[Oeva-list] A123 batteries

Gary Graunke gary at whitecape.org
Fri May 6 11:21:01 PDT 2011


A good question, but I don't have the answer for any specific battery.

Pulling higher currents will cause heat due to the internal resistance, and 
heat is the #1 enemy of battery life. The LiFePO4 cells have way more power 
than you need--just the sort of cells that you are looking for in a tiny pack. 
But if the storage temperature affects battery lifetime (in my previous email 
about batteries as fuel--NOT), increasing operating temperature while driving 
or charging is also not a good thing for battery life.

I have never seen specs on battery life with different rates of discharge.
It is probably a good idea to stay within the power that was specified (they 
*did* specify the power rate, one would hope) in the vendors battery lifetime 
data. Another way is to stay within the power spec of the battery, but often 
vendors do not tell you that the power rate for the lifetime may be well below 
the max power spec, or put another way, the lifetime expectancy is reduced at 
the max power rate.

Bill Dube drew 180A for 2 seconds out of 10 parallel A123 2.3AH cells for the 
killacycle. But he also pre-heated them to 70C. They were 100C (boiling point 
of water) by the end of the draw. (The spec says the operating range tops out 
as 60C). We also know that they can withstand 160C during a vehicle fire and 
still work from an accident involving a Hybrids Plus plug-in Prius. Doing this 
for a year, his cells lost 5% of their capacity, and he expected the loss to 
be linear--5% every year. Other cells are likely not so good. But remember, 
the drag racers are out to break world records--not maximize battery life or 
minimize operating costs. You probably want to drive for more than 7 seconds. 
;-)

So I think when a pack is properly designed for range, you would want to stay 
within the power spec, or better yet, the power specified in lifetime data 
(assuming it is reasonable--a 20 hour draw (C/20) is not reasonable for EV 
use). Take the AH of the pack, divide by the time you want to draw that 
current, and that will determine your power rate. 

The pack capacity in watt hours (now the voltage comes into play) will 
determine the energy available, and how far you can drive in that time. 
Driving faster will take more energy per mile, and your motor controller has a 
minimum and maximum voltage that it will take.

Gary

On Friday, May 06, 2011 09:57:35 AM Jon Balgley wrote:
> I should phrase this a little differently.
> 
> If I configure the pack to have severely limited range, in order to keep
> weight/costs down, how do I factor in any extra degradation due to pulling
> higher C-rates during hard acceleration?
> 
> On Fri, May 6, 2011 at 7:53 AM, Jon Balgley <jon at photodad.com> wrote:
> > In a few recent messages, different people praised and commented upon
> > A123 batteries.  I am hoping to upgrade to A123's, or similar?, when my
> > current lead-acid pack dies.
> > 
> > Someone said, re: A123's, "properly sized for range" -- since A123's can
> > tolerate much higher C-rates, what does that mean, if I only want limited
> > range?  If I design the pack to be much smaller, pull more amps from it,
> > how do I estimate how much less lifetime I'll get from them?
> > 
> > It appears that A123 20Ah pouch cells are appearing on the market, too.
> > 
> >  Anybody around here using them yet?  I know John Wayland is using pouch
> > 
> > cells from Dow Kokam.



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