# [EVDL] A123 small LiFePO4 cells, Reliability



## EVDL List (Jul 27, 2007)

Hi,

With the folk's putting 1000's of small C size cells Li-Ion in parallel/series combination, Tesla, racers etc., I can't imagine that the reliability would be good, the MTBF must be terrible for all those component points of failure. I sure wouldn't want to rely on my daily drive 74 miles a day/ 370 miles a week like I do on lots of little cells with zillions of connections all strung together!

Have a renewable (and reliable ) energy day,
Mark
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## EVDL List (Jul 27, 2007)

Hi Mark,
The only way I have seen to do this with any reliability is to have the
cells axially spring loaded in a tube, like a flashlight.

And we all have personal experiences with flashlights not working.....
Stephen

On Fri, Jul 25, 2008 at 2:11 PM, Mark Hanson <[email protected]>


> wrote:
> 
> >
> > Hi,
> ...


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## EVDL List (Jul 27, 2007)

You could engineer something similar such that each cell snapped into a
carrier and the 
carrier was wired together. So you could snap in and out cells. I would
think
that there would be quite an overhead for the "carrier" or what ever you 
want to call the unit that held the cells.

You could do this on a cell per cell basis -- or spring load several of them
like 
Mark suggested -- "flashlight" like. Maybe even like a caulk gun so that
you could increase the pressure, and release it to get the cells out.
In fact the more I think of this "solution" the more I like it.

Seems like a reasonble engineering puzzle -- but not one most of us at home
have the equipment to manufacture.

Mike






> enganear wrote:
> >
> > Hi Mark,
> > The only way I have seen to do this with any reliability is to have the
> ...


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## EVDL List (Jul 27, 2007)

> Stephen Chapman wrote:
> > Hi Mark,
> > The only way I have seen to do this with any reliability is to have the
> > cells axially spring loaded in a tube, like a flashlight.
> ...


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## EVDL List (Jul 27, 2007)

> Bill Dube wrote:
> > The weight and bulk of the carrier transforms your nifty lightweight
> > high-tech cells into the equivalent of much lower-cost, bulky, heavy
> > "ordinary" batteries.
> ...


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## EVDL List (Jul 27, 2007)

Tesla must think its reliable. I don't think they are using A123 
cells, but they have over 6000 of them in their roadster.



> Bill Dube wrote:
> 
> > Pressure contacts (like a flashlight) are not at all relaible in a
> > vehicle. They also don't work well with the large currents involved
> ...


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## EVDL List (Jul 27, 2007)

MEC has been selling solderless power tubes, using a heavy walled
plastic tube, endcaps, threaded tensioning rods, and conductive silver
paste.

I have two 8 cell packs for my RC airplane and so far no issues (been
using them about a year now). Then again the environment is a lot more
benign than in a vehicle. The cells seem to be ok with the compression
forces, though I do wonder how they'll be in 5 or 10 years.

http://www.modelelectronicscorp.com/view_products.php?tid=1&stid=2

They don't have any for sale at the moment, apparently because of some
supplier issues for the end caps, and there aren't any pics on the
site, but you can view the patent images (you may need the AlternaTIFF
plugin):

6,187,470
6,303,248

The direct url is really really long, so go here, and copy/paste to
the search field:
http://patft.uspto.gov/netahtml/PTO/srchnum.htm

About balancing them (I posted this over at rcuniverse.com forums last
year):
------------
"there is no practical way to bring out nodes for balancing, is
there?" 

It can be done. 
I used some silver plated copper strips 0.01 x 1/2 inch, cut so the
full width contacts the button top, and cut with a 1/8 wide x 1/4 inch
solder tab bent at a 90 degree angle. I used a dremel cutoff wheel and
#11 xacto and made narrow slits just wide enough to let the tab
through. I inserted the tab, then a battery with silver paste, the
next battery, etc., soldered the wires to the tabs (quickly). I
dremeled a hole/channel in the end caps for the wires (so the wires
wouldn't dangle outside), and also soldered to the end pieces. I used
two layers of kapton tape to insulate the exposed solder joints on the
the tabs. If you try this, make sure to remove all burrs from cutting
the strips. I also cleaned the strips with steelwool and alcohol/paper
towel before assembling. 

