[ausev] AGM batterys

[Peace]

I agree with Bill on LFPs.  LFPs do have higher upfront costs (though they've dropped in price substantially over the past year).  However their increased service life more than makes up for it.  EMS has tested LFPs to 4000+ cycles and the cells still have 85% of their original capacity.  That includes deep discharges which would destroy SLAs.  Consider this -- lead-acids must not be discharged too far (~50%?) or their life cycle drops dramatically.  To get 500-1000 cycles out of leads, you have to over-design the batteries by 2x (e.g. 200Ah leads can only deliver 100Ah without damage).  That means you're carrying around half dead weight in batteries.  Lead-acids' lower energy density adds insult to injury.  I'll never invest in leads again.

I've heard of people running LFPs without a BMS.  They have to manually balance the cells every dozen or so cycles.  The cost of a BMS can be fairly high, and in this respect can be viewed as a convenience item that automates cell balancing.  If you are diligent and can spend the time/effort to balance them periodically then you can avoid the cost of a BMS.  The catch is that safety concerns can arise if you don't mange the cells in a timely manner.  BMS's are still maturing and they're not all created equal, so there's something to be said for manually balancing.

More data from the ThunderSky group:
> "Scott Peterson, Jay Apt, and Jay Whitacre are preparing a paper titled 
> "[B][I]Lithium- Ion Battery Cell Degradation Resulting from Realistic Vehicle and Vehicle-to-Grid Utilization[ /I][/B]"
> 
> The paper applies some interesting NHTSA driving pattern information to
charge/discharge cycles of A123 
> LiFePo4 cells along with a simulated
Vehicle to Grid discharge cycle.
> 
> Notably the cells seemed to last LONGER than expected in all test
regimes, and MOST notably, depth of discharge 
> (DOD) did NOT have the
expected reduction in capacity in extended life cycling. Cells
discharged to 95% DOD, 
> instead of fading to 80% capacity in 1500
cycles, seemed destined to hit 5300 cycles before diminishing to that
level.
> 
> These test results would seem to indicate BETTER life cycle performance
that the manufacturers specifications for 
> the cells, and apparently to
a pretty dramatic degree.
> 
> The paper is athttp://wpweb2. tepper.cmu. edu/ceic/ papers/ceic- 09-02.asp
> 
> Jack Rickard"




----- Original Message ----

Fw: Re: [ausev] AGM batterys
Re: [ausev] AGM batterys
Wednesday, September 23, 2009 10:07:05 AM
From: 
"Bill Gooch" <goochb at alum.mit.edu>
To: 
"" <ausev at austinev.org>
Quoting ausev-request at austinev.org:

> Message: 2
> Date: Tue, 22 Sep 2009 09:50:17 -0500
> From: Willie McKemie <mckemie at spamcop.net>
> Subject: Re: [ausev] AGM batterys
> To: AustinEV News Announcements and General Discussion
>     <ausev at austinev.org>
> Message-ID: <20090922145017.GT12742 at c25>
> Content-Type: text/plain; charset=us-ascii
> 
> On Tue, Sep 22, 2009 at 09:02:57AM -0500, Bill Gooch wrote:
>> Joby,
>> 
>> I'm not an expert, but my assessment was that flooded LA batteries
>> have a better power-to-weight ratio (higher energy density) than AGM
>> batteries.  For example, I'm using Trojan T1275s in my Mr2, which
> 
> I didn't find your conversion on http://evalbum.com
> ....

Yeah it's not in there yet, but I'll get it there before too long.  Gary Ellis and I are still working on some finishing bits, such as improved cooling for the Kelly controller.  It also needs some other small things and some cosmetic work done, and I don't feel it's quite ready for showing yet.  I'll be going to Fredericksburg for Saturday only, and don't plan to take the car this time.

Regarding batteries, I'd also say that if I do another conversion, I'll be highly inclined to go with Lithium Iron Phosphate, unless something even better comes along.  I think the performance and lifespan tradeoffs are leaning in favor of LiFePO4 now, even at the much higher cost.  The main downside aside from the cost seems to be the complexity of proper battery management to insure the lifespan.

Bill