At least, that is the conclusion 
I came to when deciding what cells to use.

-- 
Willie, ONWARD!  Through the fog!
http://counter.li.org Linux registered user #228836 since 1995
Debian3.1/GNU/Linux system uptime  456 days 11 hours 36 minutes