I've also seen it done just by drilling some holes, and squeezing the
balance wires in between the cells. But that method has less contact
area than using 1/2 wide strips. A bit higher losses I'd think, but at
30 amps probably ok. My motor pulls 70 amps though, so I wanted to do
it right. 

Note about A123. The button part you normally think of as the top of a
battery and the positive terminal is actually the negative terminal.
The case is positive. Learned that the hard way and had to take it all
apart to fix...
---------------

I was thinking about making an ev battery using PVC, ABS, or
fiberglass pipe the appropriate length and diameter and trying this
with Sanyo N-4000DRL D nicads. Using like 10 cells end to end in a
tube and stacking like 20 tubes in say a 4x5 arrengement, and have big
endplates and a bunch of threaded tensioning rods. The cheapest I've
found the Sanyos is Digikey at 6.24/1000, so it would be at least a
$13,000 pack... According to one NASA doc I found, the cells are good
for 2100 to 3700 cycles at 40% DOD, which comes out to about
$0.22/mile if 40% DOD is 20 miles and it lasts 3000 cycles. I still
have lots of other debts to pay first so it's only a thought at this
point...

I'm not interested in lithium with small cells due to the balancing
issues. Charge D nicads at C/10 and you don't have to worry about
balancing. I'd still want a temperature sensor on each cell, so you
could find bad ones though (picture 2000 holes in the pipes and 2000
sensors fitted.. hehe..).

Brad Baylor


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## EVDL List (Jul 27, 2007)

"Using like 10 cells end to end in a
tube and stacking like 20 tubes in say a 4x5 arrengement"

Doh.. I'm off by a factor of 10... 
Ok.. 20 in a tube and a 10x10 stack of tubes. That's better. 

Brad Baylor


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## EVDL List (Jul 27, 2007)

Is the vibration along with the bumps and flex of the automotive
environment one of the drivers behind the need for high quality welds?



> -----Original Message-----
> From: [email protected] [mailto:[email protected]] On
> Behalf Of Lee Hart
> Sent: Friday, July 25, 2008 5:32 PM
> To: Electric Vehicle Discussion List
> Subject: Re: [EVDL] A123 small LiFePO4 cells, Reliability
> 


> > Bill Dube wrote:
> > > The weight and bulk of the carrier transforms your nifty lightweight
> > > high-tech cells into the equivalent of much lower-cost, bulky, heavy
> > > "ordinary" batteries.
> ...


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## EVDL List (Jul 27, 2007)

> Rod Fitzsimmons Frey wrote:
> >
> > Is the vibration along with the bumps and flex of the automotive
> > environment one of the drivers behind the need for high quality welds?
> ...


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## EVDL List (Jul 27, 2007)

> Rod Fitzsimmons Frey wrote:
> > Is the vibration along with the bumps and flex of the automotive
> > environment one of the drivers behind the need for high quality welds?
> 
> ...


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## EVDL List (Jul 27, 2007)

Thanks. I'm trying to determine an adequate testing regimin for
home-assembled cell packs, so that's very helpful, especially the points
about oxidation. Testing within a somewhat corrosive environment (maybe a
jar of HCl nearby would be sufficient), with vibration, over a period of
days would probably do it.

> -----Original Message-----
> From: [email protected] [mailto:[email protected]] On
> Behalf Of Lee Hart
> Sent: Saturday, July 26, 2008 8:59 AM
> To: Electric Vehicle Discussion List
> Subject: Re: [EVDL] A123 small LiFePO4 cells, Reliability
> 


> > Rod Fitzsimmons Frey wrote:
> > > Is the vibration along with the bumps and flex of the automotive
> > > environment one of the drivers behind the need for high quality
> > welds?
> ...


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## EVDL List (Jul 27, 2007)

> > Bill Dube wrote:
> >> The weight and bulk of the carrier transforms your nifty lightweight
> >> high-tech cells into the equivalent of much lower-cost, bulky, heavy
> >> "ordinary" batteries.
> ...


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