[Oeva-list] update on is battery fuel

Gary Graunke gary at whitecape.org
Fri May 6 06:22:22 PDT 2011


Steve,

Well, this is another difference between lead acid and lithium based 
chemistries. They don't have liquid with things floating around inside.

When I first converted my Insight 8 years ago, I used left-over, 12 year old 
AGM lead-acid batteries for it. Later I bought a new pack of Panasonic AGM's. 
Because I have the data acquistion system, I have lots of graphs showing 
batteries straining as I went over Sylvan, and recording their behavior as I 
drove them until they died. I had plenty of spares, and replaced them as they  
constrained my range--well before the sudden death that you experienced. 
However, I have no doubt that if I continued to drive them, reversing cells in 
the process, sudden death would occur. 

But 4 years ago I switched to A123 LiFePO4 cells, after talking to Bill Dube 
about his 1 year experience with them in the Killacycle. The A123 cells are a 
whole new experience. Now they are at least one generation behind the new 
cells, but at the time they were the best.

They store between 4 and 5 times more energy per pound than AGM's do. Also, 
they lose very little of their capacity at large current draws (the Peukert 
effect):  drawing at 40C (40 times the capacity of the battery), they lose only 
5% over the 20 hour (C/20) draw. This means that in an EV drawing 1C (50 amps 
for a 50 AH battery) gets about 6 times the energy per pound. I went from 
getting 20-25 miles with 466 lbs of lead to 60 miles with 200 lbs of LiFePO4. 

In addition, they get 7000 cycles vs 300-500 for lead AGM batteries. The 
previous winner was NiCd at 3000 cycles. Paul Wallace had SAFT flooded NiCd 
batteries for many years in his S10. If you do one full cycle a day, this is 
over 19 years. Both lead acid AGM and LiFePO4 cells have a shelf life of about 
15 years. So you will be seeing mostly defects in manufacturing (sometimes 
these result in shorter life rather than immediate failure) rather than actual 
acute death events. About 2-3% of A123 batteries had a defect that resulting 
in higher self-discharge rate, but otherwise normal performance. 

Every battery chemistry and manufacturer is a bit different, but for high 
quality manufacturers, we should see only an occasional early defect. Defects 
later should be rare (but there will still be some). 

The BMS in a commercial vehicle should identify them so they can be replaced. 
But, I guess the point is, you can replace these individual cells/modules, and 
this is a relatively minor task compared to engine work. 

For most of us, however, the pack will simply have decreased range as a result 
of all the cells degrading. At some point, we will upgrade to a new pack. 
The pack will still have a long useful life doing non-mobile applications 
before it is time to recycle/remanufacture new batteries from the old ones.

The point is that we will decide when to do this--the battery will not force 
us to do so unexpectedly unless we really decide to "drive it until it drops" 
at extremely reduced ranges.

Gary



On Thursday, May 05, 2011 08:20:08 PM Steve's Account wrote:
> > Subject: Re: [Oeva-list] Fwd:  Update on is battery fuel
> > 
> > You probably saw my reply to Myles--I won't repeat it here.
> > 
> > However, one more difference. The battery seldom just dies like an engine
> > so that you have to replace it. Sometimes you may have to replace
> > individual defective cell/modules, but it degrades slowly rather than
> > aburptly most of the time.
> 
> Gee.. Most of the batteries I have had fail, (Lead Acid) usually die fairly
> suddenly.. and most often it'd due to shorting of a cell. Sometimes you can
> break the bridge and keep them going for a while longer.. but often it's
> the beginning of the end.
> 
> You then end up with a pack you can't recharge to a 100 percent SOC and
> pretty soon the other batteries in the string die due to deep discharge
> cycling.
> 
> If you have a BMS and can remove the dead battery from the string fairly
> quickly.. you can often replace it and get many more miles from the
> remaining batteries.
> 
> > When you can't go as far as you need to, you replace the battery with a
> > new one. You might also be able to sell the old battery to someone who
> > doesn't need to go as far, who may be able to use it for a very long
> > time.
> > 
> > Gary
> 
> Steve
> 
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